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Originally published In Press as doi:10.1074/jbc.R400002200 on March 17, 2004

J. Biol. Chem., Vol. 279, Issue 22, 22787-22790, May 28, 2004
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Cotranslational Membrane Protein Biogenesis at the Endoplasmic Reticulum*

Nathan N. Alder{ddagger} and Arthur E. Johnson{ddagger}§

From the {ddagger}Department of Medical Biochemistry and Genetics, Texas A&M University System Health Science Center, College Station, Texas 77843-1114 and §Departments of Chemistry and of Biochemistry and Biophysics, Texas A&M University, College Station, Texas 77843


   INTRODUCTION
TOP
 INTRODUCTION
Translocon Structural and...
 Nascent Chain Topogenesis
 Integration of TMSs into...
 Nascent Chain Regulation of...
 Some Unresolved Issues
 REFERENCES
 
In eukaryotic cells, most polypeptides destined to become membrane proteins are initially integrated into the membrane of the endoplasmic reticulum (ER)1 before being sorted to the location at which they function. Integration occurs at sites in the ER membrane termed translocons that are comprised of a specific set of membrane proteins (1, 2). In most cases, proteins are integrated into the bilayer cotranslationally, i.e. at the same time that they are being synthesized by ribosomes. During this process, the biosynthetic machinery mediates the integration of transmembrane sequences (TMSs) into the nonpolar core of the bilayer and delivers aqueous cytoplasmic and luminal domains to the appropriate compartments. Simultaneously, a nascent protein may undergo covalent modification (e.g. signal sequence cleavage, disulfide bond formation, and N-glycosylation), folding, and interactions with other proteins (e.g. chaperones) that ultimately lead to the assembly of the polypeptide into a functional monomeric or multimeric complex (1–3). Membrane protein biogenesis is therefore exceedingly complex, especially because the mechanisms involved are further constrained by the need to maintain the permeability barrier of the membrane.

Here we highlight the most recent advances in our understanding of cotranslational integration at the ER membrane, focusing on four overlapping areas: translocon structural and functional states; nascent chain topogenesis; insertion of TMSs into the bilayer; and nascent chain regulation of integration. Other processes coupled with integration (e.g. covalent modification, folding, assembly, and quality control) and protein integration at other membranes are beyond the scope of this minireview.


   Translocon Structural and Functional States
TOP
 INTRODUCTION
 Translocon Structural and...
Nascent Chain Topogenesis
 Integration of TMSs into...
 Nascent Chain Regulation of...
 Some Unresolved Issues
 REFERENCES
 
Structure—The mammalian ER translocon consists of the core heterotrimeric Sec61 complex (Sec61{alpha}{beta}{gamma}) and TRAM along with several associated protein complexes (1). The Sec61p complex in the yeast ER membrane contains homologous core components in addition to the Sec62/63/71/72 complex, whereas the prokaryotic translocase consists of a heterotrimeric SecYEG complex that is homologous to the Sec61 complex (2). Membrane protein integration in bacteria also involves YidC, a protein that functions in both a Sec-dependent and Sec-independent manner (4). The mammalian translocon forms an aqueous pore that spans the ER membrane, and the walls of the pore are formed primarily by Sec61{alpha} (1). Cryo-EM images indicate that 2–4 copies of the Sec61 complex oligomerize to form rings with pores of ~20 Å (or indentations that may result from pore sizes below the resolution limit) that align coaxially with the nascent chain exit tunnel in the ribosome (5–9).

The first atomic level view of translocon proteins has been provided by the crystal structure of an archeal SecYE{beta} (10). Because the SecYE{beta} complex was crystallized in the absence of substrate and hence is in a translocation-inactive and sealed or "stand-by" conformation, one can only conjecture how the complex might rearrange during substrate translocation and integration. Interestingly, this SecYE{beta} structure is proposed to form a functional translocation pore with a single heterotrimer, which is at odds with the multimeric Sec61 (5–9) and SecYEG (11) complexes observed in cryo-EM studies as well as the detection of SecY oligomerization using FRET (12). Further experiments are necessary to ascertain the structure and oligomeric state of a functioning mammalian translocon (e.g. FRET studies to directly determine translocon component stoichiometry and arrangement as well as the magnitude and dynamics of translocon conformational changes).

Function—The mammalian ER translocon is a dynamic multicomponent and multifunctional complex that facilitates protein translocation across and integration into the ER membrane (1). Ribosomes synthesizing membrane or secretory proteins are identified by a signal sequence in the nascent chain that binds to SRP (Fig. 1, i) (13). A GTP-dependent interaction between SRP and SR then elicits the binding of the RNC to the translocon and the transfer of the nascent chain to the translocon (Fig. 1, ii). Although some signal sequences are cleaved from the nascent chain by signal peptidase (Fig. 1, iii), others contain additional residues and are not cleaved; instead, such sequences move laterally into the bilayer to form a TMS that is termed a "signal anchor" (SA) sequence.



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  		FIG. 1.
Functional stages in the integration of a signal-cleaved, single-spanning membrane protein. The core proteins that form the translocon are shown in yellow, whereas many of the associated proteins that comprise a fully assembled, functional translocon (e.g. oligosaccharyltransferase, calnexin) have been omitted for clarity. The BiP-mediated luminal gate is represented by the magenta oval; it is not yet known whether BiP binds directly to the pore to plug it or whether closure is effected indirectly through BiP binding to other proteins that effect closure. SR and signal peptidase (SPase) are shown at only a single step in the process but may be proximal to the translocon at other steps. SRP binds a signal sequence (cylinder), arrests elongation, and targets the RNC to the translocon via the SR (i). After targeting, the RNC binds tightly to the translocon (yellow), and translation resumes (ii). Once the nascent chain reaches a threshold length (14), the BiP-mediated luminal gate (magenta) is opened and later the signal sequence is cleaved by signal peptidase (iii). The functional mode of the RNC-translocon machinery switches from translocation (iv) to integration (v) shortly after synthesis of a TMS (wavy line). The TMS folds into a stable compact conformation compatible with an {alpha}-helix within the exit tunnel near one end of the extended ribosomal protein L17 (red), resulting in the closure of the luminal end of the pore (v), presumably through a conformational change in L17 that in turn elicits a conformational change in a transmembrane translocon or translocon-associated protein (cyan) that leads to the binding of BiP and/or other proteins and the formation of the BiP-mediated gate. The RNC-translocon junction opens when the folded TMS reaches L39 (green)(vi). Following synthesis of the cytosolic domain (vii) and termination of translation, the TMS of the mature protein is released from the translocon and integrated into the bilayer (viii). In contrast, a nascent secretory protein passes through the ribosome in an extended conformation and is exposed only to L4 (orange) (iv). The colors of L4, L17, and L39 are shown as unshaded when they photocross-link (are proximal to) a nascent chain TMS. The molecules depicted are not to scale.

 
As translation proceeds, different regions of the nascent chain must be directed to the cytosol, lumen, or bilayer without disrupting the permeability barrier of the membrane. To this end, the ribosome and translocon undergo a series of conformational changes that are designed to alternately gate the cytosolic or luminal end of the pore and thereby direct the nascent chain into the appropriate aqueous compartment. Following RNC targeting to the translocon, an ion-tight RNC-translocon junction seals off the nascent chain from the cytosol, thereby directing the nascent chain into the ER lumen (Fig. 1, iv) (14, 15). Upon detection of a TMS (see below), the luminal end of the pore is closed by the action of the luminal Hsp70 chaperone BiP (Fig. 1, v), and shortly thereafter the RNC-translocon seal is opened to allow the cytoplasmic domain of the membrane protein to move into the cytosol while the luminal end of the pore remains closed (Fig. 1, vi and vii) (15, 16). For a polytopic membrane protein with multiple TMSs, we presume that the translocon pore is alternately sealed at its cytosolic and luminal ends by the RNC and BiP, respectively, though this has yet to demonstrated experimentally. Upon termination of translation and release of the membrane protein into the bilayer, the ribosome-free translocon remains assembled, and its pore is sealed on the luminal side by the BiP-mediated gate (Fig. 1, viii) (17). The translocon also undergoes dramatic conformational changes during its functional cycle. Not only do TMSs of the translocon proteins part to allow nascent chain TMSs to move laterally into the bilayer, but the inner diameter of the pore also increases from 9 to 15 Å in the ribosome-free closed state (e.g. Fig. 1, i) to 40–60 Å in the RNC-bound state (e.g. Fig. 1, iv) to a smaller diameter when the pore is sealed by both BiP and the RNC (Fig. 1, vii) (16, 18). One reason for this dynamic flexibility may be the need to accommodate multiple TMSs that may leave the translocon in pairs or groups (1).

The above mechanisms that maintain the permeability barrier of the ER membrane were determined by measuring the accessibility of fluorescent-labeled nascent membrane proteins to hydrophilic collisional quenching agents located on either the cytosolic side or on both sides of the ER membrane. These studies revealed that the pore is never in an aqueous continuum with both the cytosol and lumen; during translocation of luminal domains, the pore is sealed by the RNC-translocon junction, and during synthesis of cytosolic domains, the luminal end of the pore is sealed by the BiP-mediated gate (15, 16). Moreover, large collisional quenching agents have been observed to move completely through a RNC-bound mammalian translocon pore occupied by a translocating nascent chain, thereby indicating that there is no constriction in the pore during translocation (18). The fluorescence-detected reduction in pore size observed with a translocon that is sealed by the action of BiP may be accomplished by a conformational change in the translocon core that creates a constriction, but whatever the structural basis of the smaller pore, it does not completely block ion movement through the pore (17). It is important to emphasize that this fluorescence-based experimental approach directly measures ion movement and accessibility and therefore reveals how an ion-tight seal is maintained at the translocon throughout integration to maintain ion gradients such as the Ca2+ stores in the ER lumen.

In contrast, the existence of a gap between the ribosome and translocon in cryo-EM images (7–9) led to the suggestion that the permeability barrier is maintained by a constriction in the translocon pore, a view supported by the crystal structure of the monomeric SecYE{beta} (10). The differing interpretations are most likely explained by the differences in the samples being analyzed. Whereas the fluorescence experiments examine integration or translocation intermediates that are functional and fully assembled in the membrane, the translocons used for high resolution cryo-EM images are detergent-solubilized and hence devoid of lipids, the translocon core protein TRAM, and other translocon-associated proteins, any of which may be important in maintaining a functional ion-tight RNC-translocon junction. Also, because an archeal organism would have to seal an unoccupied SecYE{beta} pore without the assistance of a BiP-like molecule on the other side of the membrane, a constriction that completely closes the pore may be required for such organisms.


   Nascent Chain Topogenesis
TOP
 INTRODUCTION
 Translocon Structural and...
 Nascent Chain Topogenesis
Integration of TMSs into...
 Nascent Chain Regulation of...
 Some Unresolved Issues
 REFERENCES
 
TMS Orientation—The orientation of a TMS in the bilayer, Nexo/Ccyt (type I, N terminus toward the lumen) or Ncyt/Cexo (type II, N terminus toward the cytoplasm), is determined by multiple features of the topogenic sequence (19). Charged residues that flank the hydrophobic core of TMSs are the primary topogenic determinants, with the more positive end typically located in the cytoplasm in accordance with the "positive-inside rule" (20)(although not all eukaryotic membrane proteins strictly adhere to this rule (1)). In addition, the hydrophobic core of the TMS influences orientation because greater total hydrophobicity and length of the nonpolar region of the TMS both promote Nexo/Ccyt orientation (19) as does the hydrophobicity gradient within the TMS (21). Folding of the nascent chain prior to arrival of a SA TMS at the translocon can also influence orientation (19).

Although these properties of a TMS have been shown to be important in topogenesis, the components of the biosynthetic machinery that decipher this information have yet to be identified. The fate of a TMS (its orientation and whether it is integrated or not) could be decided in part by soluble and membrane-bound proteins associated with the core components of the translocon (22); the existence of such factors has been demonstrated by both biochemical (e.g. Ref. 23) and genetic (24) approaches. In addition, phospholipids may affect topography (25) and influence substrate folding (26). Thus, the unique topography adopted by most native membrane proteins cannot be simplified or generalized to a single or very few interactions but instead results from the summation of several physicochemical properties of the TMSs and the combined influences of nascent chain interactions with the translocon and associated components. The existence of alternate topographical isoforms for some proteins (see Ref. 1 for examples) presumably arises from a delicate balancing of different regulatory factors (22).

The flanking regions can also influence the recognition of a TMS during biogenesis. The inversion of the charges that flank the N-terminal TMS of the Glut1 glucose transporter (27), the removal of positive charges from an internal loop (28), or the introduction of basic residues into an exofacial loop (29) results in the exclusion of hydrophobic TMSs from the membrane.

Topogenesis of Polytopic Membrane Proteins—In the simplest model of polytopic protein topogenesis, the orientation of the first TMS inserted in the bilayer dictates the orientation of succeeding TMSs because of the requirement to alternate TMS orientations. However, the integration of many multispanning proteins appears to be more complex; polytopic proteins have multiple topogenic determinants, and proper integration and orientation often require the coordinated interaction of multiple topogenic segments of the nascent chain. Furthermore, an increasing number of studies has detected instances of delayed TMS insertion into the bilayer and of TMS insertion and reorientation that is nascent chain context-dependent.

The topogenesis of many polytopic proteins has been studied by analyzing the topogenic activity of individual TMSs and by examining the coordinate topogenic activity of multiple TMSs using various reporter domains (30). Many topogenic sequences, when examined separately, adopt an orientation that is opposite to their orientation in the native protein; hence, these TMSs require the presence of adjacent TMSs for proper topogenesis. In other cases, individual TMSs can assume the proper orientation in the translocon but must be paired with another TMS for efficient integration into the bilayer (30).

In several cases, a downstream TMS is required for the integration of a preceding TMS with weak topogenic activity (Fig. 2A), as has been observed for Sec61{alpha} (31), CFTR (32), and band 3 protein (33). Such downstream determinants can even impart a transmembrane orientation onto a preceding hydrophilic segment (34). In other cases, TMSs with weak topogenic determinants require a strongly hydrophobic upstream TMS for integration (35) either because an interaction between the two TMSs is required for integration and/or because the first TMS alters the translocon so as to promote its interaction with the second TMS (Fig. 2B). Interestingly, in one case increasing the loop length between the two TMSs resulted in translocation of the weak TM domain, suggesting that the strong TMS may have integrated prior to the arrival of the second TMS at the translocon (35). Also, TMSs bearing charged residues have been shown to require specific electrostatic interactions (charge pairing) with neighboring TMSs for stable integration (3, 36).



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  		FIG. 2.
Integration of polytopic proteins sometimes requires adjacent topogenic determinants. A, integration requires a downstream TMS. Some weakly hydrophobic Ncyt/Cexo TMSs (blue)(i) do not integrate in the absence of downstream determinants (step i'). The requirement for the presence of a C-terminal TMS (red)(ii) suggests two possible scenarios. First, the weakly hydrophobic segment may reside transiently in the cytoplasm until it interacts with the C-terminal TMS after which the two insert together (iii). Alternatively, the presence of the downstream TMS in the ribosome may alter the translocon characteristics via long range signal transduction (e.g. Fig. 1, v) to stabilize the upstream TMS within the translocon, perhaps by reforming the tight RNC-translocon junction (iii'), or to effect its integration before the downstream TMS reaches the translocon (not shown). The presence of both sequences allows for proper integration (iv), resulting in the native topography of the protein (v). B, integration requires an upstream TMS. An upstream TMS (red) may reside in the translocon long enough to allow necessary interactions with the downstream TMS (blue) (ii to iii), facilitating proper integration to achieve native topography (iv). However, lengthening of the loop size between the two TMSs (ii') may result in the integration of the first TMS prior to the arrival of the second (iii') and hence translocation of the downstream TMS (step iv').

 
Time-dependent Reorientation—Goder and Spiess (37) recently showed that each SA TMS initially orients Nexo/Ccyt in the translocon and that any inversion to the opposite orientation must occur within seconds. Consistent with previous studies, they also found that the final distribution of Nexo/Ccyt and Ncyt/Cexo orientations could be altered by adding or deleting a charge residue adjacent to a particular SA TMS or by altering its hydrophobicity. Moreover, TMS reorientation was hindered by glycosylation and by an increased length of intervening loop, suggesting that such inversion can be sterically prohibited and/or that the translocation of a certain length of polypeptide commits the substrate to a particular topography (38). Whether each TMS in a polytopic protein experiences a similar charge-, hydrophobicity-, and time-dependent evaluation and possible reorientation in the translocon has yet to be determined.

Some membrane proteins undergo delayed TMS insertion or rearrangement. For example, internal TMSs (here defined as any TMS synthesized after the initial signal or SA sequence) of nascent CFTR (32) and K+ channel (39) are required for efficient targeting. Moreover, aquaporin 1 (AQP1) is integrated as a loosely folded, four-TMS protein with extramembranous hydrophobic regions and during maturation undergoes topographical reorientation of multiple internal TMSs and peptide loops to assume its native six-TMS structure (40). The related AQP4 protein assumes the mature six-spanning topography cotranslationally, and the relevant differences in the topogenic determinants of AQP1 and AQP4 that dictate these disparate biosynthetic pathways have been identified (41). Similarly, TMS10 of band 3 protein is translocated to the lumen and does not fully integrate into the membrane until both TMS12 and TMS1–3 have been assembled in the bilayer (42).


   Integration of TMSs into the Bilayer
TOP
 INTRODUCTION
 Translocon Structural and...
 Nascent Chain Topogenesis
 Integration of TMSs into...
Nascent Chain Regulation of...
 Some Unresolved Issues
 REFERENCES
 
The molecular environment of the lateral TMS pathway from the translocon pore into the bilayer has been experimentally assessed using photoreactive probes positioned within the TMS of intermediates at defined stages of integration. Upon insertion into the translocon pore, internal and SA TMSs are proximal to both translocon proteins and phospholipids (1, 43–46). These studies showed that TMSs have immediate access to phospholipid, but the length of time a TMS remains adjacent to a translocon protein(s) during integration is controversial.

One proposed mechanism for TMS integration is a partitioning of the nonpolar TMSs into the hydrophobic lipid bilayer from the aqueous pore. This mechanism is supported by photocross-linking experiments that showed TMSs were only transiently proximal to translocon proteins (43, 45, 47). In such a scenario, translocon proteins gate TMS access to the lipid phase without specifically recognizing or interacting with the TMS, thereby creating a one-step integration process in which a SA TMS moves directly from the pore into the bilayer.

An alternative mechanism for TMS integration is based on other photocross-linking studies that showed internal and SA TMSs from a wide range of substrates were adjacent to translocon proteins for prolonged times (44, 46, 48, 49). Some TMSs remain adjacent to Sec61{alpha} and/or TRAM (46, 48) as well as the recently identified PAT-10 (44) until translation terminates. Because in some cases more than 500 Å of fully extended polypeptide separates the TMS from the peptidyl-tRNA (46, 48), the nascent chain tether is sufficiently long to allow a TMS to diffuse away from the translocon, and the fact that it does not do so indicates that the translocon is actively regulating TMS release into the bilayer.

Furthermore, because the TMSs of several different substrates form {alpha}-helices that occupy a fixed position in the translocon relative to Sec61{alpha} and TRAM, it appears that nascent chain TMSs are not able to rotate or move freely within the translocon and hence are bound to a translocon protein(s) during integration (46). The non-random positioning of each TMS within the translocon differed for different TMSs examined in this study, which indicates that individual TMSs bind differently (most likely by van der Waals contacts along complementary surfaces) to Sec61{alpha}, TRAM, and/or other proteins prior to being released into the bilayer (46). Moreover, TMSs of polytopic proteins interact with distinct translocon proteins during integration in a manner dependent on their relative order of insertion (44). Taken together, these results indicate that TMS traffic through the translocon is controlled by the binding of TMSs to translocon proteins and hence that the translocon plays an active regulatory role in membrane protein assembly in part by controlling the lateral movement of TMSs into the bilayer.


   Nascent Chain Regulation of Integration
TOP
 INTRODUCTION
 Translocon Structural and...
 Nascent Chain Topogenesis
 Integration of TMSs into...
 Nascent Chain Regulation of...
Some Unresolved Issues
 REFERENCES
 
A recent FRET study has revealed that inside the ribosomal exit tunnel close to the peptidyltransferase center the ribosome induces a nascent chain TMS to fold into a conformation compatible with an {alpha}-helix (50). In contrast, a nascent secretory protein passes through the ribosome in an extended conformation. Furthermore, the folded TMS of a nascent membrane protein is sequentially proximal to two ribosomal proteins, L17 and L39, that are not exposed to a nascent secretory protein in the tunnel (50). These interactions coincide with changes in the gating of the aqueous translocon pore (15). Therefore, it appears that the recognition of TMS folding by L17 initiates a transmembrane communication pathway that elicits the BiP-mediated closure of the luminal end of the pore (Fig. 1, v) and that subsequent TMS recognition by L39 mediates the opening of the RNC-translocon junction (Fig. 1, vi) (50). Thus, maintenance of the ER membrane permeability barrier and the TMS-dependent conversion of the operational mode of the translocon from translocation to integration are apparently regulated by the nascent chain via a sophisticated series of interactions that involve components of both the ribosome and translocon as well as at least one luminal protein (15, 16, 50). Moreover, the ability of the ribosome to recognize TMSs and effect long range conformational changes may provide a basis for some aspects of topogenesis such as the mechanism by which a C-terminal TMS promotes integration of a weakly hydrophobic upstream TMS (Fig. 2A). The ribosome, translocon, and associated proteins therefore constitute a functional unit in which nascent chain-dependent structural features are recognized and communicated from one component to another, thereby effecting a coupled and ordered response that dictates membrane protein topogenesis, insertion, and assembly.


   Some Unresolved Issues
TOP
 INTRODUCTION
 Translocon Structural and...
 Nascent Chain Topogenesis
 Integration of TMSs into...
 Nascent Chain Regulation of...
 Some Unresolved Issues
REFERENCES
 
The biosynthesis of a fully folded, functional membrane protein at the ER involves complex interactions of the nascent chain with the ribosome, translocon, membrane, and several other proteins. This complexity is then magnified by the differing requirements of a multitude of individual proteins for proper integration and assembly. Because we have information about the integration of only a few substrates, it is fair to say that our understanding of membrane protein biogenesis is still fragmentary and that many structural and mechanistic aspects of integration have yet to be discovered. Among the many important and fundamental questions that remain are the following.

What is the oligomeric state of the intact mammalian translocon when bound to a RNC engaged in translocation or integration? How and where is an internal TMS rotated into an Ncyt/Cexo orientation? By what mechanism does a TMS promote the insertion of another TMS into the bilayer? Does this involve direct interactions between two TMSs? How and when do major rearrangements such as the transition from four to six TMSs in AQP1 occur? Is this structural change translocon-mediated? Do the TMSs of a polytopic protein leave the translocon singly, in pairs, or in a group? When do the TMSs of a polytopic protein assemble into their native structure? Do the ribosome and BiP alternate closing the pore during polytopic membrane protein integration? If so, does the ribosome induce the folding of each TMS to regulate changes at the translocon while the TMS is far inside the ribosome? What happens when the TMSs are separated by short loops (only a few residues)?

Finally, the kinetics of cotranslational integration remain largely unexplored because of the difficulty in obtaining a synchronized and homogeneous sample to examine the time dependence of TMS translation, integration, and assembly. Yet by regulating the kinetics of nascent chain-translocon interactions at different points along the integration pathway, the biosynthetic machinery could, among other things, avoid misfolded proteins and allow TMSs to dynamically reorient within the translocon and to facilitate the insertion of TMSs of intermediate hydrophobicity and thereby profoundly influence the final folded state of the substrate protein.

In conclusion, recent progress has demonstrated that the functional stages of membrane protein integration involve dynamic conformational changes of the ribosome-translocon machinery and that substrate integration is subject to regulation by the translocon, the ribosome, and the nascent chain itself. A major challenge for the future will entail determining how well the paradigms identified to date apply to every protein. Addressing the above questions will require the use of complex biochemical and biophysical approaches applied to an increasing array of substrates as well as structural studies on different functional states of the translocon and biophysical studies of the dynamics of those states.


   FOOTNOTES
 
* This minireview will be reprinted in the 2004 Minireview Compendium, which will be available in January, 2005. Work in the authors' laboratory was supported by National Institutes of Health Grant GM 26494 and by the Robert A. Welch Foundation. Back

To whom correspondence should be addressed: College of Medicine, Texas A&M University System Health Science Center, 1114 TAMU, 116 Reynolds Medical Bldg., College Station, TX 77843-1114. Tel.: 979-862-3188; Fax: 979-862-3339; E-mail: aejohnson{at}tamu.edu.

1 The abbreviations used are: ER, endoplasmic reticulum; TMS, transmembrane sequence; cryo-EM, cryo-electron microscopy; FRET, fluorescence resonance energy transfer; SRP, signal recognition particle; SR, SRP receptor; RNC, ribosome-nascent chain complex; SA, signal anchor; CFTR, cystic fibrosis transmembrane conductance regulator. Back


   ACKNOWLEDGMENTS
 
We thank David W. Andrews and William R. Skach for valuable suggestions.



   REFERENCES
TOP
 INTRODUCTION
 Translocon Structural and...
 Nascent Chain Topogenesis
 Integration of TMSs into...
 Nascent Chain Regulation of...
 Some Unresolved Issues
 REFERENCES
 

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