The JAK family of protein-tyrosine kinases (JAK1, JAK2, JAK3, and TYK2; EC 2.7.1.112) appears to play a crucial role in the signal transduction cascade initiated by activation of numerous cytokine receptors(2) . Cytokines that signal through a common chain, such as interleukin-2, interleukin-4, and interleukin-7, trigger the tyrosine phosphorylation of JAK3 and may also induce the activation of other Janus kinases(3, 4, 5) . Interferons and require both JAK1 and TYK2 in their signaling cascade(6) , and interferon signaling requires both JAK1 and JAK2(7) . A number of cytokine receptors are associated with JAK2, and activation of these receptors by their cognate ligands results in the rapid tyrosine phosphorylation of JAK2. Such receptor systems include those for erythropoietin(8) , interleukin-3(9) , interleukin-6(10) , leukemia inhibitory factor, ciliary neurotrophic factor, oncostatin M(11) , granulocyte-macrophage colony-stimulating factor(12) , growth hormone(13) , and prolactin(14, 15) . It is noteworthy that deletion of the membrane-proximal domains of most of these cytokine receptors abrogates not only the transduction of a cytokine-initiated proliferation but also the cytokine-triggered activation of JAK2(8, 10, 12, 16, 17) . The specific mechanism by which cytokine receptor activation converts these kinases from a latent to an active state remains unclear, but in systems involving the prolactin receptor(14, 15) , the leukemia inhibitory factor receptor chain, and the interleukin-6 signal transducer gp130(11) , JAK2 is associated with receptor before ligand stimulation of the receptor. Thus, it does not appear that receptor association per se is sufficient to activate the kinase. The detailed enzymologic behavior of the ``activated'' Janus kinases has yet to be characterized, and site-directed mutagenesis studies assessing the functional importance of some of the structural domains of murine JAK2 (1) and of human TYK2 (18) have only recently appeared in the literature.

As an initial step toward understanding the mechanism of enzymatic activation of the JAK protein-tyrosine kinases, we overexpressed the rat jak2 cDNA clone in Sf21 cells via the baculovirus expression vector system. We also overexpressed two deletion mutants of JAK2 that were predicted to be active and inactive forms based on the retention and deletion, respectively, of a highly conserved tyrosine kinase motif. The characterization of the resultant protein products is described.

MATERIALS AND METHODS

Construction of Recombinant Baculoviruses

Solubilization and Immunoprecipitation of Recombinant Proteins

()

Immunoblot Analyses of Recombinant Proteins

In Vitro Kinase Assay

RESULTS

A recombinant baculovirus was created with a transfer vector (pBacPAK9:rJAK2) that contained approximately 150 base pairs between the polyhedron promoter and the initiation codon of rat Jak2. The mutants (N661)rJAK2 and rJAK2(C795) were derived from restriction endonuclease digests and religation of the transfer vector pBacPAK9:rJAK2. A schematic diagram showing the JAK homology domains retained in each of the three forms, rJAK2, (N661)rJAK2, and rJAK2(C795), is presented in Fig. 1.

Figure 1: Schematic comparison of recombinant rJAK2 forms. rJAK2 is the protein that contains all seven JAK homology (JH) domains as defined by Harpur et al.(34) ; the rat JAK2 amino acid residues contained within each domain are JH7 (16-138), JH6 (144-284), JH5 (288-309), JH4 (322-440), JH3 (450-538), JH2 (542-824), and JH1 (836-1125). rJAK2 (C795) terminates after Arg and thus lacks 10% of JH2 and all of JH1. (N661)rJAK2 begins at Met and thus includes 60% of JH2 and all of JH1. The antibody recognition site is contained within the JH2 segment retained in all three constructs. A map of the relevant restriction sites of pBacPAK9:rJAK2 is aligned over the rJAK2 protein; the dark arrow represents the polyhedron promoter, and the boxed area represents the jak2 coding region.

A time course of protein production in Sf21 cells infected with the baculovirus-expressing rJAK2 is shown in Fig. 2. The cells were harvested at various times p.i., solubilized in a buffer containing 1% Triton X-100, pre-cleared with protein A-Sepharose CL-4B, then immunoprecipitated with antiserum directed against a synthetic peptide DSQRKLQFYEDKHQLPAPK(C) corresponding to amino acid residues 758-776 of murine and rat JAK2. At the saturating levels of virus used for this experiment, production of rJAK2 was observed as early as 27 h p.i. (Fig. 2, panelA), while tyrosine phosphorylation of rJAK2 was not detected at this time point (Fig. 2, panelB). Tyrosine phosphorylation of rJAK2 was first observed in this experiment at 51 h p.i. (Fig. 2, panelB), at a stage when rJAK2 production had increased (Fig. 2, panelA). The level of rJAK2 production and tyrosine phosphorylation of rJAK2 appeared to plateau at 71 h p.i. and was stable through 123 h p.i. The molecular mass deduced from the rat jak2 cDNA sequence is 130 kDa, but the apparent molecular mass of the native rat JAK2 is 120 kDa(23, 24) , which corresponds to the apparent molecular mass of the recombinant rat JAK2 produced via baculovirus expression vector system.

Figure 2: Time course of rJAK2 production and tyrosine phosphorylation. Plates containing 2 10 Sf21 were infected with saturating (M.O.I. = 50) levels of recombinant baculovirus expressing rJAK2 and then harvested 27, 51, 71, 97, or 123 h post-infection as indicated. Control plates were left non-infected or were infected with recombinant baculovirus expressing human prolactin receptor (hPRLR) and harvested 27, 51, 71, 97, or 123 h post-infection. Cellular contents were solubilized and immunoprecipitated with anti-JAK2 as described under ``Materials and Methods.'' Sample aliquots equivalent to approximately 6.7 10 cells were subjected to SDS-PAGE and transferred to Immobilon-P membranes and immunodecorated with anti-JAK2 (panelA) or with anti-phosphotyrosine (panelB).

The JH1 domain contains a motif that is recognized as a protein-tyrosine kinase consensus domain(25) . Recombinant baculovirus producing rJAK2(C795), a mutant of rat JAK2 containing amino acid residues 1 through 795 and thus lacking the JH1 domain and most of the JH2 domain, was created with the initial intention of testing whether active rJAK2 could transphosphorylate an inactive rJAK2(C795). A time course of protein production in Sf21 cells infected with baculovirus producing rJAK2(C795) is presented in Fig. 3. The molecular mass of rJAK2(C795) deduced from its cDNA is 91 kDa. As seen in panelA, a 90-kDa protein was produced and recognized by anti-JAK2 antisera (Fig. 3, panelA) but not by the antiphosphotyrosine monoclonal antibody (Fig. 3, panelB). As in Fig. 2, Sf21 cells infected with recombinant baculovirus producing rJAK2 contained a 120-kDa protein that was recognized by both polyclonal anti-JAK2 antisera and by the monoclonal antiphosphotyrosine antibody. When Sf21 cells were co-infected with equal M.O.I. levels of both baculoviruses, the production of both rJAK2 and rJAK2(C795) was detected by anti-JAK2 immunodecoration (Fig. 3, panelA), but neither form was recognized by antiphosphotyrosine antibody (Fig. 3, panelB). One interpretation of these data is that the inactive rJAK2(C795) and rJAK2 form an inactive oligomer, an interpretation originally proposed by Zhuang et al.(1) , to account for similar observations obtained in a COS cell expression system. Another concept that derives from such an interpretation is that the amino-terminal portion of the enzyme may contain an inhibitory or regulatory domain.

Figure 3: Time course of rJAK2 and rJAK2(C795) coproduction and tyrosine phosphorylation. 13 plates containing 1 10 Sf21 were infected either individually or simultaneously with recombinant baculoviruses (M.O.I. = 20) expressing rJAK2 and rJAK2(C795). A control plate was left non-infected. Samples were harvested 22, 47, 65, or 77 h post-infection as indicated. Cellular contents were solubilized and immunoprecipitated with anti-JAK2 as described under ``Materials and Methods.'' Sample aliquots equivalent to approximately 3.3 10 cells were subjected to SDS-PAGE and transferred to Immobilon-P membranes and immunodecorated with anti-JAK2 (panelA) or with anti-phosphotyrosine (panelB). The blackarrows point to the 120-kDa rJAK2; the whitearrow points to the 90-kDa rJAK2(C795).

We sought to test this concept, as well as to determine if sufficient information was contained in the JH1/JH2 domains to confer catalytic activity, by constructing the recombinant baculovirus that produced (N661)rJAK2. (N661)rJAK2, which contained amino acid residues 661 through 1132 and thus retained all of JH1 but only 60% of JH2, was predicted to be a 472-amino acid protein with a molecular mass of 55 kDa. This protein comigrated with rabbit immunoglobulin G heavy chain, and therefore immunodecoration of (N661)rJAK2 with anti-JAK2 anti-sera following anti-JAK2 immunoprecipitation was obscured. A demonstration of the time course of (N661)rJAK2 production was achieved by anti-JAK2 immunodecoration of detergent-solubilized proteins that had not been immunoprecipitated before electrophoresis (Fig. 4, panelA). Under these conditions, an endogenous cross-reactive 97-kDa protein was also observed. More importantly, the (N661)rJAK2 protein was detected as a 56-kDa protein (or as a doublet of protein bands) between 27 and 49 h post-infection (Fig. 4, panelA) and became tyrosine phosphorylated by 49 h p.i. (Fig. 4, panelB). As the infection progressed, a distinct 62-kDa protein in the anti-JAK2 immunoprecipitation complex was detected by anti-phosphotyrosine immunodecoration at 72 and 93 h p.i. (Fig. 4, panelB). A Coomassie-stained band was observed at this mobility in immunoblots obtained from cells infected with baculoviruses producing rJAK2, rJAK2(C795), or (N661)rJAK2 (data not shown), but tyrosine phosphorylation of the 62-kDa protein was only observed in immunoblots from cells infected with baculovirus producing (N661)rJAK2. It is not yet clear whether the common Coomassie-detectable band, which was not detectable by anti-JAK2 (data not shown), was a non-phosphorylated version of the tyrosine-phosphorylated 62-kDa protein or whether it coincidentally comigrated with it.

Figure 4: Time course of (N661)rJAK2 production and tyrosine phosphorylation. Four plates of 4 10 Sf21 cells were infected at a 4-fold M.O.I. with recombinant baculovirus expressing (N661)rJAK2. Cells were harvested at 27, 49, 72, and 93 h p.i. Cellular contents were solubilized as described under ``Materials and Methods.'' PanelA, following sample pre-clearance with protein A-Sepharose CL-4B but prior to immunoprecipitation, aliquots equivalent to 1.5 10 cells were subjected to SDS-PAGE. The resolved proteins were transferred to Immobilon-P membrane and then immunodecorated with anti-JAK2. The blackarrow points to the (N661)rJAK2. PanelB, cellular contents were immunoprecipitated with anti-JAK2, aliquots equivalent to 2 10 cells subjected to SDS-PAGE, transferred to Immobilon-P membrane, and then immunodecorated with anti-phosphotyrosine. The blackarrow points to (N661)rJAK2, and the whitearrow points to an unidentified 62-kDa protein.

To demonstrate the relative intensities of the anti-phosphotyrosine signal of rJAK2 and of (N661)rJAK2, as well as to test whether co-expression of these two forms would influence each other's phosphotyrosine state, co-infection studies were carried out (Fig. 5). Sf21 cells were infected with varying proportions of baculoviruses producing rJAK2 and (N661)rJAK2, then harvested and detergent-solubilized 88 h post-infection. The presence of rJAK2 was determined via Western blot analysis of proteins that had not been immunoprecipitated before electrophoresis (Fig. 5, panelA). As the proportion of baculovirus producing rJAK2 increased, so did the intensity of the 120-kDa rJAK2 protein band. The decrease in intensity of the 56-kDa (N661)rJAK2 protein band did not correlate as well with the decrease in the proportion of (N661)rJAK2-producing baculovirus. ()It was still apparent that the amount of rJAK2 produced at an M.O.I. of 18 or 20 equaled or exceeded the amount of (N661)rJAK2 produced at an M.O.I. of 2, and overall, the amounts of anti-JAK2 cross-reactive material were comparable between the two proteins. This was not the case, however, when the anti-phosphotyrosine signals were compared (Fig. 5, panelB). A 120-kDa phosphotyrosine signal was first observed at an rJAK2 M.O.I. of 15, slightly intensified at an rJAK2 M.O.I. of 18, and was clearly visible at an rJAK2 M.O.I. of 20. In contrast, a very intense phosphotyrosine signal was apparent in all samples in which (N661)rJAK2 was produced, and this intensity only slightly decreased when the (N661)rJAK2 M.O.I. was reduced to 2. The very intense signal was a combination of two phosphotyrosine bands, as demonstrated by the data in Fig. 4. It should be noted that other minor bands (migrating at approximately 135, 105, and 45 kDa) were detectable as phosphotyrosine-containing proteins in the anti-JAK2 immunoprecipitate from (N661)rJAK2-producing cells.

Figure 5: Co-production and tyrosine phosphorylation of (N661)rJAK2 and rJAK2. Seven plates of 2 10 Sf21 cells were infected simultaneously with recombinant baculoviruses expressing (N661)rJAK2 and rJAK2 at a total M.O.I. of 20. The proportion of the two viruses are listed above each gel lane; the lane at the extremeleft was derived from Sf21 cells left uninfected. Cells were harvested at 88 h p.i. Cellular contents were solubilized as described under ``Materials and Methods.'' PanelA, following sample pre-clearance with protein A-Sepharose CL-4B but prior to immunoprecipitation, aliquots equivalent to 1 10 cells were subjected to SDS-PAGE. The resolved proteins were transferred to Immobilon-P membrane and then immunodecorated with anti-JAK2. PanelB, cellular contents were immunoprecipitated with anti-JAK2, aliquots equivalent to 1 10 cells subjected to SDS-PAGE, transferred to Immobilon-P membrane, and then immunodecorated with anti-phosphotyrosine. In both panels, the blackarrow points to (N661)rJAK2 and the whitearrow points to the rJAK2.

The variable intensity of a phosphotyrosine signal that correlated to the retention or loss of a putative catalytic domain suggested that the rJAK2 and (N661)rJAK2 proteins were active enzymes but that the rJAK2(C795) was not. In an attempt to verify that these proteins were indeed active enzymes and not merely substrates for phosphorylation, in vitro kinase assays were conducted with proteins purified by immunoprecipitation (Fig. 6). This assay demonstrated incorporation of radioactive phosphorus into the 120-kDa protein when rJAK2 was incubated with [-P]ATP and a conspicuously greater incorporation of radioactive phosphorus into the 56-kDa protein when (N661)rJAK2 was incubated with [-P]ATP. However, no autophosphorylation of the 90-kDa protein was observed upon incubation of the rJAK2(C795) with [-P]ATP, as was anticipated. This demonstration of autophosphorylation verified that the rJAK2 and (N661)rJAK2 were active enzyme forms and that the rJAK2(C795) was an inactive form.

Figure 6: Autophosphorylation of immunoprecipitated rJAK2 forms. Four plates of 4 10 Sf21 cells were infected at a 20-fold M.O.I. with recombinant baculoviruses expressing one of the following: human prolactin receptor (hPRLR), (N661)rJAK2, rJAK2(C795), or rJAK2. Infected cells were harvested 112 h p.i. and then assayed for in vitro kinase activity as described under ``Materials and Methods.'' Immunoprecipitated equivalents of 2 10 Sf21 cells were loaded in each gel lane; the dried gel was exposed to film for 9 h. The blackarrow points to the full-length (120 kDa) rJAK2 in the gel autoradiograph, and the whitearrow points to the (56 kDa) (N661)rJAK2 mutant.

DISCUSSION

The tyrosine phosphorylation of JAK2 overexpressed via baculovirus expression vector system has been previously reported(12) , and it appeared that the tyrosine phosphorylation occurs constitutively in such an overexpression system. By observing the time course of rJAK2 protein production (Fig. 2) and by observing JAK2 production at a fixed post-infection harvest time under varied multiplicities of infection (Fig. 5), one can amend this interpretation. It appears that once the JAK2 production exceeds a critical level, tyrosine autophosphorylation occurs spontaneously.

A simple, plausible explanation of this observation is that monomers of JAK2 begin to form catalytically competent oligomers (possibly simple dimers) once the JAK2 concentration approaches a value that shifts the JAK2(oligomer)/JAK2(monomer) equilibrium to a detectable amount of oligomer. This explanation is currently plausible because it is consistent with several pieces of information. First, the observation that inactive JAK2 mutants are capable of inhibiting autophosphorylation of wild-type JAK2 (initially observed by Zhuang, et al.(1) and demonstrated here in Fig. 3) could be explained by the formation of an inactive hetero-oligomer. Second, it has also been shown that JAK2-related receptors such as growth hormone receptor (26, 27) dimerize subsequent to ligand binding. Activation of tyrosine kinase activity is mediated by dimerization of JAK2-related receptors such as prolactin receptor (28) and gp130(29) , which are associated with JAK2 before ligand binding(11, 14, 15) , leading to the presumption that bringing the JAK2 proteins into close physical proximity is crucial for the ``activation'' of the JAK2. Finally, there is evidence that correlates a significant increase in jak2 mRNA (19) in the Nb2-SP pre-T lymphoma cell line (as compared to the parental Nb2-11C cell line) to an increase of tyrosine-phosphorylated JAK2 protein, ()a phenomenon that may be relevant to the loss of absolute prolactin dependence in the Nb2-SP cell line(30) .

The concept that the oligomeric form of JAK2 is the catalytically active form must be verified by additional independent experiments before it is acceptable as a part of the mechanism of enzymatic activation. It should be noted that there is no evidence yet to support transphosphorylation or inter-JAK2 phosphorylation; the full-length rJAK2 does not phosphorylate the inactive rJAK2(C795), nor does the ``hyperactive'' (N661)rJAK2 appear to phosphorylate rJAK2. These data are admittedly inconclusive since we have not shown that rJAK2(C795) contains suitable phosphorylation sites nor have we demonstrated that (N661)rJAK2 and rJAK2 can form hetero-oligomers. Without a ``transphosphorylation mechanism,'' many scenarios remain that would link oligomerization to activation. These include cooperativity between multiple catalytic sites (e.g. the proton-translocating ATP synthase(31) ), catalytic sites formed at subunit interfaces (e.g. lipoamide dehydrogenase(32) ), or relief of interactions between inhibitory domains and catalytic domains (e.g. CaM kinase II(33) ). The last scenario may also help to explain the apparent hyperactivity of the (N661)rJAK2 mutant.

There are three pieces of evidence that suggest that the (N661)rJAK2 mutant is both hyperphosphorylated and hyperactive. These data are as follows: 1) the exaggerated anti-phosphotyrosine signal intensity of this form relative to that of the full-length protein (Fig. 5), 2) the significant increase in radiolabel incorporation observed in in vitro kinase assay autoradiography (Fig. 6), and 3) the appearance of a novel 62-kDa protein that is recognized by anti-phosphotyrosine, but not by anti-JAK2, on Western blots ( Fig. 4and Fig. 5). Although the variance in the 5`-untranslated region (which does not vary between rJAK2 and rJAK2(C795)) may have slightly increased expression of (N661)rJAK2, the apparent equal intensity of the anti-JAK2 signal in samples that have obvious differences in anti-phosphotyrosine signal intensities (Fig. 5) leads to the conclusion that (N661)rJAK2 is hyperphosphorylated relative to rJAK2. One plausible explanation for hyperactivity is that an inhibitory or regulatory domain is contained within the amino-terminal portion of the enzyme. This model is consistent with data from Zhuang, et al.(1) that show that kinase-deficient JAK2 mutants inhibit autophosphorylation of the wild-type JAK2 in the COS cell expression system, as we also show in the baculoviral expression vector system (Fig. 3). The concept of such an inhibitory or regulatory domain requires additional experimental support, since alternative interpretations for the ``trans-inhibitory'' properties of an inactive JAK2 still exist.

If such inhibitory or regulatory domains exist in JAK2, one might expect other Janus kinases to possess them as a consequence of the high degree of sequence conservation in these proteins. Expression of TYK2 mutants TK (lacking JH1) and KL (lacking JH2) in cells derived from the 2fTGH cell line resulted in the generation of inactive kinases (18) , which is consistent with the presence of an inhibitory domain in the amino-terminal domains JH3 through JH7. A useful test of the inhibitory/regulatory hypothesis will be to determine if expression of only the TK (JH1), KL (JH2), or combined KL/TK (JH2/JH1) domains is sufficient to produce an active kinase.

The nature and relevance of the 62-kDa band observed in Fig. 4pose intriguing questions. Although identification of this protein is in progress, ()the identification process is not yet complete. Thus, there remains some ambiguity as to whether this protein is virally encoded, is produced in response to viral infection, or whether it is a constitutive cellular product. It is also unknown whether the appearance of an intense phosphotyrosine signal associated with the 62-kDa protein in (N661)rJAK2 (but not rJAK2) immunoprecipitation complexes is due to an alteration in substrate specificity or whether the apparent increase in catalytic turnover of the (N661)rJAK2 mutant increased the phosphotyrosyl forms of the 62-kDa protein beyond the capacity of endogenous phosphatases.

The dual hypotheses that the oligomeric form of JAK2 is the catalytically active form and that there is an inhibitory or regulatory domain in the amino terminus of the enzyme are currently under investigation in our laboratory. These hypotheses do, however, allow for the speculative prediction that dysfunctional regulation of JAK2 expression may be responsible for factor-independent and receptor-independent proliferation and hence may predict circumstances for JAK2-mediated oncogenesis.

FOOTNOTES

*

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked ``advertisement'' in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

§

To whom correspondence should be addressed: Biological Carcinogenesis and Development Program, Science Applications International Corp., National Cancer Institute-Frederick Cancer Research and Development Ctr., P. O. Box B, Frederick, MD 21702-1201. Tel.: 301-846-1508; Fax: 301-846-6107; DUHE{at}ncifcrf.gov.

()

The abbreviations used are: M.O.I., multiplicity of infection; p.i., post-infection; rJAK2, rat type 2 Janus kinase; (N661)rJAK2, rJAK2 mutant initiating at Met; rJAK2(C795), rJAK2 mutant terminating after Arg; PAGE, polyacrylamide gel electrophoresis.

()

A plausible explanation for this is that the (N661)rJAK2-producing baculovirus produces (N661)rJAK2 more efficiently than does the rJAK2-producing baculovirus due to differences in the 5`-untranslated region. Since the rJAK2-producing baculovirus and the rJAK2(C795)-producing baculovirus do not differ in the 5`-untranslated region, one would not anticipate such a discrepancy in protein production levels when cells are infected at equal M.O.I. levels. Indeed, the data in Fig. 3, panelA, support this contention.

()

H. Rui and W. L. Farrar, unpublished results.

()

R. J. Duhé, R. J. Fisher, and W. L. Farrar, unpublished results.

ACKNOWLEDGEMENTS

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