Val200 residue in Lys189-Lys205 outermost loop on the A domain of sarcoplasmic reticulum Ca2+-ATPase is critical for rapid processing of phosphoenzyme intermediate after loss of ADP sensitivity.

Possible roles of the Lys(189)-Lys(205) outermost loop on the A domain of sarcoplasmic reticulum Ca(2+)-ATPase were explored by mutagenesis. Both nonconservative and conservative substitutions of Val(200) caused very strong inhibition of Ca(2+)-ATPase activity, whereas substitutions of other residues on this loop reduced activity only moderately. All of the Val(200) mutants formed phosphoenzyme intermediate (EP) from ATP. Isomerization from ADP-sensitive EP (E1P) to ADP-insensitive EP (E2P) was not inhibited in the mutants, and a substantially larger amount of E2P actually accumulated in the mutants than in wild-type sarcoplasmic reticulum Ca(2+)-ATPase at steady state. In contrast, decay of EP formed from ATP in the presence of Ca(2+) was strongly inhibited in the mutants. Hydrolysis of E2P formed from P(i) in the absence of Ca(2+) was also strongly inhibited but was faster than the decay of EP formed from ATP, indicating that the main kinetic limitation of the decay comes after loss of ADP sensitivity but before E2P hydrolysis. On the basis of the well accepted mechanism of the Ca(2+)-ATPase, the limitation is likely associated with the Ca(2+)-releasing step from E2P.Ca(2). On the other hand, the rate of activation of dephosphorylated enzyme on high affinity Ca(2+) binding was not altered by the substitutions. In light of the crystal structures, the present results strongly suggest that Val(200) confers appropriate interactions of the Lys(189)-Lys(205) loop with the P domain in the Ca(2+)-released form of E2P. Results further suggest that these interactions, however, do not contribute much to domain organization in the dephosphorylated enzyme and thus would be mostly lost on E2P hydrolysis.

Sarcoplasmic reticulum Ca 2ϩ -ATPase (SERCA1a) 1 is a 110-kDa membrane protein and a representative member of P-type ion-transporting ATPases. SERCA1a catalyzes Ca 2ϩ transport coupled with ATP hydrolysis (Refs. 1 and 2, and for recent reviews, see Refs. 3 and 4). According to the E1/E2 transport mechanism ( Fig. 1) (3-7), the enzyme is activated by the binding of two Ca 2ϩ ions (E1⅐Ca 2 , steps 1-2) and then autophosphorylated by MgATP to form an ADP-sensitive phosphoenzyme (E1P, step 3). On formation of this EP, the bound Ca 2ϩ ions are occluded in the transport sites. The subsequent isomeric transition to the ADP-insensitive form (E2P, step 4) will result in a reduction in affinity and a change in orientation of the Ca 2ϩ binding sites and thus a Ca 2ϩ release into lumen (step 5). Finally, hydrolysis takes place and returns the enzyme into an unphosphorylated and Ca 2ϩ -unbound form (E2, step 6). The main kinetic limitation in this cycle is associated with the mechanism of Ca 2ϩ release before the hydrolysis of E2P (8,9). E2P can also be formed from P i in the presence of Mg 2ϩ and absence of Ca 2ϩ by reversal of the hydrolysis of E2P.
The three-dimensional structure of Ca 2ϩ -ATPase with bound Ca 2ϩ (E1⅐Ca 2 ) and, very recently, the structure without bound Ca 2ϩ and with bound thapsigargin (E2(TG)), were solved by x-ray crystallography at the atomic level (10,11). The enzyme has three cytoplasmic domains (A, N, and P) that are widely separated in E1⅐Ca 2 and associated in E2(TG). The modeling with a low resolution map of tubular crystals formed with decavanadate (E2V) revealed (10) that three cytoplasmic domains gather to form a most compactly organized single headpiece in E2V (see Fig. 7). Our previous limited and systematic proteolysis experiments showed (12,13) that E2V is very similar to the Ca 2ϩ -released form of E2P in the domain organization and that this EP is the intermediate with the most compactly organized headpiece in the catalytic cycle. The results further indicated that a large motion of the A domain (i.e. rotation by ϳ90° (10)) and the strong association of the A domain with the P and N domains most likely occur during the isomerization of EP and Ca 2ϩ release and suggested that the stabilization energy provided by intimate contacts between all three cytoplasmic domains in E2P provides energy for moving transmembrane helices and releases the bound Ca 2ϩ ions.
To substantiate such changes in cytoplasmic domain organization and their roles in the Ca 2ϩ transport, it is essential to find the regions and residues involved in the domain-domain interactions and reveal their actual roles in the catalytic steps. The conserved outermost TGES loop (Thr 181 -Ser 184 ) on the A domain is situated at the interface of the A and P domains in E2V. This loop was previously found to be essential for the isomerization of EP (14) and predicted by iron-catalyzed cleavage with Na ϩ /K ϩ -ATPase to participate in Mg 2ϩ binding with specific residues in the conserved TGDGVND loop (starting from Thr 701 with Ca 2ϩ -ATPase) on the P domain in E2P and E2 (15). One more outermost loop is located on the outer surface of the A domain in E1⅐Ca 2 but situated at the A-P domain interface in E2V, the Lys 189 -Lys 205 loop that includes the tryptic T2 site (Arg 198 ) (see Fig. 7). We had actually indicated by sitedirected chemical modification (16) and mutations (17) as well as by the limited proteolysis (12,13) that Arg 198 comes very close to the phosphorylation site when E2P without bound Ca 2ϩ is formed and that the positive charge of this residue is important for its rapid hydrolysis. These findings suggest that the Lys 189 -Lys 205 loop may have critical roles in domain organization, particularly in the Ca 2ϩ -released form of E2P.
In the present study, we therefore further explored possible roles of the Lys 189 -Lys 205 outermost loop by mutagenesis and found that Val 200 is critical for rapid processing of E2P, most likely in steps 5 and 6. With the crystal structures, our results suggest that Val 200 confers appropriate interactions between polar residues on the Lys 189 -Lys 205 loop with those on the P domain in the Ca 2ϩ -released form of E2P and, further, that these interactions would be mostly lost on the hydrolysis of E2P.

EXPERIMENTAL PROCEDURES
Mutagenesis and Expression-Overlap extension PCR (18) was used for the substitution of residues in the Lys 189 -Lys 205 region in rabbit SERCA1a cDNA. The ApaI-KpnI restriction fragments were excised from the PCR products and ligated back into the corresponding region in the full-length SERCA1a cDNA in the pMT2 expression vector (19). The pMT2 DNA was transfected into COS-1 cells by the liposomemediated transfection method. Microsomes were prepared from the cells as described previously (20). The "control microsomes" were prepared from COS-1 cells transfected with the pMT2 vector containing no SERCA1a cDNA. The amount of expressed SERCA1a was quantified by a sandwich enzyme-linked immunosorbent assay as described previously (21). The expression levels of all the mutants in the microsomes were comparable with those of the wild type.
ATPase Activity-The rate of ATP hydrolysis was determined at 25°C in the presence and absence of 0.5 M thapsigargin in a mixture containing 20 g/ml microsomal protein, 1 M A23187, 5 mM [␥-32 P]ATP, 7 mM MgCl 2 , 0.1 M KCl, 50 mM MOPS/Tris (pH 7.0), 0.55 mM CaCl 2 , and 0.5 mM EGTA. The specific ATPase activity/mg of expressed SERCA1a protein was calculated from the amount of expressed SERCA1a and the ATPase activity of expressed SERCA1a, which was obtained by subtraction of the thapsigargin-sensitive ATPase activity of the control microsomes from that of the microsomes expressing SERCA1a. This background level with the control microsomes was as low as 4% of the activity of microsomes expressing the wild-type SERCA1a.
Formation and Hydrolysis of EP-Phosphorylation of SERCA1a in microsomes with [␥-32 P]ATP or 32 P i and dephosphorylation of 32 P-labeled SERCA1a were performed under the conditions described in the legends to the figures and tables. The reactions were quenched with ice-cold trichloroacetic acid containing P i . Rapid kinetics measurements of phosphorylation and dephosphorylation were performed with a handmade rapid mixing apparatus (22) and otherwise as described above. The precipitated proteins were separated at pH 6.0 by 5% SDS-PAGE according to Weber and Osborn (23). The radioactivity associated with the separated Ca 2ϩ -ATPase was quantitated by digital autoradiography as described (17). The amount of EP formed with the expressed SERCA1a was obtained by subtracting the background radioactivity with the control microsomes. This background was less than 5% of the radioactivity of EP formed with the expressed wild-type SERCA1a. The amount of EP/mg of SERCA1a protein was calculated from the amount of EP thus obtained and the amount of expressed SERCA1a.
Miscellaneous-Protein concentrations were determined by the method of Lowry et al. (24) with bovine serum albumin as a standard. Data were analyzed by nonlinear regression using the program Origin (Microcal Software, Inc., Northampton, MA). Three-dimensional models were reproduced by the program Swiss-PdbViewer (25).  (4,17,26), and the results are in essential agreement. Substitutions of Glu 192 and Pro 195 caused no loss of function (4). We then focused on Val 200 , of which substitutions gave distinguishably strong inhibition, and substituted with various amino acids including nonpolar and polar (positively charged, negatively charged, and uncharged) amino acids (Ala, Ile, Arg, Lys, Asp, Thr, and Gln). The activity was strongly inhibited in the mutants V200I, V200D, V200T, and V200Q and further reduced in the mutants V200A, V200R, and V200K (Fig. 3).

Effects of Substitutions in Lys
Formation of EP from ATP-We then performed kinetic analysis on the formation and decay of EP with these Val 200 mutants at 0°C under conditions otherwise similar to those for the ATPase assay. All of the mutants possessed the ability to form EP, and the amount formed was comparable with that of wild type (Table I). As shown in Fig. 4, the time course of E2P formation from ATP was examined in the presence and absence of K ϩ . Under both sets of conditions, the total amount of EP reached its maximum level within 1 s after the addition of ATP and remained unchanged for the time periods of observation (120 s) (data not shown). Thus, the time course actually reflects E2P accumulation from E1P. In the presence of K ϩ , the amount of accumulated E2P at steady state in wild type was very low (Fig. 4A), in agreement with the previous observations on the effects of K ϩ (27). In contrast, a substantial amount of E2P was accumulated with the mutants, especially V200A and V200I. In the absence of K ϩ , a large amount of E2P accumulated at steady state in wild type as well as in the mutants (Fig. 4B), thus the time courses can be clearly compared. The E2P accumulation apparently proceeded with first-order kinetics, and the apparent rate to reach the steady state (k formation ) was estimated (Table II). The rates obtained in the mutants in the absence of K ϩ were comparable with the rate in wild type, with somewhat lower rates in V200R and V200K. These rates obtained in the mutants are also comparable with their rates in the presence of K ϩ . The results indicate that the E1P to E2P isomerization (i.e. loss of ADP sensitivity) is not significantly inhibited by the Val 200 substitutions. Results also suggest that the substitutions (especially in V200A and V200I) affected the equilibrium in step 4 in favor of E2P⅐Ca 2 .
Decay of EP-Decay of EP formed from ATP (Fig. 5) and hydrolysis of E2P formed from P i (Fig. 6) were then examined at 0°C under conditions otherwise similar to those for the ATPase assay (i.e. in the presence of K ϩ ). Decay of EP formed from ATP was determined first by phosphorylation with [␥-32 P]ATP in the presence of K ϩ and Ca 2ϩ for 15 s and then termination of phosphorylation by the addition of excess EGTA to prevent further phosphorylation and thus allow the decay of 32 P-labeled EP (Fig. 5). The EP decay was markedly slowed by the Val 200 substitutions. The time courses were apparently fitted very well with a single exponential, although a substantial amount of E2P (in addition to E1P) was accumulated in the mutants at the start of the decay reaction (see Fig. 4A). The results indicate that processing of EP after loss of ADP sensitivity (i.e. step 5 and/or step 6) was strongly inhibited by the    Table II. Wild type (q) and mutants V200A (), V200I (ƒ), V200R (‚), V200K (f), V200D (Ⅺ), V200T (ࡗ), and V200Q (छ) are indicated.

Rates of formation and decay of EP
The rate to reach the steady state level of E2P after addition of ATP (k formation ) was obtained in the presence of K ϩ (ϩK ϩ ) and absence of K ϩ ((Ϫ)) in the experiments shown in Fig. 4. The rate of decay of EP formed from ATP in the presence of Ca 2ϩ (k decay ) and that of hydrolysis of E2P formed from P i in the absence of Ca 2ϩ (k hydrolysis ) were obtained in the presence of K ϩ (ϩK ϩ ) in the experiments shown in Figs. 5 and 6, respectively. In parentheses, the k decay and the k hydrolysis values obtained with the wild type are normalized to 100%. Val 200 in SR Ca 2ϩ -ATPase substitutions. The apparent decay rate (k decay ) was strongly reduced in V200D, V200Q, V200T, and V200I to ϳ30 -20% of that of the wild type and was further reduced to ϳ10% in V200A, V200K, and V200R (Table II). It should be noted that the extent of reduction in k decay agreed well with that of inhibition of ATPase activity (cf . Table II and Fig. 3). Also, in each mutant the k decay value is much lower than the k formation value.
Hydrolysis of E2P Formed from P i -To examine the hydrolysis of E2P without bound Ca 2ϩ (step 6), the enzyme was first phosphorylated by 32 P i without Ca 2ϩ in the absence of K ϩ and the presence of 35% (v/v) Me 2 SO, which extremely favors E2P formation (28), and then the phosphorylated samples were diluted with a large volume of a solution containing K ϩ and non-radioactive P i (Fig. 6). The conditions were thus made otherwise identical to those used for the E2P formation from ATP in Fig. 4A and the decay of EP formed from ATP in Fig. 5.
The hydrolysis of 32 P-labeled E2P proceeded with first-order kinetics, and its rate (k hydrolysis ) was summarized in Table II. The rate was moderately reduced in V200D and largely reduced in V200T and V200Q (8 and 7% that of wild type, respectively). It was further reduced in V200A, V200I, V200K, and V200R (less than 3% that of wild type).
Kinetic Limitation in the Mutants for Decay of EP Formed from ATP-For the mutants, the above kinetic results show that the decay of EP formed from ATP in the presence of Ca 2ϩ is much slower than the accumulation of E2P from ATP and also that the EP decay is substantially slower than the E2P hydrolysis in the absence of Ca 2ϩ (as is most clearly shown with V200K, V200D, V200T, and V200Q). The results indicate for the mutants that the main kinetic limitation (although not the only one) in the decay of EP formed from ATP comes after loss of ADP sensitivity (step 4) but before E2P hydrolysis (step 6). On the basis of the well accepted mechanism of the Ca 2ϩ -ATPase (Fig. 1), it is therefore likely that step 5 associated with Ca 2ϩ release is greatly slowed by the Val 200 substitutions and is the main kinetic limitation in the mutants. 2 For the mutants, the actual rate of formation of E2P from E1P on isomerization (k isomerization ) in the presence of K ϩ can be estimated by assuming the equilibrium in step 4 without further processing of EP (because of the subsequent very slow processing as described above for the mutants). The k isomerization value thus estimated for the mutants (s Ϫ1 ) by an equation, k isomerization ϭ k formation ϫ (fraction of E2P at steady state), was 0.063 (V200A), 0.108 (V200I), 0.011 (V200R), 0.013 (V200K), 0.032 (V200D), 0.036 (V200T), and 0.084 (V200Q). These values are in fact substantially larger than the k decay values (as most clearly shown with V200A, V200I, V200R, V200T, and V200Q).
Transition from E2 to E1⅐Ca 2 -Ca 2ϩ dependence of phosphorylation with ATP was determined in the presence of K ϩ at various concentrations of Ca 2ϩ under conditions as described in the legend to Table I and thus similar to those for the ATPase assay. The experiments were performed with the four representative mutants (V200A, V200I, V200R, V200T) and wildtype SERCA1a. The dissociation constant for Ca 2ϩ and Hill coefficient obtained by least square fits for these mutants (0.20 -0.29 M and 1.8 -2.0, respectively) were nearly the same as those for wild type (0.27 M and 1.9, respectively).
To compare the rate of the E2 to E1⅐Ca 2 transition, the four mutants and wild type were preincubated in the absence of Ca 2ϩ at pH 6, where equilibrium between E1 and E2 is most shifted to E2 (30) and then phosphorylated by the simultaneous addition of saturating concentrations of Ca 2ϩ and ATP under otherwise identical conditions as described above. The time course of EP formation was well described by the first order kinetics (data not shown). The rates (s Ϫ1 ) obtained with the mutants (0.178 (V200A), 0.122 (V200I), 0.140 (V200R), and 0.204 (V200T)) were almost the same as or only slightly lower than that of the wild type (0.204). When ATP was added to the wild-type and mutant enzymes were preincubated with Ca 2ϩ , the EP formation was much faster and reached its maximal level within 1 s (data not shown). These results show that the Ca 2ϩ -induced E2 to E1⅐Ca 2 transition, which is rate-limiting for the EP formation, takes place at almost the same rate in the mutants and wild type. 2 The results with the wild type (the very low accumulation of E2P in spite of the slow k decay ) are accounted for by the equilibrium in step 4 favoring E1P⅐Ca 2 (29). To account for this very low E2P accumulation, it is also possible to assume the rate-limiting E1P to E2P isomerization (step 4) followed by rapid Ca 2ϩ release (step 5) and hydrolysis (step 6). In any case, however, the same conclusion as described in text will be made for the mutants that the step after loss of ADP sensitivity but before hydrolysis of E2P is greatly slowed by the Val 200 substitutions.  Table I. Phosphorylation was terminated by the addition of EGTA to give 4 mM, and the decay reaction was quenched with trichloroacetic acid at different times after the addition of EGTA. The amounts of EP obtained at zero time are normalized to 100%. Solid lines show the least squares fit to a single exponential decay, and the first-order rate constants (k decay ) thus obtained are given in Table II 1 (V200Q). These values are normalized to 100%. Solid lines show the least squares fit to a single exponential, and the first-order rate constants (k hydrolysis ) obtained are given in Table  II. Inset, hydrolysis during the short period of time.

DISCUSSION
In the present study, we explored possible roles of the Lys 189 -Lys 205 loop on the A domain by mutagenesis and found that Val 200 is critical for rapid processing of E2P, most likely in step 5, and also for rapid hydrolysis of E2P in step 6 in the Ca 2ϩ transport cycle (Fig. 1). During isomerization of EP and Ca 2ϩ release, a large movement of the A domain (i.e. rotation by ϳ90°(10)) and its strong association with the P and N domains occur to form the most compactly organized cytoplasmic domains in E2P without bound Ca 2ϩ (12,13). In this E2P, a hydrophobic atmosphere (28,(31)(32)(33) is thus realized around the phosphorylation site, and a specific water molecule can now attack the acylphosphate bond. It is likely that the Lys 189 -Lys 205 loop, especially Val 200 , is critical for the appropriate domain interactions and proper structure in E2P without bound Ca 2ϩ .
In E2V that is very similar to E2P without bound Ca 2ϩ in the cytoplasmic domain organization (12,13), the Lys 189 -Lys 205 loop forms an A-P domain interface (Fig. 7). The side chain of Val 200 protrudes toward the P domain and is in van der Waals contact with the side chain hydrocarbon parts of Arg 198 , Gln 202 , and Asp 203 , which interact with polar residues on the P domain by hydrogen bonding or salt bridge formation (broken green lines in Fig. 7). It is likely that Val 200 coordinates spatial arrangement of these surrounding residues by the nonpolar interactions so as to produce the most favorable configuration for appropriate interactions at the terminal N and O atoms with the polar residues on P domain and thus for concurrent contribution of these residues to intimate contact between the A and P domains in E2P without bound Ca 2ϩ .
In agreement with this view, the substitutions of Val 200 by any examined conservative or nonconservative amino acids resulted in strong inhibition of the Ca 2ϩ -ATPase activity because of inhibition of the EP decay, whereas substitutions of each of the above surrounding three residues caused only moderately reduced activity (Fig. 2 and Refs. 17 and 26). Actually, the positive charge of Arg 198 was previously shown to be important for rapid E2P hydrolysis, and its nonconservative substitutions partially reduce the k hydrolysis value (by 62%) (17). Mutations of Glu 680 (4) and of Arg 678 on the P domain (see Fig.  7) also cause only partial loss of function. 3 It is likely that the substitutions at Val 200 would seriously affect coordination of the surrounding residues and thus appropriate interactions of the Lys 189 -Lys 205 loop with the P domain.
The observed high Ca 2ϩ affinity and rate of the Ca 2ϩ -induced activation in the Val 200 mutants being comparable with those in the wild type (see "Results") indicate that the interactions between the Lys 189 -Lys 205 loop and the P domain likely do not contribute much to cytoplasmic domain organization in the unphosphorylated states and thus would be mostly lost on E2P hydrolysis. In fact, in E2(TG) that is very similar to E2 in the cytoplasmic domain organization (12), no hydrogen bonds or salt bridges are found between the Lys 189 -Lys 205 loop and the P domain (Fig. 7). Consistently, the enzyme in E2 state is rapidly attacked and degraded by proteinase K and V8 protease, whereas E2P without bound Ca 2ϩ is completely resistant to all proteinase K, V8 protease, and trypsin at the T2 site (Arg 198 ) (12,13). E2 is, however, partially resistant to tryptic attack at the T2 site, indicating that the A domain in this state is still partially associated with the P and N domains and has not yet completely rotated back to the widely separated state found in E1⅐Ca 2 (12,13), as is in fact seen in the E2(TG) structure. Possible interactions between the A and N domains and between the TGES loop and the P domain (possibly through ligation of Mg 2ϩ (15)), may be significant for holding the A domain in such an organized state in E2. On hydrolysis of E2P to E2, the atmosphere around the phosphorylation site again becomes hydrophilic (28,(31)(32)(33). It is possible that loss of the interactions between the Lys 189 -Lys 205 loop and the P domain contribute at least in part to such changes in hydrophobicity.
In summary, we conclude that the Lys 189 -Lys 205 loop, especially Val 200 , is critical for the intimate contact between the A and P domains and the appropriate cytoplasmic domain organization in E2P without bound Ca 2ϩ . Although the structure of E2P⅐Ca 2 (ADP-insensitive EP with occluded Ca 2ϩ (8)) is not yet characterized, the final process of appropriate gathering and intimate contact of the A domain with the P and N domains will likely be accomplished in step 5 after the loss of ADP sensitivity, and this change is likely essential for Ca 2ϩ release into lumen.