Chimaeras reveal the role of the catalytic core in the activation of the plasma membrane Ca2+ pump

Isoform 2b of the plasma membrane calcium pump differs from the ubiquitous isoform 4b in the following: (a) higher basal activity in the absence of ca lmodulin; (b) higher affinity for calmodulin; and (c) higher affinity for C a2+ in the presence of calmodulin [Elwess, Filoteo, Enyedi and Penniston (1 997) J, Biol, Chem. 272, 17981-17986]. To investigate which parts of the mo lecule determine these kinetic differences, we made four chimaeric construc ts in which portions of isoform 2b were grafted into isoform 4b: chimaera I contains only the C-terminal regulatory region of isoform 2b; chimaera II contains the N-terminal moiety of isoform 2b, including both cytoplasmic lo ops; chimaera III contains the sequence of isoform 2b starting from the N-t erminus to after the end of the first (small) cytoplasmic loop; and chimaer a IV contains only the second (large) cytoplasmic loop. Surprisingly, chima era I showed low basal activity in the absence of calmodulin and low affini ty for calmodulin, unlike isoform 2b, In contrast, the chimaera containing both loops showed high basal activity, and Ca2+ activation curves (both in the absence and in the presence of calmodulin) similar to those of isoform 2b. The rates of activation by calmodulin and of inactivation by calmodulin removal were measured, and the apparent K-d for calmodulin was calculated from the ratio between these rate constants. The order of affinity was. 2b = II > 4b = IV > III = I. From these results it is clear that the construct that most closely resembles isoform 2b is chimaera II, This shows that, in order to obtain an enzyme with properties similar to those of isoform 2b, both cytoplasmic loops are needed.