The effects of mutation on the regulatory properties of phospholamban in co-reconstituted membranes.

Reconstitution into proteoliposomes is a powerful method for studying calcium transport in a chemically pure membrane environment. By use of this approach, we have studied the regulation of Ca(2+)-ATPase by phospholamban (PLB) as a function of calcium concentration and PLB mutation. Co-reconstitution of PLB and Ca(2+)-ATPase revealed the expected effects of PLB on the apparent calcium affinity of Ca(2+)-ATPase (K(Ca)) and unexpected effects of PLB on maximal activity (V(max)). Wild-type PLB, six loss-of-function mutants (L7A, R9E, I12A, N34A, I38A, L42A), and three gain-of-function mutants (N27A, L37A, and I40A) were evaluated for their effects on K(Ca) and V(max). With the loss-of-function mutants, their ability to shift K(Ca) correlated with their ability to increase V(max). A total loss-of-function mutant, N34A, had no effect on K(Ca) of the calcium pump and produced only a marginal increase in V(max). A near-wild-type mutant, I12A, significantly altered both K(Ca) and V(max) of the calcium pump. With the gain-of-function mutants, their ability to shift K(Ca) did not correlate with their ability to increase V(max). The "super-shifting" mutants N27A, L37A, and I40A produced a large shift in K(Ca) of the calcium pump; however, L37A decreased V(max), while N27A and I40A increased V(max). For wild-type PLB, phosphorylation completely reversed the effect on K(Ca), but had no effect on V(max). We conclude that PLB increases V(max) of Ca(2+)-ATPase, and that the magnitude of this effect is sensitive to mutation. The mutation sensitivity of PLB Asn(34) and Leu(37) identifies a region of the protein that is responsible for this regulatory property.