THE HYDROPHILIC DOMAIN OF PHOSPHOLAMBAN INHIBITS THE CA2-ATPASE( TRANSPORT STEP OF THE CA2+)

The peptide MEKVQYLTRSAIRRASTIEMPQQAR-Cys corresponding to residues 1- 25 of phospholamban was found to inhibit the ATPase activity of skelet al muscle Ca2+-ATPase, but to have no effect on the Ca2+-dependence of its activity. The peptide was found to decrease the rate of the Ca2transport step (E1PCa(2) --> E2P) by a factor of 2.4. The rate of this same step was decreased by poly(L-Arg) by a factor of 2.2. The peptid e shifted the E2-E1 equilibrium of the ATPase towards E1 by a factor 4 due to stronger binding to the E1 than to the E2 conformation of the ATPase; dissociation constants for binding to E1 and E2 were estimated as 3 and 10 mu M respectively. The peptide had no effect on the level of phosphorylation by P-i in the absence of Ca2+ or on the rate of ph osphorylation by ATP in the presence of Ca2+.