IDENTIFICATION OF CRITICAL POSITIVE CHARGES IN XIP, THE NA CA EXCHANGE INHIBITORY PEPTIDE/

The peptides XIP (RRLLFYKYVYKRYRAGKQRG) and C28R2 (LRRGQILWFRGLNRIQTQI RVVKAFRSS) correspond to the autoinhibitory domains of the Na-Ca excha nger and the plasma membrane Ca pump, respectively. An increase of ion ic strength reduced the inhibition of exchange activity by XIP and C28 R2, consistent with an important role for electrostatic interactions. Sulfosuccunimidyl acetate (SNA)-modified XIP did not inhibit Na-Ca exc hange. Because SNA modifies lysines, we conclude that at least one of the positive charges at the XIP lysine positions (7, 11, or 17) is imp ortant for inhibition. 2CK-XIP (RRLLFYRYVYRCYCAGRQKG) has cysteines at 12 and 14 and only one lysine (at 19). 2CK-XIP inhibited Na-Ca exchan ge; thus positive charges at 12 and 14 are not essential. SNA-modified 2CK-XIP did not inhibit; thus a positive charge at 19 is important, I odoacetic acid-modified 2CK-XIP inhibits the Na-Ca exchanger but not t he PM Ca pump. These results show that the structural determinants for inhibition of the Na-Ca exchanger and the PM Ca pump are different, t hat positive charges at 7, 11, or 17 (or some combination) are more im portant than positive charges at 12 and 14 for inhibition by XIP of th e Na-Ca exchanger. (C) 1997 Academic Press.