POSITIVE CHARGE MODIFICATIONS ALTER THE ABILITY OF XIP TO INHIBIT THEPLASMA-MEMBRANE CALCIUM-PUMP

Exchange inhibitory peptide (XIP; RRLLFYKYVYKRYRAGKQRG) is the shortes t peptide that inhibits the plasma membrane Ca pump at high Ca (A. Eny edi, T. Vorherr, P. James, D. J. McCormick, A. G. Filoteo, E. Carafoli , and J. T. Penniston, J. Biol. Chem. 264: 12313-12321, 1989). Sulfosu ccinimidyl acetate (SNA)-modified XIP does not inhibit the Ca pump; SN A neutralizes the positive charge on Lys at positions 7, 11, and 17. P eptide 2CK-XIP (RRLLFYRYVYRCYCAGRQKG) inhibits the pump, but the iodoa cetamido-modified peptide does not inhibit. Three peptide analogues, i n which 7, 11, and 17 were Ala, Cys, or Lys, inhibited about as well a s XIP. SNA modification of these analogues (each with 1 Lys) did not i nhibit. SNA modification of 2CK-XIP results in a peptide that does not inhibit; thus position 19 is important. Our results suggest that it i s critical that position 19 be positively charged, that positions 7, 1 1, and 17 are important contact points between XIP and the Ca pump (wi th at least one positively charged), and that, whereas it is not essen tial that residues 12 and 14 be positive, they cannot be negative.