Modifications of XIP, the autoinhibitory region of the Na-Ca exchanger, alter its ability to inhibit the Na-Ca exchanger in bovine sarcolemmal vesicles

The molecular determinants of ionic regulatory differences between brain and kidney Na+/Ca2+ exchanger (NCX1) isoforms

The Na(+)/Ca(2+) exchanger gene NCX1 undergoes alternative splicing leading to several isoforms that differ in a small portion of the large cytoplasmic loop. This loop is involved in many regulatory processes of NCX1, including ionic regulation by the transported substrates Na(+) and Ca(2+). High intracellular Ca(2+) can alleviate intracellular Na(+)-dependent inactivation in exon A (NCX1.4)-containing isoforms but not in those containing the mutually exclusive exon B (NCX1.3). Giant excised patches from Xenopus oocytes expressing various NCX1 constructs were used to examine the specific amino acids responsible for these observed regulatory differences. Using a chimeric approach, the region responsible was narrowed down to the small central part of exon A (IDDEEYEKNKTF). Replacing the second aspartic acid of this sequence with arginine (the corresponding amino acid in exon B) in an exon A background completely prevented the effect of Ca(2+) on intracellular Na(+)-dependent inactivation. Mutating the second lysine to cysteine (exon B) had a similar, but only partial, effect. The converse double mutant, but neither single mutation alone, introduced into an exon B background (arginine to aspartic acid and cysteine to lysine) was able to restore the NCX1.4 regulatory phenotype. These data demonstrate that aspartic acid 610 and lysine 617 (using the rat NCX1.4 numbering scheme) are critical molecular determinants of the unique Ca(2+) regulatory properties of NCX1.4.

Identification of critical positive charges in XIP, the Na/Ca exchange inhibitory peptide

The peptides XIP (RRLLFYKYVYKRYRAGKQRG) and C28R2 (LRRGQILWFRGLNRIQTQIRVVKAFRSS) correspond to the autoinhibitory domains of the Na-Ca exchanger and the plasma membrane Ca pump, respectively. An increase of ionic strength reduced the inhibition of exchange activity by XIP and C28R2, consistent with an important role for electrostatic interactions. Sulfosuccinimidyl acetate (SNA)-modified XIP did not inhibit Na-Ca exchange. Because SNA modifies lysines, we conclude that at least one of the positive charges at the XIP lysine positions (7, 11, or 17) is important for inhibition. 2CK-XIP (RRLLFYRYVYRCYCAGRQKG) has cysteines at 12 and 14 and only one lysine (at 19).2CK-XIP inhibited Na-Ca exchange; 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. Iodoacetic acid-modified 2CK-XIP inhibits the Na-Ca exchanger but not the PM Ca pump. These results show that the structural determinants for inhibition of the Na-Ca exchanger and the PM Ca pump are different, that positive charges at 7, 11, or 17 (or some combination) are more important than positive charges at 12 and 14 for inhibition by XIP of the Na-Ca exchanger.

Positive charge modifications alter the ability of XIP to inhibit the plasma membrane calcium pump

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