Regulation of Calcyclin (S100A6) Binding by Alternative Splicing in the N-terminal Regulatory Domain of Annexin XI Isoforms

Annexin XI is a Ca 2+ /phospholipid-binding protein that interacts with a member of S100 protein family, calcyclin (S100A6), in a Ca 2+ -dependent manner. There are two isoforms of annexin XI, annexin XI-A and -B, generated by alternative splicing in the N-terminal regulatory domain. To determine the role of the alternative splicing region in the calcyclin-binding, we identified and characterized its calcyclin binding site. Experiments with glutathione S -transferase fusion proteins with N-terminal sites of annexin XI-A showed the calcyclin binding site to be in residues Gln 49 -Thr 62 of rabbit annexin XI-A, which contains part of the splicing region. A synthesized peptide corresponding to Tyr 43 -Thr 62 of annexin XI-A inhibited the interaction of annexin XI with calcyclin in liposome co-pelleting assay. The calcyclin binding site possesses a hydrophobic residue cluster conserved among S100 binding sites of annexin I and II. Recombinant annexin XI isoforms were expressed in Sf9 cells using a baculovirus expression system. In contrast to annexin XI-A, it was found that annexin XI-B protein could not bind to calcyclin by the liposome co-pelleting assay. In Sf9 cells coexpressing calcyclin with annexin XI isoforms, the calcyclin binding was observed only for annexin XI-A isoform. These results indicate that the calcyclin binding ability of annexin XI is an annexin XI-A isoform-specific character, suggesting that annexin XI isoforms might play distinct roles in cells through each alternative splicing regions.