Binding site of annexin XI on the calcyclin molecule

We purified rabbit calcyclin of S100 family protein and a calcyclin associated protein which has proved to be a novel annexin, annexin XI. Using a co-precipitation assay of annexin XI with phospholipid, the binding site of annexin XI on calcyclin was examined. The peptide fragment of calcyclin, CNBr-3 (residues 1-57), digested with cyanogen bromide completely inhibited the interaction of native calcyclin with annexin XI, while CNBr-l (residues 83-90) and CNBr-2 (residues 58-82) did not affect the binding. We then constructed and expressed recombinant cDNAs for wild type and four different deletion mutants lacking N-terminal portions. The wild type (wt) and mt1 mutant lacking three amino acids from N-terminal bound to annexin XI with phosphatidylserine and Ca²⁺, whereas mt2, mt3 and mt4 with seven, twelve and eighteen amino acids deleted, respectively, did not bind to annexin XI. Moreover, the truncated mutant from residues 4 to 7 (mt5) decreased the binding capacity. These observations suggest that four amino acids (residues 4-7) at the N-terminal portion of calcyclin play an important role in the interaction of calcyclin with annexin XI.