Crystal structures of sodium-bound annexin A4

Annexin A4 (Anx4) possesses four repeat domains with one Ca(2+)-binding site (CBS) in each domain. In this study, we resolved two crystal structures of the Na(+)-bound form at high resolution (1.58 and 1.35 A). This is the first report that Anx4 binds the Na(+) ion in CBSs. Electron density maps, valence screening, and atomic absorbance spectrometry confirmed that Anx4 bound the Na(+) ion. One structure (1.58 A) bound the Na(+) ion in CBS I, whereas another structure (1.35 A) bound the Na(+) ion in CBS II and CBS III. We compared the two Na(+)-bound forms by superimposing their C(alpha) traces. The C(alpha) atoms of CBS III largely moved by coordination of the Na(+) ion. In the C(alpha) atoms of CBS I, however, little change resulted from Na(+)-coordination. Only the side chain of Glu71 was moved by Na(+)-coordination in CBS I. These results indicate that Anx4 also binds not only Ca(2+) but also Na(+) ion in the CBS.