Crystal Structures of Sodium-Bound Annexin A4

Abstract

Annexin A4 (Anx4) possesses four repeat domains with one Ca²⁺-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 Å). 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 Å) bound the Na⁺ ion in CBS I, whereas another structure (1.35 Å) bound the Na⁺ ion in CBS II and CBS III. We compared the two Na⁺-bound forms by superimposing their C^α traces. The C^α atoms of CBS III largely moved by coordination of the Na⁺ ion. In the C^α 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 Ca²⁺ but also Na⁺ ion in the CBS.