Calcium signaling and annexins.

The annexins, are a family of calcium ion (Ca2+)-binding proteins whose physiological functions are poorly understood. Although many diverse functions have been proposed for these proteins, such as in vesicle trafficking, this review focuses on their proposed roles as Ca2+ or other ion channels, or as intracellular ion channel regulators. Such ideas are founded mainly on in vitro and structural analyses, but there is increasing evidence that at least some members of this protein family may indeed play a part in intracellular Ca2+ signaling by acting both as atypical ion channels and as modulators of ion channel activity. This review first introduces the annexin family, then discusses intracellular localization, developmental regulation, and modes of membrane association of annexins, which suggest roles in Ca2+ homeostasis. Finally, it examines the structural and electrophysiological data that argue for key roles for annexins in the control of ion fluxes.