Annexins as intracellular calcium sensors

The annexins are a multigene family of Ca2+- and charged phospholipid-binding proteins. Although they have been ascribed with diverse functions, there is no consensus about the role played by this family as a whole. We have mapped the Ca2+-induced translocations of four members of the annexin family and of two truncated annexins in live cells, and demonstrated that these proteins interact with the plasma membrane as well as with internal membrane systems in a highly coordinated manner. Annexin 2 was the most Ca2+ sensitive of the studied proteins, followed by annexins 6, 4 and 1. The calcium sensitivity of annexin 2 increased further following co-expression with S100A10. Upon elevation of [Ca2+]i, annexins 2 and 6 translocated to the plasma membrane, whereas annexins 4 and 1 also became associated with intracellular membranes and the nuclear envelope. The NH2-terminus had a modulatory effect on plasma membrane binding: its truncation increased the Ca2+ sensitivity of annexin 1, and decreased that of annexin 2. Given the fact that several annexins are present within any one cell, it is likely that they form a sophisticated [Ca2+] sensing system, with a regulatory influence on other signaling pathways.