Annexin 5 mediates a peroxide-induced Ca(2+) influx in B cells.

Annexin 5 is a Ca(2+)-binding protein, the function of which is poorly understood. Structural and electrophysiological studies have shown that annexin 5 can mediate Ca(2+) fluxes across phospholipid membranes in vitro [1]. There is, however, no direct evidence for the existence of annexin 5 Ca(2+) channels in living cells. Here, we show that annexin 5 inserts into phospholipid vesicle membranes at neutral pH in the presence of peroxide. We then used targeted gene disruption to explore the role of annexin 5 in peroxide-induced Ca(2+) signaling in DT40 pre-B cells. DT40 clones lacking annexin 5 exhibited normal Ca(2+) responses to both thapsigargin and B-cell receptor stimulation, but lacked the sustained phase of the response to peroxide. This late phase was due to Ca(2+) influx from the extracellular space, demonstrating that annexin 5 mediates a peroxide-induced Ca(2+) influx. Thus, peroxide induces annexin 5 membrane insertion in vitro, and peroxide-induced Ca(2+) entry in vivo in DT40 cells requires annexin 5. Our results are consistent with a role for annexin 5 either as a Ca(2+) channel, or as a signaling intermediate in the peroxide-induced Ca(2+)-influx pathway.