Cyclic 3′‐5′‐adenosine monophosphate binds to annexin I and regulates calcium‐dependent membrane aggregation and ion channel activity

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Abstract
The annexin (Anx) gene family comprises a set of calcium‐dependent membrane binding proteins, which have been implicated in a wide variety of cellular processes including membrane fusion and calcium channel activity. We report here that cAMP activates Ca2+‐dependent aggregation of both phosphatidylserine (PS) liposomes and bovine chromaffin granules driven by [des 1–12]annexin I (lipocortin I, AnxI). The mechanism of cAMP action involves an increase in AnxI‐dependent cooperativity on the rate of such a reaction without affecting the corresponding k1/2 values. Cyclic AMP causes the values of the Hill coefficient (n H) for AnxI to change from 3 to 6 in both PS liposomes and chromaffin granules. By contrast, ATP inhibits the rate of aggregation activity without affecting the cooperativity or the extent of aggregation process. We were also able to photolabel AnxI specifically with an 8‐azido analogue of cAMP by a calcium‐independent process. Such a process is saturable, yielding a Kd = 0.8 μM by Scatchard analysis. Specific displacement occurs in the presence of cAMP and ATP. Finally, we found that cAMP alters the conductance of calcium channels formed by AnxI in planar lipid bilayers. We interpret these data to indicate that AnxI binds both calcium and cAMP independently, and that both actions have functional consequences. This is the first report of a nucleotide binding function for a member of the annexin gene family.