CYCLIC 3'-5'-ADENOSINE MONOPHOSPHATE BINDS TO ANNEXIN-I AND REGULATESCALCIUM-DEPENDENT MEMBRANE AGGREGATION AND ION-CHANNEL ACTIVITY

The annexin (Anx) gene family comprises a set of calcium-dependent mem brane binding proteins, which have been implicated in a wide variety o f cellular processes including membrane fusion and calcium channel act ivity. We report here that cAMP activates Ca2+-dependent aggregation o f both phosphatidylserine (PS) liposomes and bovine chromaffin granule s driven by [des 1-12]annexin I (lipocortin I, AnxI). The mechanism of cAMP action involves an increase in Ansi-dependent cooperativity on t he rate of such a reaction without affecting the corresponding k(1/2) values, Cyclic AMP causes the values of the Hill coefficient (n(H)) fo r AnxI to change from 3 to 6 in both PS liposomes and chromaffin granu les, By contrast, ATP inhibits the rate of aggregation activity withou t affecting the cooperativity or the extent of aggregation process, We were also able to photolabel Ansi specifically with an 8-azido analog ue of cAMP by a calcium-independent process, Such a process is saturab le, yielding a K-d = 0.8 mu M by Scatchard analysis, Specific displace ment occurs in the presence of cAMP and ATP. Finally, me found that cA MP alters the conductance of calcium channels formed by AnxI in planar lipid bilayers, We interpret these data to indicate that AnxI binds b oth calcium and cAMP independently, and that both actions have functio nal consequences, This is the first report of a nucleotide binding fun ction for a member of the annexin gene family.