Purification and Mode of Action of Synexin: A Protein Enhancing Calcium‐Induced Membrane Aggregation

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Abstract
Synexin, a protein from the cytosol of the bovine adrenal medulla, selectively increases the ability of Ca2+ to aggregate chromaffin granules and other membrane-bound particles. The ability of synexin to self-aggregate in the presence of Ca2+ can be employed in the purification of the protein by monitoring purification with parallel assays that utilize the aggregation of both chromaffin granule membranes and phosphatidylserine liposomes. The enhancement of the Ca2+-induced aggregation of both liposomes and chromaffin granule membranes is a property associated with a 47,000 MW protein. Trypsin inactivated synexin. If granule membranes were well washed after trypsin treatment, they were still excellent substrates for synexin aggregation. This finding cannot be explained by extinction changes owing to synexin self-aggregation. The 47,000 MW protein enhanced Ca2+ aggregation of phosphatidylserine liposomes containing up to 40% phosphatidylcholine, liposomes made from lipids extracted from chromaffin granule membranes, and trypsin-treated chromaffin granule emmbranes, thus suggesting that synexin activity in vivo may be independent of specific membrane proteins but dependent on the presence of acidic phospholipids in the membrane.