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Differential recognition of secretory vesicles by annexins. European Molecular Biology Organization Course "Advanced Techniques for Studying Secretion".

The interactions of synexin, calpactin, and p68 (annexins VII, II, and VI) with zymogen granules and adrenal medullary chromaffin granules were compared. Synexin promoted the aggregation and fusion of both types of organelles. Calpactin inhibited the action of synexin on zymogen granules but would aggregate only chromaffin granules. p68 inhibited aggregation of chromaffin granules by calpactin and fusion of zymogen granules by synexin. The results demonstrate that these annexins, all of which bind acidic phospholipids, interact differently with different biological membranes and therefore may play specific and non-overlapping roles in cells where they are co-expressed.

Adrenal Medulla↗

Calcium-dependent KEX2-like protease found in hepatic secretory vesicles converts proalbumin to albumin.

The yeast KEX2 protease is the only enzyme that has a proven role in the activation of polypeptide hormones through cleavage at pairs of basic residues. The enzyme that fulfils this role in higher eukaryotes has yet to be unequivocally identified. In this investigation, a KEX2-like calcium-dependent protease has been identified in rat hepatic microsomes. The enzyme is membrane-bound, has a pH optimum of 5-6 and converts proalbumin to albumin. More importantly, like the KEX2 protease, it meets two other exacting criteria defined by specific mutations in humans. Namely, it does not process proalbumin Christchurch (-1 Arg----Gln) which lacks one of the requisite basic residues and, whilst not itself a serine protease, it is inhibited by the reactive center variant, alpha 1-antitrypsin Pittsburgh (358 Met----Arg) but not by normal alpha 1-antitrypsin.

Autoradiography↗

Identification of the secretory vesicle membrane binding region of chromogranin A.

Since the conserved near N-terminal region (residues 18-37) of chromogranin A (CGA) has tentatively been identified as the vesicle membrane interacting region using synthetic peptides, it was necessary to confirm this finding with CGA deletion proteins. In order to address this need and to clarify the discrepancies of the published amino acid sequences of CGA, we cloned a CGA gene and produced CGA deletion proteins of various sizes. The recombinant CGA protein lacking the first 16 amino acid residues bound to the vesicle membrane as well as the full-length CGA at pH 5.5. However, the CGA protein lacking the first 39 amino acid residues, which include the conserved near N-terminal region (residues 17-38), failed to interact with the vesicle membrane at pH 5.5, clearly indicating the essential role of the conserved near N-terminal region in the pH-dependent interaction of CGA with the vesicle membrane.

Amino Acid Sequence↗

Identification of the secretory vesicle membrane binding region of chromogranin B.

In the past a synthetic peptide representing the conserved near N-terminal region of chromogranin B (CGB) (residues 17-36) has been shown to interact with the vesicle membrane. Hence, it was necessary to confirm this observation with CGB deletion proteins. In order to address this need and to confirm the published sequence of CGB, we cloned a CGB gene and produced various CGB deletion proteins. The recombinant CGB protein lacking the first 15 (rCGB 16-626) or 16 amino acid residues (rCGB 17-626) bound to the vesicle membrane as well as the full-length CGB. However, rCGB 49-626, lacking the conserved near N-terminal region, failed to interact with the vesicle membrane, indicating an anchor role for the conserved near the N-terminal region.

Amino Acid Sequence↗