Membrane Association Domains in Ca super(2+)-dependent Activator Protein for Secretion Mediate Plasma Membrane and Dense-core Vesicle Binding Required for Ca super(2+)-dependent Exocytosis

• Published in: The Journal of biological chemistry 2002-06, Vol.277 (24), p.22025-22034
• Main Authors: Grishanin, R N, Klenchin, V A, Loyet, K M, Kowalchyk, JA, Ann, K, Martin, TFJ
• Format: Article
• Language: English
• Subjects: dense core vesicles
• ISSN: 0021-9258

Ca super(2+)-dependent activator protein for secretion (CAPS) is a cytosolic protein essential for the Ca super(2+)-dependent fusion of dense-core vesicles (DCVs) with the plasma membrane and the regulated secretion of a subset of neurotransmitters. The mechanism by which CAPS functions in exocytosis and the means by which it associates with target membranes are unknown. We identified two domains in CAPS with distinct membrane-binding properties that were each essential for CAPS activity in regulated exocytosis. The first of these, a centrally located pleckstrin homology domain, exhibited three properties: charge-based binding to acidic phospholipids, binding to plasma membrane but not DCV membrane, and stereoselective binding to phosphatidylinositol 4,5-bisphosphate. Mutagenesis studies revealed that the former two properties but not the latter were essential for CAPS function. The central pleckstrin homology domain may mediate transient CAPS interactions with the plasma membrane during Ca super(2+)-triggered exocytosis. The second membrane association domain comprising distal C-terminal sequences mediated CAPS targeting to and association with neuroendocrine DCVs. The CAPS C-terminal domain was also essential for optimal activity in regulated exocytosis. The presence of two membrane association domains with distinct binding specificities may enable CAPS to bind both target membranes to facilitate DCV-plasma membrane fusion.