Apactin is Involved in Remodeling of the Actin Cytoskeleton during Regulated Exocytosis

Abstract

Apactin is an 80-kDa type I membrane glycoprotein derived from pro-Muclin, a precursor that also gives rise to the zymogen granule protein Muclin. Previous work showed that apactin is efficiently removed from the regulated secretory pathway and targeted to the actin-rich apical plasma membrane of the pancreatic acinar cell. The cytosolic tail (C-Tail) of apactin consists of 16 amino acids, has Thr casein kinase II and Ser protein kinase C phosphorylation sites, and a C-terminal PDZbinding domain. Secretory stimulation of acinar cells causes a decrease in Thr phosphorylation and an increase in Ser phosphorylation of apactin. Fusion peptides of the C-Tail domain pulldown actin, ezrin, and EBP50/NHERF in a phosphorylation-dependent manner. HIV TAT-C-Tail fusion peptides were used as dominant negative constructs on living pancreatic cells to study effects on the actin cytoskeleton. During secretory stimulation, TAT-C-Tail-Thr/Asp phosphomimetic peptide caused an increase in actin-coated zymogen granules at the apical surface, while TAT-C-Tail-S/D phosphomimetic peptide caused a broadening of the actin cytoskeleton. These data indicate that stimulation-mediated Thr dephosphorylation allows decreased association of apactin with EBP50/NHERF and fosters actin remodeling to coat zymogen granules.