Doc2b Serves as a Scaffolding Platform for Concurrent Binding of Multiple Munc18 Isoforms in Pancreatic Islet Beta Cells

Biphasic glucose-stimulated insulin secretion (GSIS) from pancreatic beta cells involves SNARE protein-regulated exocytosis. SNARE complex assembly further requires the regulatory proteins Munc18c, Munc18-1 and Doc2b. Munc18-1 and Munc18c are required for 1st- and 2nd-phase GSIS, respectively. These distinct Munc18-1 and Munc18c roles are related to their transient high-affinity binding with their cognate t-SNAREs; Syntaxin 1A and Syntaxin 4, respectively. Doc2b is essential for both phases of GSIS, yet the molecular basis for this remains unresolved. Because Doc2b binds to Munc18-1 and Munc18c via it’s distinct C2A and C2B domains, respectively, we hypothesized that Doc2b may provide a plasma membrane-localized scaffold/platform for transient docking of these Munc18 isoforms during GSIS. Toward this, macromolecular complexes composed of Munc18c, Doc2b, and Munc18-1 were detected in beta cells. In vitro interaction assays indicated that Doc2b is required to bridge the interaction between Munc18c and Munc18-1 in the macromolecular complex; Munc18c and Munc18-1 failed to associate in the absence of Doc2b. Competition-based GST-Doc2b interaction assays revealed that Doc2b could simultaneously bind both Munc18-1 and Munc18c. Hence, these data support a working model wherein Doc2b functions as a docking platform/scaffold for transient interactions with the multiple Munc18 isoforms operative in insulin release, promoting SNARE assembly.
Source: BJ Cell - Category: Biochemistry Authors: Tags: BJ Metabolism Source Type: research