Abstract
Vesicle fusion is an indispensable cellular process required for eukaryotic cargo delivery. The Sec/Munc18 protein Munc18c is essential for insulin-regulated trafficking of glucose transporter4 (GLUT4) vesicles to the cell surface in muscle and adipose tissue. Previously, our biophysical and structural studies have used Munc18c expressed in SF9 insect cells. However to maximize efficiency, minimize cost and negate any possible effects of post-translational modifications of Munc18c, we investigated the use of Escherichia coli as an expression host for Munc18c. We were encouraged by previous reports describing Munc18c production in E. coli cultures for use in in vitro fusion assay, pulldown assays and immunoprecipitations. Our approach differs from the previously reported method in that it uses a codon-optimized gene, lower temperature expression and autoinduction media. Three N-terminal His-tagged constructs were engineered, two with a tobacco etch virus (TEV) or thrombin protease cleavage site to enable removal of the fusion tag. The optimized protocol generated 1–2 mg of purified Munc18c per L of culture at much reduced cost compared to Munc18c generated using insect cell culture. The purified recombinant Munc18c protein expressed in bacteria was monodisperse, monomeric, and functional. In summary, we developed methods that decrease the cost and time required to generate functional Munc18c compared with previous insect cell protocols, and which generates sufficient purified protein for structural and biophysical studies.
Highlights
Sec/Munc18 (SM) and SNARE proteins are essential for vesicle exocytosis in eukaryotes [1,2,3,4,5,6,7]
The assembly of the fusogenic SNARE complex is regulated in part by SM proteins through interaction with their cognate SNARE Syntaxin (Sx), in distinct vesicle transport pathways [3,8,9,10,11,12,13,14,15]
The interaction between Munc18 and Sx proteins is a major focus for understanding the molecular basis of vesicle fusion
Summary
Sec/Munc (SM) and SNARE proteins are essential for vesicle exocytosis in eukaryotes [1,2,3,4,5,6,7]. Deletion and over-expression of SM proteins have shown both positive and negative effects in each step of vesicle fusion [3,16,17,18]. Three of the seven SM isoforms expressed in mammals are involved in exocytosis: Munc18a, Munc18b and Munc18c [19,20,21]. Munc18a is expressed on neuronal cells and is required for neurotransmission. Munc18c is ubiquitously expressed [22] and is required for GLUT4 translocation to the cell surface in adipose/muscle tissues in response to insulin signaling, and is important in endothelial cell activation [23,24,25,26,27]
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