title: Alpha-sarcoglycan is recycled from the plasma membrane in the absence of sarcoglycan complex assembly.
abstract: The sarcoglycan complex consists of four subunits in skeletal muscle (alpha, beta, gamma, and delta-SG). Mutations in alpha-sarcoglycan (alpha-SG) result in the most common form of limb girdle muscular dystrophy. However, the function of alpha-SG remains unknown. In this report we attempt to clarify its function by delineating the trafficking pathway of alpha-SG in live cells. We present evidence, utilizing total internal reflection microscopy, fluorescence recovery after photobleaching and photoactivation of green fluorescent protein (GFP) constructs, that pools of alpha-SG are able to translocate to the plasma membrane in the absence of the remaining sarcoglycans. Internalization assays and drug treatment experiments demonstrate that alpha-SG recycles from the plasma membrane and accumulates in recycling endosomes. We also establish that alpha-SG utilizes well-described clathrin mediated mechanisms and microtubules to traffic within the cell. Finally, we show that the most commonly reoccurring limb girdle muscular dystrophy (R77C) mutation causes a fundamental defect in protein biosynthesis, trapping the mutant protein in the endoplasmic recticulum (ER). These results demonstrate that alpha-SG requires assembly into the sarcoglycan complex for stability at the plasma membrane rather than export out of the ER. Furthermore, this data suggests that alpha-SG utilizes known trafficking machinery to control deposition at the plasma membrane through recycling.
authors.affiliation: Department of Cell biology and Molecular Physiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA.
authors.forename: Romesh A
authors.initials: RA
authors.lastname: Draviam
authors.forename: Bing
authors.initials: B
authors.lastname: Wang
authors.forename: Stuart H
authors.initials: SH
authors.lastname: Shand
authors.forename: Xiao
authors.initials: X
authors.lastname: Xiao
authors.forename: Simon C
authors.initials: SC
authors.lastname: Watkins
mesh_terms: D001120:Arginine / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism*; D002966:Clathrin / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D011992:Endosomes / Q000378:metabolism; D006801:Humans; D008870:Microtubules / Q000378:metabolism; D009132:Muscles / Q000378:metabolism; D009154:Mutation / Q000235:genetics; D011485:Protein Binding; D021381:Protein Transport; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D049031:Sarcoglycans / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D002966:Clathrin; D011994:Recombinant Proteins; D049031:Sarcoglycans; D001120:Arginine