Novel links between protein sorting and cytoskeletal dynamics in the secretory pathway

Title: Novel links between protein sorting and cytoskeletal dynamics in the secretory pathway
Authors: Prosser, Derek C
Date: 2008
Abstract: Sorting of proteins and lipids is an essential process for maintaining organelle composition and integrity in eukaryotic cells. In many compartments, proteins are sequestered into membrane subdomains from which vesicle budding and scission occur, thereby allowing transport to other compartments. Protein sorting and vesicle transport are closely coupled to the cytoskeleton, where actin and/or tubulin dynamics are involved in multiple steps of this process. However, links between sorting and the cytoskeleton remain unclear. Within the endocytic pathway, sorting nexins (SNXs) play a role in sorting of internalized receptors for recycling or degradation. In this study, SNX1 and SNX2 are shown to interact with Kalirin-7 (Kal7), a neuronal guanine nucleotide exchange factor (GEF) that activates the actin-modulating GTPases Rac1 and RhoG. Overexpression of either SNX with Kal7 in epithelial cells potentiated lamellipodia formation in a GEF- and RhoG-dependent manner, while SNX depletion inhibited this phenotype. Furthermore, SNX2 enhanced Kal7-dependent neurite extension in nerve growth factor-treated PC12 cells. Finally, SNX1 and SNX2 interact with inactive RhoG, and could thus recruit the small GTPase to its GEF. In the exocytic pathway, proteins are synthesized at the endoplasmic reticulum (ER), where they are folded prior to transport and maturation in post-ER compartments. ER tubules extend along the microtubule network, and this link is critical for ER structure and function. The VAMP-associated proteins VAPA and VAPB are ER transmembrane proteins that interact with microtubules through a conserved major sperm protein (MSP) domain. This link between ER and microtubules could create immobile obstacles in the ER membrane, which could adversely affect membrane protein diffusion. In this study, overexpression of VAPA, but not VAPB, inhibited lateral diffusion of the transmembrane cargo VSVG, suggesting differential effects on cargo sorting. Furthermore, a P56S mutation in the VAPB MSP domain, which is linked to adult-onset amyotrophic lateral sclerosis, caused ER aggregation that blocked anterograde VSVG transport. These defects could be rescued by co-expression of a FFAT (two phenylalanines in an acidic tract) motif found in numerous VAP-interacting proteins, which dissociated VAPs from microtubules. Overall, these findings demonstrate novel links between protein sorting and cytoskeletal dynamics in multiple transport processes.
CollectionTh├Ęses, 1910 - 2010 // Theses, 1910 - 2010
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