Role of small GTPase Arl8b and its effector proteins in regulating cargo trafficking to lysosomes

Abstract

Eukaryotic cells have an elaborate endocytic system that is specialized totake up newlinematerials from the environment and route it to the lysosomes fordegradation. The newlineendocytic pathway is marked by multiple fusion andfission events whose regulation newlineinvolves an interplay of small GTPases,tethering factors and SNAREs. HOmotypic newlinefusion and Protein Sorting (HOPS)complex is an evolutionarily conserved newlinemultisubunit tethering factor thatmediates vesicle fusion with lysosomes. The newlinemechanism of mammalian HOPSaction and its crosstalk with other lysosome proteins newlineis only beginningto be understood. In the first part of this thesis, we demonstrate newlinethatthe small GTPase Arl8b interacts with, and recruits HOPS complex tolysosome newlinemembranes. Depletion of HOPS subunit Vps41 results in defects in newlinecargo trafficking to lysosomes that were rescued upon expression ofwild-type but not newlinean Arl8b-binding-defective mutant, suggesting thatArl8b-dependent localization of newlineHOPS complex to lysosomes is required for cargo degradation. Since the discovery of newlineArl8b, an ever-increasing numberof its interaction partners have come into light, newlineinclusive of the RUNdomain-containing proteins. In the second section of the thesis, newlinewe haveidentified that Arl8b interacts with the RUN and FYVE (RUFY)domain- newlinecontaining proteins, Rabip4 and Rabip4/RUFY1, via their RUNdomains. Arl8b newlinedepletion results in striking displacement of endogenousRabip4(s) from the newlineendosomal membranes to the cytosol that can be rescuedupon expression of siRNA- newlineresistant Arl8b. Future studies will be useful togain insights into how Arl8b regulates newlinethe membrane localization ofRabip4(s) and significance of Rabip4(s) interaction with newlineArl8b in membranetrafficking. newline