Cellular Function Of The ADP-ribosylation Factor 6 (Arf6

Heart, Lung, And Blood Institute

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Abstract

The Arf6 GTPase regulates membrane traffic between the plasma membrane (PM) and an endosomal compartment and influences the dynamics of the cortical actin cytoskeleton. In many cells this pathway is the route followed by PM proteins that lack clathrin localization sequences and hence are endocytosed into cells independently of clathrin. PM proteins that are endocytosed via this clathrin-independent pathway can either be recycled back to the PM or routed to convergence with the "classical" early endosomal pathway and on to degradation in lysosomes. Among the PM proteins that enter cells through this non-clathrin pathway is major histocompatibility complex class I (MHCI). The tubular endosomal membranes that recycle MHCI back to the PM have associated with them the Eps15 homology domain-containing protein, EHD1, which enhances MHCI recycling when overexpressed. Expression of constitutively active Arf6 mutant, Arf6Q67L, sequesters non-clathrin cargo in vacuoles that are enriched in PIP2 and blocks their trafficking to Rab5/EEA1 endosomes and degradation but does not affect trafficking of clathrin-derived cargo. These observations suggest that inactivation of Arf6, PIP2 turnover and acquisition of PI3P are required for fusion of endosomes arising from the Arf6 pathway with the Rab5/EEA1 early endosome system. We have found that some lipid raft-associated proteins, those anchored to the membrane by a glycosylphosphatidyl inositol (GPI) moiety, also enter cells via this clathrin-independent mechanism and converge with the Rab5 early endosome or recycle back to the PM with MHCI. Assays are being developed to reconstitute in vitro these membrane trafficking events.