Interaction of SAP97 with minus-end-directed actin motor myosin VI. Implications for AMPA receptor trafficking

Myo6 facilitates the translocation of endocytic vesicles from cell peripheries

Immunolocalization studies in epithelial cells revealed myo6 was associated with peripherally located vesicles that contained the transferrin receptor. Pulse-chase experiments after transferrin uptake showed that these vesicles were newly uncoated endocytic vesicles and that myo6 was recruited to these vesicles immediately after uncoating. GIPC, a putative myo6 tail binding protein, was also present. Myo6 was not present on early endosomes, suggesting that myo6 has a transient association with endocytic vesicles and is released upon early endosome fusion. Green fluorescent protein (GFP) fused to myo6 as well as the cargo-binding tail (M6tail) alone targeted to the nascent endocytic vesicles. Overexpression of GFP-M6tail had no effect on a variety of organelle markers; however, GFP-M6tail displaced the endogenous myo6 from nascent vesicles and resulted in a significant delay in transferrin uptake. Pulse-chase experiments revealed that transferrin accumulated in uncoated vesicles within the peripheries of transfected cells and that Rab5 was recruited to the surface of these vesicles. Given sufficient time, the transferrin did traffic to the perinuclear sorting endosome. These data suggest that myo6 is an accessory protein required for the efficient transportation of nascent endocytic vesicles from the actin-rich peripheries of epithelial cells, allowing for timely fusion of endocytic vesicles with the early endosome.

Cytoskeletal motors and cargo in membrane trafficking: opportunities for high specificity in drug intervention

Membrane trafficking comprises the directed transport of vesicle and/or organelle cargos to specific locations throughout the cell, and is primarily driven by molecular motors tracking along microtubules and microfilaments. The mechanisms by which specific motor complexes attach to their respective vesicular cargo is of great interest, and is only now starting to be unraveled. The proteins identified as links between the molecular motors and the vesicular cargo are viable drug targets and represent opportunities to regulate small groups of related proteins or even single proteins, such as receptors and transporters, at the cytosolic trafficking level. Ultimately, continued development in this area will lead to greater success in directing endocytosed drugs to the desired intracellular targets, such as the cell nucleus or the basolateral membrane.

Direct interaction with a kinesin-related motor mediates transport of mammalian discs large tumor suppressor homologue in epithelial cells

Membrane-associated guanylate kinase homologues (MAGUKs) are generally found under the plasma membrane of cell-cell contact sites and function as scaffolding proteins by linking cytoskeletal and signaling molecules to transmembrane receptors. The correct targeting of MAGUKs is essential for their receptor-clustering function; however, the molecular mechanism of their intracellular transport is unknown. Here, we show that the guanylate kinase-like domain of human discs large protein binds directly within the amino acids 607-831 of the stalk domain of GAKIN, a kinesin-like protein of broad distribution. The primary structure of the binding segment, termed MAGUK binding stalk domain, is conserved in Drosophila kinesin-73 and some other motor and non-motor proteins. This stalk segment is not found in GKAP, a synaptic protein that interacts with the guanylate kinase-like domain, and unlike GKAP, the binding of GAKIN is not regulated by the intramolecular interactions within the discs large protein. The recombinant motor domain of GAKIN is an active microtubule-stimulated ATPase with k(cat) = 45 s(-1), K(0.5 (MT)) = 0.1 microm. Overexpression of green fluorescent protein-fused GAKIN in Madin-Darby canine kidney epithelial cells induced long projections with both GAKIN and endogenous discs large accumulating at the tip of these projections. Importantly, the accumulation of endogenous discs large was eliminated when a mutant GAKIN lacking its motor domain was overexpressed under similar conditions. Taken together, our results indicate that discs large is a cargo molecule of GAKIN and suggest a mechanism for intracellular trafficking of MAGUK-laden vesicles to specialized membrane sites in mammalian cells.

Myosin VI, an actin motor for membrane traffic and cell migration

The actin cytoskeleton and associated myosin motor proteins are essential for the transport and steady-state localization of vesicles and organelles and for the dynamic remodeling of the plasma membrane as well as for the maintenance of differentiated cell-surface structures. Myosin VI may be expected to have unique cellular functions, because it moves, unlike almost all other myosins, towards the minus end of actin filaments. Localization and functional studies indicate that myosin VI plays a role in a variety of different intracellular processes, such as endocytosis and secretion as well as cell migration. These diverse functions of myosin VI are mediated by interaction with a range of different binding partners.

PDZ domain interaction controls the endocytic recycling of the cystic fibrosis transmembrane conductance regulator

The C terminus of CFTR contains a PDZ interacting domain that is required for the polarized expression of cystic fibrosis transmembrane conductance regulator (CFTR) in the apical plasma membrane of polarized epithelial cells. To elucidate the mechanism whereby the PDZ interacting domain mediates the polarized expression of CFTR, Madin-Darby canine kidney cells were stably transfected with wild type (wt-CFTR) or C-terminally truncated human CFTR (CFTR-DeltaTRL). We tested the hypothesis that the PDZ interacting domain regulates sorting of CFTR from the Golgi to the apical plasma membrane. Pulse-chase studies in combination with domain-selective cell surface biotinylation revealed that newly synthesized wt-CFTR and CFTR-DeltaTRL were targeted equally to the apical and basolateral membranes in a nonpolarized fashion. Thus, the PDZ interacting domain is not an apical sorting motif. Deletion of the PDZ interacting domain reduced the half-life of CFTR in the apical membrane from approximately 24 to approximately 13 h but had no effect on the half-life of CFTR in the basolateral membrane. Thus, the PDZ interacting domain is an apical membrane retention motif. Next, we examined the hypothesis that the PDZ interacting domain affects the apical membrane half-life of CFTR by altering its endocytosis and/or endocytic recycling. Endocytosis of wt-CFTR and CFTR-DeltaTRL did not differ. However, endocytic recycling of CFTR-DeltaTRL was decreased when compared with wt-CFTR. Thus, deletion of the PDZ interacting domain reduced the half-life of CFTR in the apical membrane by decreasing CFTR endocytic recycling. Our results identify a new role for PDZ proteins in regulating the endocytic recycling of CFTR in polarized epithelial cells.