Loss of Myosin VI No Insert Isoform (NoI) Induces a Defect in Clathrin-mediated Endocytosis and Leads to Caveolar Endocytosis of Transferrin Receptor

Abstract

Myosin VI is a motor protein that moves toward the minus end of actin filaments. It is involved in clathrin-mediated endocytosis and associates with clathrin-coated pits/vesicles at the plasma membrane. In this article the effect of the loss of myosin VI no insert isoform (NoI) on endocytosis in nonpolarized cells was examined. The absence of myosin VI in fibroblasts derived from the Snell's waltzer mouse (myosin VI knock-out) gives rise to defective clathrin-mediated endocytosis with shallow clathrin-coated pits and a strong reduction in the internalization of clathrin-coated vesicles. To compensate for this defect in clathrin-mediated endocytosis, plasma membrane receptors such as the transferrin receptor (TfR) are internalized by a caveola-dependent pathway. Moreover the clathrin adaptor protein, AP-2, necessary for TfR internalization, follows the receptor and relocalizes in caveolae in Snell's waltzer fibroblasts.In eukaryotic cells diverse endocytic uptake pathways occur at the plasma membrane; these have crucial roles in signal transduction, immune surveillance, antigen presentation, cellcell communication, and cellular homeostasis. Clathrin-mediated endocytosis is by far the best characterized pathway by which cells internalize protein and lipid molecules, although at least three other basic mechanisms, caveola-mediated endocytosis, macropinocytosis, and clathrin-and caveola-independent endocytosis, are also known to operate (1, 2). Recent results have suggested that signaling receptors such as epidermal growth factor receptor (EGFR) 2 and other receptor tyrosine kinases could be endocytosed not only by the clathrin-dependent pathway but also by alternative pathways involving circular dorsal ruffles (3) or by caveolin/raft-dependent endocytosis (4 -7). Moreover, it was recently observed that the transferrin receptor (TfR), a nonsignaling receptor that is internalized canonically by clathrin-mediated endocytosis, can also be endocytosed by a clathrin/dynamin-independent, cholesterol-dependent pathway (8).Caveolae were first observed more than 50 years ago on the surface of endothelial cells; caveola-mediated endocytosis is the best described clathrin-independent uptake route. Caveolae, flask-shaped invaginations of the plasma membrane, are specialized lipid raft domains that are rich in cholesterol and sphingolipids, with shape and structural organization maintained by the integral membrane protein caveolin. Although not all mammalian cells express caveolin, caveolae are very abundant at the plasma membrane of fibroblasts, adipocytes, and endothelial and smooth muscle cells; however, the precise uptake mechanism involved and how it is triggered and regulated are thus far rather poorly understood.Motor proteins such as myosin VI have been shown to be involved in clathrin-mediated endocytosis (9 -13). Myosin VI is a unique actin-based motor protein that moves toward the minus end of actin filaments, in the opposite direction to all other myosins characterized thus far (14). In mammalian cells, four alternat...

Myosin VI Must Dimerize and Deploy Its Unusual Lever Arm in Order to Perform Its Cellular Roles

“…It has been shown that fibroblasts derived from the myosin VI-null Snell’s waltzer ( sv/sv ) mice exhibit a significant defect in clathrin-mediated endocytosis, which is rescued by expression of the isoform of full-length myosin VI that is missing the large insert (NI) that follows residue 1036 (Puri, 2009). Since removal of a large portion of the SAH (MVI FL -ΔSAH construct) did not impact gating and processive movement under in vitro conditions, we investigated whether endocytosis is rescued in sv/sv fibroblasts expressing this mutant construct (NI isoform).…”

Clathrin-Independent Pathways of Endocytosis

“…Compensation can be the explanation but because these mice lack caveola-like invaginations, vesicular transport would have to occur through other pathways. An interesting example of compensation, which might provide more evidence for an endocytic role of caveolae, comes from work on a specific isoform of myosin VI (Puri 2009). Loss of this protein blocks transferrin uptake through clathrin-coated pits but, remarkably, transferrin receptors are instead concentrated in AP2-positive caveolae, which mediate transferrin uptake (Puri 2009).…”

Dynamic of Ion Channel Expression at the Plasma Membrane of Cardiomyocytes

“…Myosin VI has been shown to be important in the forward trafficking of AMPA receptors in hippocampal neurons (296) in a process involving the MAGUK protein SAP97, and has been implicated in the exocytosis of CFTR in endothelial and other cell types (20,79). It is involved also in clathrin-mediated endocytosis (54 -55, 287,340,408) and is important in the endocytosis of CFTR (419). Myosin VI has also been shown to be involved in recycling of internalized plasma membrane proteins back to that locale (70) and has recently been implicated in secretory vesicle fusion with the plasma membrane (44).…”