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

Claudia Puri

doi:10.1074/jbc.m109.012328

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.

Highlights

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
The Myosin VI no insert (NoI) Isoform Is Targeted to coated structures (CCS) at the Plasma Membrane—To investigate the role of myosin VI in clathrinmediated endocytosis at the ultrastructural level, skin fibroblasts were isolated from wild type and Snell’s waltzer mice that lack myosin VI
Similar results were obtained in wild type mouse fibroblasts for the endogenous myosin VI and overexpressing full-length myosin VI NoI or the NoI tail

Summary

Introduction

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. The full-length myosin VI NoI splice isoform shows reduced targeting to clathrin-coated structures in nonpolarized rat fibroblast (NRK) cells and in the ARPE-19 cell line and a reduced ability to bring the uncoated vesicles out of the plasma membrane actin-rich region [17]. Both myosin VI LI and NoI isoforms are, able to bind the endocytic protein Disabled-2 (Dab2) [10]. Several different mutations in the myosin VI gene have been identified in three Pakistani families suffering from recessive deafness [30]

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Conclusion