Function of Myosin VI

National Heart, Lung, And Blood Institute

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Abstract

During the process of constitutive secretion, proteins synthesized at the endoplasmic reticulum are transported to the Golgi complex for processing and then to the plasma membrane for incorporation or release into the extracellular environment. The molecular motor myosin VI and its binding partner optineurin have been implicated in constitutive secretion, but the precise roles of these proteins in this process remain unknown. This study uses a novel live cell secretion assay with a constitutively secreted, GFP-tagged reporter molecule to establish roles for myosin VI and optineurin in discrete stages of the secretory pathway. siRNA-based knockdown of myosin VI leads to a kinetic delay in ER to Golgi transport, suggesting an unexpected function for myosin VI in the early secretory pathway. Depletion of myosin VI or optineurin has no effect on the number of vesicles budding from the trans-Golgi network (TGN), indicating that these proteins do not play a role in vesicle formation at the TGN. Live cell Total Internal Reflection Fluorescence (TIRF) microscopy analysis demonstrates, however, that depletion of myosin VI or optineurin reduces the total number of secretory vesicle fusion events at the plasma membrane and increases both the proportion of incomplete fusion events and the number of docked vesicles in this region. Taken together, these results suggest a novel role for myosin VI and optineurin during the final stages of the secretory pathway in the regulation of the fusion pores formed between secretory vesicles and the plasma membrane.