Arabidopsis Myosin XIK Interacts with the Exocyst Complex to Facilitate Vesicle Tethering during Exocytosis

ABSTRACT Myosin motors are essential players in secretory vesicle trafficking and exocytosis in yeast and mammalian cells; however, similar roles in plants remain a matter for debate, at least for diffusely-growing cells. Here, we demonstrate that Arabidopsis ( Arabidopsis thaliana ) myosin XIK, via its globular tail domain (GTD), participates in the vesicle tethering step of exocytosis through direct interactions with the exocyst complex. Specifically, myosin XIK GTD bound directly to the SEC5B subunit of exocyst in vitro and functional fluorescently-tagged XIK colocalized with multiple exocyst subunits at plasma membrane (PM)-associated stationary foci. Moreover, genetic and pharmacological inhibition of myosin XI activity reduced the frequency and lifetime of stationary exocyst complexes at the PM. By tracking single exocytosis events of cellulose synthase (CESA) complexes (CSCs) with high spatiotemporal resolution imaging and pair-wise colocalization analysis of myosin XIK, exocyst subunits and CESA6, we demonstrated that XIK associates with secretory vesicles earlier than exocyst and is required for the recruitment of exocyst to the PM tethering site. This study reveals an important functional role for myosin XI in secretion and provides new insights about the dynamic regulation of exocytosis in plants.