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Myosin 18Aα targets guanine nucleotide exchange factor β-Pix to dendritic spines of cerebellar Purkinje neurons to promote spine maturation

Dendritic spines are signaling microcompartments that serve as the primary site of synapse formation in neurons. Actin assembly and myosin 2 contractility play major roles in the maturation of spines from filopodial precursors, as well as in the subsequent, activity-dependent changes in spine morphology that underly learning and memory. Myosin 18A is a myosin 2-like protein conserved from flies to man that lacks motor activity, is sub-stochiometric to myosin 2, and co-assembles with myosin 2 to make mixed filaments. Myosin 18A is alternatively spliced to create multiple isoforms that contain unique N- and C-terminal extensions harboring both recognizable and uncharacterized protein: protein interaction domains. These observations suggest that myosin 18A serves to recruit proteins to mixed filaments of myosin 2 and myosin 18A. One protein known to bind to myosin 18A is β-Pix, a guanine nucleotide exchange factor (GEF) for Rac1 and Cdc42. Notably, β-Pix has been shown to promote spine maturation by activating both Arp2/3 complex-dependent branched actin filament assembly and myosin 2 contractility within spines. Here we show that myosin 18Aα is expressed in cerebellar Purkinje neurons and concentrates in spines along with myosin 2 and F-actin. Myosin 18Aα is targeted to spines by co-assembling with myosin 2 and by an actin binding site present in its N-terminal extension. miRNA-mediated knockdown of myosin 18Aα results in a significant defect in spine maturation that is rescued by an RNAi-immune version of myosin 18Aα. Importantly, β-Pix co-localizes with myosin 18Aα in spines, and its spine localization is lost upon myosin 18Aα knockdown or when its myosin 18Aα binding site is deleted. Finally, we show that the spines of myosin 18Aα knockdown Purkinje neurons contain significantly less F-actin and myosin 2. Together, these data demonstrate that mixed filaments of myosin 2 and myosin 18Aα form a complex with β-Pix in Purkinje neuron spines that promotes spine maturation by enhancing the assembly of actin and myosin filaments downstream of β-Pix’s GEF activity.