Myosin Va phosphorylated on Ser¹⁶⁵⁰ is found in nuclear speckles and redistributes to nucleoli upon inhibition of transcription

Abstract

Nuclear actin and nuclear myosins have been implicated in the regulation of gene expression in vertebrate cells. Myosin V is a class of actin-based motor proteins involved in cytoplasmic vesicle transport and anchorage, spindle-pole alignment and mRNA translocation. In this study, myosin-Va, phosphorylated on a conserved serine in the tail domain (phospho-ser(1650) MVa), was localized to subnuclear compartments. A monoclonal antibody, 9E6, raised against a peptide corresponding to phosphoserine(1650) and flanking regions of the murine myosin Va sequence, was immunoreactive to myosin Va heavy chain in cellular and nuclear extracts of HeLa cells, PC12 cells and B16-F10 melanocytes. Immunofluorescence microscopy with this antibody revealed discrete irregular spots within the nucleoplasm that colocalized with SC35, a splicing factor that earmarks nuclear speckles. Phospho-ser(1650) MVa was not detected in other nuclear compartments, such as condensed chromatin, Cajal bodies, gems and perinucleolar caps. Although nucleoli also were not labeled by 9E6 under normal conditions, inhibition of transcription in HeLa cells by actinomycin D caused the redistribution of phospho-ser(1650) MVa to nucleoli, as well as separating a fraction of phospho-ser(1650) MVa from SC35 into near-neighboring particles. These observations indicate a novel role for myosin Va in nuclear compartmentalization and offer a new lead towards the understanding of actomyosin-based gene regulation.

Ancient animal ancestry for nuclear myosin

“…Nevertheless, there is increasing evidence that myosins and actins are associated with transcription and other nuclear functions. For example, myosin Va (Pranchevicius et al, 2008), myosin VI (Vreugde et al, 2006), myosin XVIb (Cameron et al, 2007) and actin, in addition to NMI, are found in eukaryotic nuclei. NMI interacts with topoisomerase II (Smukste et al, 2006), and MreB, an actin-like protein, interacts with RNA polymerase in bacteria (Kruse et al, 2006).…”

What we talk about when we talk about nuclear actin

“…[21][22][23][24][25][26] In Dundr et al 27 overexpression of a nucleus-targeted, non-polymerizing actin mutant (R62D) in HeLa cells inhibited long-range (2-3 micron) movement of an array of activated U2 snRNA minigenes to Cajal bodies. Chuang et al 28 observed a similar effect on the motion of a transgenic DHFR gene in CHO cells in the presence of a nucleustargeted G13R actin mutant, also defective in polymerization.…”

Nuclear actin and myosins at a glance

“…Although myosin II has been described in the nucleus (Rodgers, 2005;Li and Sarna, 2009), myosin II filaments are yet to be described in the nucleus. Nuclear myosin Va is found in nuclear speckles, which are (See poster insert) sites of RNA processing in transcriptionally active cells (Pranchevicius et al, 2008). By contrast, myosin Vb interacts with actin and RNA polymerase I in nucleoli (Lindsay and McCaffrey, 2009).…”