Myosin16b: The COOH‐tail region directs localization to the nucleus and overexpression delays S‐phase progression

Abstract

Rat Myo16a and Myo16b comprise the founding members of class XVI myosin and are characterized by an N-terminal ankyrin repeat domain thought to mediate an association with protein phosphatase 1 catalytic subunits 1alpha and 1gamma. Myo16b is the principal isoform and reveals predominant expression in developing neural tissue. Here, we use COS-7 cells as a model system to develop an understanding of Myo16b function. We find that Myo16b displays predominant localization in the nucleus of cells transitioning through interphase, but is not associated with processes of mitosis. Using a panel of EGFP-Myo16b-expression plasmids in transient transfection studies, we identified the COOH-terminal residues 1616-1912 as necessary and solely sufficient to target Myo16b to the nucleus. We show that the Myo16b-tail region directs localization to a nuclear compartment containing profilin and polymerized actin, which appears to form a three-dimensional meshwork through the depth of the nucleus. Further, we demonstrate that this compartment localizes within euchromatic regions of the genome and contains proliferating cell nuclear antigen (PCNA) and cyclin A, both markers of S-phase of the cell cycle. Cells transiently expressing Myo16b or Myo16b-tail region show limited incorporation of BrdU, delayed progression through S-phase of the cell cycle, and curtailed cellular proliferation.

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).…”

Myosin Vb localises to nucleoli and associates with the RNA polymerase I transcription complex

“…NMIb also binds the transcription activator TIF1A and it has been proposed that the NMIb-TIF1A complex activates RNA polymerase I . Other myosins have since been discovered in the nucleus, including myosin 16B [Cameron et al, 2007] and myosin VI [Vreugde et al, 2006]. Recently Pranchevicius et al have found phosphorylated myosin Va localising to nuclear speckles [Pranchevicius et al, 2008].…”

“…A number of other myosins have now also been found in the nucleus. These include myosin 16B [ Cameron et al, 2007], and myosin VI [Vreugde et al, 2006]. Recently, Pranchevicius et al have observed myosin Va in the nucleus, and they further demonstrate that the nuclear fraction of myosin Va is phosphorylated on serine 1650 [Pranchevicius et al, 2008].…”

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.…”