Mouse Nuclear Myosin I Knock-Out Shows Interchangeability and Redundancy of Myosin Isoforms in the Cell Nucleus

Abstract

BackgroundNuclear myosin I (NM1) is a nuclear isoform of the well-known “cytoplasmic” Myosin 1c protein (Myo1c). Located on the 11th chromosome in mice, NM1 results from an alternative start of transcription of the Myo1c gene adding an extra 16 amino acids at the N-terminus. Previous studies revealed its roles in RNA Polymerase I and RNA Polymerase II transcription, chromatin remodeling, and chromosomal movements. Its nuclear localization signal is localized in the middle of the molecule and therefore directs both Myosin 1c isoforms to the nucleus.Methodology/Principal FindingsIn order to trace specific functions of the NM1 isoform, we generated mice lacking the NM1 start codon without affecting the cytoplasmic Myo1c protein. Mutant mice were analyzed in a comprehensive phenotypic screen in cooperation with the German Mouse Clinic. Strikingly, no obvious phenotype related to previously described functions has been observed. However, we found minor changes in bone mineral density and the number and size of red blood cells in knock-out mice, which are most probably not related to previously described functions of NM1 in the nucleus. In Myo1c/NM1 depleted U2OS cells, the level of Pol I transcription was restored by overexpression of shRNA-resistant mouse Myo1c. Moreover, we found Myo1c interacting with Pol II. The ratio between Myo1c and NM1 proteins were similar in the nucleus and deletion of NM1 did not cause any compensatory overexpression of Myo1c protein.Conclusion/SignificanceWe observed that Myo1c can replace NM1 in its nuclear functions. Amount of both proteins is nearly equal and NM1 knock-out does not cause any compensatory overexpression of Myo1c. We therefore suggest that both isoforms can substitute each other in nuclear processes.

The Toxoplasma Acto-MyoA Motor Complex Is Important but Not Essential for Gliding Motility and Host Cell Invasion

“…Interestingly, a triple KO for myoA/B/C resulted in a more severe phenotype compared to the single myoA KO, providing strong support for overlapping functions of parasite myosins. Such interchange ability and redundancy has recently been demonstrated for the mouse nuclear myosin I (NM1) that can be complemented by Myo1c [34]. When the MyoA-associated and -regulatory partner MLC1 was depleted, we observed mislocalisation of MyoA.…”

The Toxoplasma Acto-MyoA Motor Complex Is Important but Not Essential for Gliding Motility and Host Cell Invasion

“…Interestingly, a triple KO for myoA,B/C resulted in a more severe phenotype compared to single myoA KO, in strong support for overlapping functions of parasite myosins. Such interchange ability and redundancy has recently been demonstrated for mouse nuclear myosin I (NM1) that can be complemented by Myo1c (Venit et al, 2013). When the MyoA-associated and -regulatory partner MLC1 was depleted, we observed a mislocalisation of, MyoA.…”

Nuclear myosin 1 activates p21 gene transcription in response to DNA damage through a chromatin-based mechanism

“…Initial experiments were performed on primary MEFs derived from 13.5 days NM1 WT and KO embryos 46 . In other experiments were used immortalized MEFs (ATCC® CRL-2752) or cell lines derived from these immortalized cells.…”