Mouse nuclear myosin I knock-out shows interchangeability and redundancy of myosin isoforms in the cell nucleus.

Nuclear myosin I (NM1) is a nuclear isoform of the well-known
"cytoplasmic" Myosin 1c protein (Myo1c). Located on the 11(th)
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. In 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. We 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.