BACKGROUND: 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. METHODOLOGY/PRINCIPAL FINDINGS: 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. CONCLUSION/SIGNIFICANCE: 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.

• MeSH
• Cell Nucleus metabolism   MeSH
• DNA Primers genetics   MeSH
• Phenotype * MeSH
• Genotype MeSH
• Immunoprecipitation MeSH
• Myosin Type I genetics   metabolism   MeSH
• Mice, Knockout MeSH
• Mice MeSH
• Plasmids genetics   MeSH
• Reverse Transcriptase Polymerase Chain Reaction MeSH
• Protein Isoforms genetics   metabolism   MeSH
• Blotting, Western MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• DNA Primers MeSH
• Myo1c protein, mouse MeSH Browser
• Myosin Type I MeSH
• Protein Isoforms MeSH

Lamins are the best characterized cytoskeletal components of the cell nucleus that help to maintain the nuclear shape and participate in diverse nuclear processes including replication or transcription. Nuclear actin is now widely accepted to be another cytoskeletal protein present in the nucleus that fulfills important functions in the gene expression. Some viruses replicating in the nucleus evolved the ability to interact with and probably utilize nuclear actin for their replication, e.g., for the assembly and transport of capsids or mRNA export. On the other hand, lamins play a role in the propagation of other viruses since nuclear lamina may represent a barrier for virions entering or escaping the nucleus. This review will summarize the current knowledge about the roles of nuclear actin and lamins in viral infections.

• Keywords
• viruses, cytoskeleton, lamin, nuclear actin, nuclear lamina, nucleus,
• MeSH
• Actins metabolism   MeSH
• Baculoviridae metabolism   pathogenicity   MeSH
• Cell Nucleus metabolism   virology   MeSH
• Cytoskeleton MeSH
• Herpesviridae metabolism   pathogenicity   MeSH
• Herpesviridae Infections metabolism   pathology   virology   MeSH
• Lamins metabolism   MeSH
• Humans MeSH
• Virus Replication * MeSH
• Retroviridae metabolism   pathogenicity   MeSH
• Retroviridae Infections metabolism   pathology   virology   MeSH
• Virus Assembly * MeSH
• Virus Diseases metabolism   virology   MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH
• Names of Substances
• Actins MeSH
• Lamins MeSH

BACKGROUND: Nuclear myosin I (NM1) was the first molecular motor identified in the cell nucleus. Together with nuclear actin, they participate in crucial nuclear events such as transcription, chromatin movements, and chromatin remodeling. NM1 is an isoform of myosin 1c (Myo1c) that was identified earlier and is known to act in the cytoplasm. NM1 differs from the "cytoplasmic" myosin 1c only by additional 16 amino acids at the N-terminus of the molecule. This amino acid stretch was therefore suggested to direct NM1 into the nucleus. METHODOLOGY/PRINCIPAL FINDINGS: We investigated the mechanism of nuclear import of NM1 in detail. Using over-expressed GFP chimeras encoding for truncated NM1 mutants, we identified a specific sequence that is necessary for its import to the nucleus. This novel nuclear localization sequence is placed within calmodulin-binding motif of NM1, thus it is present also in the Myo1c. We confirmed the presence of both isoforms in the nucleus by transfection of tagged NM1 and Myo1c constructs into cultured cells, and also by showing the presence of the endogenous Myo1c in purified nuclei of cells derived from knock-out mice lacking NM1. Using pull-down and co-immunoprecipitation assays we identified importin beta, importin 5 and importin 7 as nuclear transport receptors that bind NM1. Since the NLS sequence of NM1 lies within the region that also binds calmodulin we tested the influence of calmodulin on the localization of NM1. The presence of elevated levels of calmodulin interfered with nuclear localization of tagged NM1. CONCLUSIONS/SIGNIFICANCE: We have shown that the novel specific NLS brings to the cell nucleus not only the "nuclear" isoform of myosin I (NM1 protein) but also its "cytoplasmic" isoform (Myo1c protein). This opens a new field for exploring functions of this molecular motor in nuclear processes, and for exploring the signals between cytoplasm and the nucleus.

• MeSH
• Adenosine Diphosphate metabolism   MeSH
• Actins metabolism   MeSH
• Active Transport, Cell Nucleus MeSH
• Cell Nucleus metabolism   MeSH
• Cell Line MeSH
• Nuclear Localization Signals MeSH
• Calmodulin metabolism   MeSH
• Karyopherins metabolism   MeSH
• Humans MeSH
• Molecular Sequence Data MeSH
• Myosin Type I chemistry   metabolism   MeSH
• Myosins chemistry   metabolism   MeSH
• Mice MeSH
• Amino Acid Sequence MeSH
• Protein Structure, Tertiary MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Adenosine Diphosphate MeSH
• Actins MeSH
• Nuclear Localization Signals MeSH
• Calmodulin MeSH
• Karyopherins MeSH
• Myo1c protein, mouse MeSH Browser
• Myosin Type I MeSH
• Myosins MeSH