Identification and characterization of a novel myosin Ic isoform that localizes to the nucleus

In vertebrates, two myosin Ic isoforms that localize to the cytoplasm and to the nucleus have been characterized. The isoform that predominantly localizes to the nucleus is called nuclear myosin I (NMI). NMI has been identified as a key factor involved in nuclear processes such as transcription by RNA polymerases I and II and intranuclear transport processes. We report here the identification of a previously uncharacterized third MYOIC gene product that is called isoform A. Similar to NMI, this isoform contains a unique N-terminal peptide sequence, localizes to the nucleus and colocalizes with RNA polymerase II. However, unlike NMI, upon exposure to inhibitors of RNA polymerase II transcription the newly identified isoform translocates to nuclear speckles. Furthermore, in contrast to NMI, this new isoform is absent from nucleoli and does not colocalize with RNA polymerase I. Our results suggest an unexpected diversity among nuclear myosin Ic isoforms in respect to their intranuclear localization and interaction with nuclear binding partners that could provide new insights into the regulation of myosin-dependent nuclear processes.