Actin-related proteins (Arps), which are divergent, but apparently homologues to actin, are categorized into 10 classes. While Arps belonging to classes 1-3 were found to be localized in the cytoplasm across eukaryotic phyla, other classes of Arps were found mostly in invertebrates and suggested to contribute to structural modulation of chromatin. Here we report the identification and the characterization of two human isoforms of an Arp not belonging to classes 1-3, which we designated hArpNα and hArpNβ. Both proteins were expressed in HeLa cells and they were found localized within the nucleus. Most interestingly, in different human tissues, hArpNα and β were found to be expressed mutually exclusively, and the expression of hArpNα was absolutely restricted to the brain. These findings suggest that, in vertebrates, members of distantly related Arps might have tissue-specific functions in the nucleus, possibly through structural modulation of chromatin.

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