Cell cycle-dependent association of Arabidopsis actin-related proteins AtARP4 and AtARP7 with the nucleus.

Arabidopsis encodes at least eight actin-related proteins (ARPs) most of which have orthologs in other distant organisms. To gain insight into the role of ARPs in plants, we have examined the spatial expression and subcellular distribution of two highly divergent Arabidopsis ARPs, AtARP4 and AtARP7. AtARP4 is a homolog of human BAF53 and yeast Arp4, and AtARP7 is a novel, ancient and plant-specific actin-related protein that is not distinctly related to any known ARPs from other kingdoms. Analysis of both these proteins with AtARP4- and AtARP7-specific antibodies revealed that they were most abundant in young meristematic and floral tissues, but were expressed constitutively in all organs and cell types irrespective of their developmental stage. Immunofluorescence studies showed that both AtARP4 and AtARP7 were localized predominantly to the nucleus during interphase. In mitotic cells lacking a nuclear envelope (e.g. metaphase, anaphase, and early telophase stages), these ARPs were excluded from the condensed chromosomes and dispersed throughout the cytoplasm. In contrast, a putative Arabidopsis histone H2B protein remained associated with the interphase nuclei as well as chromosomes throughout the cell cycle. Based on our results and data on the yeast ortholog of AtARP4, these two nuclear plant ARPs may be involved in the modulation of chromatin structure and transcriptional regulation mainly in interphase cells.