THE ACTIN-RELATED PROTEIN ACT3P OF SACCHAROMYCES-CEREVISIAE IS LOCATED IN THE NUCLEUS

Actin-related proteins, a group of protein families that exhibit about 50% sequence identity among each other and to conventional actin, hav e been found in a variety of eukaryotic organisms. In the budding yeas t Saccharomyces cerevisiae, genes for one conventional actin (ACT1) an d for three actin-related proteins (ACT2, ACT3, and ACT5) are known. A CT3, which we recently discovered, is an essential gene coding for a p olypeptide of 489 amino acids (Act3p), with a calculated molecular mas s of 54.8 kDa. Besides its homology to conventional actin, Act3p posse sses a domain exhibiting weak similarity to the chromosomal protein HM G-14 as well as a potential nuclear localization signal. An antiserum prepared against a specific segment of the ACT3 gene product recognize s a polypeptide band of approximately 55 kDa in yeast extract. Indirec t immunofluorescence experiments with this antiserum revealed that Act 3p is located in the nucleus. Nuclear staining was observed in all cel ls regardless of the stage of the cell cyle. Independently, immunoblot ting experiments with subcellular fractions showed that Act3p is indee d highly enriched in the nuclear fraction. We suggest that Act3p is an essential constituent of yeast chromatin.