IDENTIFICATION OF ACT4, A NOVEL ESSENTIAL ACTIN-RELATED GENE IN THE YEAST SACCHAROMYCES-CEREVISIAE

Actin molecules are major cytoskeleton components of all eukaryotic ce lls. All conventional actins that have been identified so far are 374- 376 amino acids in size and exhibit at least 70% amino acid sequence i dentity when compared with one another. In the yeast Saccharomyces cer evisiae, one conventional actin gene ACT1 and three so-called actin-re lated genes, ACT2, ACT3 and ACT5, have been identified. We report here the discovery of a new actin-related gene in this organism, which we have named ACT4. The deduced protein, Act4, of 449 amino acids, exhibi ts only 33.4%, 26.7%, 23.4% and 29.2% identity to Act1, Act2, Act3 and Act5, respectively. In contrast, it is 684% identical to the product of the Schizosaccharomyces pombe Act2 gene and has a similar level of identity to other Sch. pombe Act2 homologues. This places Act4 in the Arp3 family of actin-related proteins. ACT4 gene disruption and tetrad analysis demonstrate that this gene is essential for the vegetative g rowth of yeast cells. The act4 mutants exhibit heterogenous morphologi cal phenotypes. We hypothesize that Act4 may have multiple roles in th e cell cycle. The sequence has been deposited in the Genome Sequence D ata Base under Accession Number L37111.