An essential gene of Saccharomyces cerevisiae coding for an actin-related protein

Actin filaments provide the internal scaffold of eukaryotic cells; they are involved in maintenance of cell shape, cytokinesis, organelle movement, and cell motility. The major component of these filaments, actin, is one of the most well-conserved eukaryotic proteins. Recently genes more distantly related to the conventional actins were cloned from several organisms. In the budding yeast, Saccharomyces cerevisiae, one conventional actin gene, ACT1 (coding for the filament actin), and a so-called actin-like gene, ACT2 (of unknown function), have so far been identified. We report here the discovery of a third member of the actin gene family from this organism, which we named ACT3. The latter gene is essential for viability and codes for a putative polypeptide, Act3, of 489 amino acids (M(r) = 54,831). The deduced amino acid sequence of Act3 is less related to conventional actins than is the deduced amino acid sequence of Act2, mainly because of three unique hydrophobic segments. These segments are found inserted into a part of the sequence corresponding to a surface loop of the known three-dimensional structure of the actin molecule. According to sequence comparison, the basal core structure of conventional actin may well be conserved in Act3. Our findings demonstrate that, unexpectedly, there exist three members of the diverse actin protein family in budding yeast that obviously provide different essential functions for survival.