AN ESSENTIAL GENE OF SACCHAROMYCES-CEREVISIAE CODING FOR AN ACTIN-RELATED PROTEIN

Actin filaments provide the internal scaffold of eukaryotic cells; the y are involved in maintenance of cell shape, cytokinesis, organelle mo vement, and cell motility. The major component of these filaments, act in, is one of the most well-conserved eukaryotic proteins. Recently ge nes more distantly related to the conventional actins were cloned from several organisms. In the budding yeast, Saccharomyces cerevisiae, on e conventional actin gene, ACT1 (coding for the filament actin), and a so-called actin-like gene, ACT2 (of unknown function), have so far be en identified. We report here the discovery of a third member of the a ctin gene family from this organism, which we named ACT3. The latter g ene is essential for viability and codes for a putative polypeptide, A ct3, of 489 amino acids (M(r) = 54,831). The deduced amino acid sequen ce of Act3 is less related to conventional actins than is the deduced amino acid sequence of Act2, mainly because of three unique hydrophobi c segments. These segments are found inserted into a part of the seque nce corresponding to a surface loop of the known three-dimensional str ucture of the actin molecule. According to sequence comparison, the ba sal core structure of conventional actin may well be conserved in Act3 . Our Findings demonstrate that, unexpectedly, there exist three membe rs of the diverse actin protein family in budding yeast that obviously provide different essential functions for survival.