There is accumulating evidence for a large, highly conserved gene family of putative ATPases. We have identified 12 different members of this novel gene family (the YTA family) in yeast and determined the nucleotide sequences of nine of these genes. All of the putative gene products are characterized by the presence of a highly conserved domain of 300 amino acids containing specialized forms of the A and B boxes of ATPases. YTA1, YTA2, YTA3 and YTA5 exhibit significant similarity to proteins involved in human immunodeficiency virus Tat-mediated gene expression but more significantly to subunits of the human 26S proteasome. YTA1 and YTA2 are essential genes in yeast. Remarkably, the cDNA of human TBP-1 can compensate for the loss of YTA1. Preliminary experiments indicate that YTA1 is a component of the 26S protease complex from yeast. Our findings lead us to propose that YTA1, YTA2, YTA3 and YTA5 function as regulatory subunits of the yeast 26S proteasome. YTA10, YTA11 and YTA12 share significant homology with the Escherichia coli FtsH protein, and together with YTA4 and YTA6 may constitute a separate subclass within this family of putative ATPases.

Incompletely synthesized polypeptides in the mitochondrial inner membrane are subject to rapid proteolysis. We demonstrate that Ytal 0p, a mitochondrial homologue of a conserved family of putative ATPases in Saccharomyces cerevisiae, is essential for this proteolytic process. Ytal0p-dependent degradation requires divalent metal ions and the hydrolysis of ATE Ytal0p is an integral protein of the inner mitochondrial membrane exposing the carboxy terminus to the mitochondrial matrix space. Based on the presence of consensus binding sites for ATE and for divalent metal ions found in a number of metal dependent endopeptidases, a direct role of Yta 10p in the proteolytic breakdown of membrane-associated polypeptides in mitochondria is suggested.

Akstraet The yeast gene, YTAIO, encodes a member of a novel family of putative ATPases. Ytal0p, as deduced from the nucleotide sequence, is 761 amino acids in length (predicted molecular mass 84.5 kDa). The amino acid sequence of Yta 10p exhibits high similarity to two other yeast proteins, Ytal 1 and Ytal2, and to E. coli FtsH. Several features of Ytal0p are compatible with its localization in mitochondria. We report here that Ytal0p is a yeast mitochondrial protein and that import is dependent on a membrane potential and accompanied by processing to a protein of approximately 73 kDa. Disruption of YTAIO leads to a nuclear petite phenotype and to a loss of respiratory competence, as shown by spectrophotometric measurement of the activities of respiratory complexes I-III and IV, respectively. These findings together with the high similarity of Ytal0p to several ATP-dependent proteases suggest that Ytal0p is a mitochondrial component involved, directly or indirectly, in the correct assembly and/or maintenance of active respiratory complexes.