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.

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