YEAST COUNTERPARTS OF SUBUNITS S5A AND P58 (S3) OF THE HUMAN 26S PROTEASOME ARE ENCODED BY 2 MULTICOPY SUPPRESSORS OF NIN1-1

Nin1p, a component of the 26S proteasome of Saccharomyces cerevisiae, is required for activation of Cdc28p kinase at the G(1)-S-phase and G( 2)-M boundaries. By exploiting the temperature-sensitive phenotype of the min1-1 mutant, we have screened for genes encoding proteins with r elated functions to Nin1p and have cloned and characterized two new mu lticopy suppressors, SUN1 and SUN2, of the nin1-1 mutation. SUN1 can s uppress a null nin1 mutation, whereas SUN2, an essential gene, does no t. Sun1p is a 268-amino acid protein which shows strong similarity to MBP1 of Arabidopsis thaliana, a homologue of the S5a subunit of the hu man 26S proteasome. Sun1P binds ubiquitin-lysozyme conjugates as do S5 a and MBP1. Sun2p (523 amino acids) was found to be homologous to the p58 subunit of the human 26S proteasome. cDNA encoding the p58 compone nt was cloned. Furthermore, expression of a derivative of p58 from whi ch the N-terminal 150 amino acids had been removed restored the functi on of a null allele of SUN2. During glycerol density gradient centrifu gation, both Sun1p and Sun2p comigrated with the known proteasome comp onents. These results, as well as other structural and functional stud ies, indicate that both Sun1p and Sun2p are components of the regulato ry module of the yeast 26S proteasome.