N-alpha-acetylation and proteolytic activity of the yeast 20 S proteasome

N-alpha-Acetylation, catalyzed co-translationally with N-alpha-acetyltransf erase (NAT), is the most common modifications of eukaryotic proteins. In ye ast, there are at least three NATs: NAT1, MAK3, and NAT3, The 20 S proteaso me subunits were purified from the normal strain and each of the deletion m utants, nat1, mak3, and nat3. The electrophoretic mobility of these subunit s was compared by two-dimensional gel electrophoresis. Shifts toward the al kaline side of the gel and unblocking of the N terminus of certain of the s ubunits in one or another of the mutants indicated that the alpha 1, alpha 2, alpha 3, alpha 4, alpha 7, and beta 3 subunits were acetylated with NAT1 , the alpha 5 and alpha 6 subunits were acetylated with MAK3, and the beta 4 subunit was acetylated with NAT3. Furthermore, the Ac-Met-Phe-Leu and Ac- Met-Phe-Arg termini of the alpha 5 and alpha 6 subunits, respectively, exte nded the known types of MAK3 substrates, Thus, nine subunits were N (alpha) -acetylated, whereas the remaining five were processed, resulting in the lo ss of the N-terminal region. The 20 S proteasomes derived from either the n atl mutant or the normal strain were similar in respect to chymotrypsinlike , trypsin-like, and peptidylglutamyl peptide hydrolyzing activities in vitr o, suggesting that N-alpha-acetylation does not play a major functional rol e in these activities. However, the chymotrypsin-like activity in the absen ce of sodium dodecyl sulfate was slightly higher in the natl mutant than in the normal strain.