Rat liver proteasome (multicatalytic proteinase complex) is a 20S-ring shaped particle having a molecular mass of 750 kDa, and iscomposed of at least 13 non-identical components ranging from 21 to 31 kDa in size. We found here that the NH2-terminal residues of all the known 13 components, except for C5, are not reactive to phenylisothiocyanate. Among them, components C2, C3 and C8 are blocked in their NH2-termini with Nα-acetyl-Met, Nα-acetyl-Ala, and Nα-acetyl-Ser, respectively. The NH2-terminal portions of C2, C3, and C8 exhibit sequence similarity to one another, but that of the non-blocked component C5 differs from those of C2, C3, and C8.
