THE ACIDIC PHOSPHOPROTEINS FROM SACCHAROMYCES-CEREVISIAE RIBOSOMES - NH(2)-TERMINAL ACETYLATION IS A CONSERVED DIFFERENCE BETWEEN P1-PROTEIN AND P2-PROTEIN

Isoelectrofocusing gels of acidic ribosomal proteins from most yeast s trains reveal the presence of up to 10 bands which are the product of only 4 genes. The proteins have been characterized by NH2-terminal ami no acid sequencing, specific antibodies, HPLC, and by taking advantage of acidic protein-defective yeast strains obtained by gene disruption methods. The four most basic proteins coincide with the phosphorylate d and dephosphorylated forms of the YP2 proteins, YP2alpha and YP2beta , formerly named L44 and L45. Amino-terminal sequencing has shown that these two polypeptides have free amino-terminal ends starting at the first methionine residue. The bands defined earlier as L44' correspond to the phosphorylated and dephosphorylated processed forms of protein YB1beta lacking the first eight amino acids. The formation of this tr uncated YP1beta form seems to be stimulated by salt during protein ext raction and is also favored by some modifications at the amino termini of the protein. On the other hand, the previously uncharacterized ban d, called Ax, corresponds to an NH2-terminal acetylated form of YP1bet a which starts at the serine in the second position of the nucleotide- derived sequence, Finally, the most acidic band is the phosphorylated product of the fourth acidic protein gene. This protein, called YP1alp ha, which is very poorly stained by silver and Coomassie blue, has not been characterized in detail previously. It is also monophosphorylate d in the ribosome and, like YP1beta, is present as an NH2-terminal ace tylated form starting at the second serine residue. YP1 proteins, but not YP2 proteins, show a pI change when obtained from Saccharomyces ce revisiae NAT1, an NH2-terminal acetyltransferase-defective mutant, con firming the N(alpha)-acetylated condition of the first polypeptides. T he difference in the amino end of the two acidic protein groups parall els the situation found in bacterial acidic proteins L7 and L12 and su ggests that this conserved structural feature must have a significant role in the acidic proteins' function.