The acidic phosphoproteins from Saccharomyces cerevisiae ribosomes. NH2-terminal acetylation is a conserved difference between P1 and P2 proteins.

Isoelectrofocusing gels of acidic ribosomal proteins from most yeast strains reveal the presence of up to 10 bands which are the product of only 4 genes. The proteins have been characterized by NH2-terminal amino 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 phosphorylated and dephosphorylated forms of the YP2 proteins, YP2 alpha and YP2 beta, 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 YB1 beta lacking the first eight amino acids. The formation of this truncated YP1 beta form seems to be stimulated by salt during protein extraction and is also favored by some modifications at the amino termini of the protein. On the other hand, the previously uncharacterized band, called Ax, corresponds to an NH2-terminal acetylated form of YP1 beta 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 YP1 alpha, which is very poorly stained by silver and Coomassie blue, has not been characterized in detail previously. It is also monophosphorylated in the ribosome and, like YP1 beta, is present as an NH2-terminal acetylated form starting at the second serine residue.(ABSTRACT TRUNCATED AT 250 WORDS)