Assembly of Saccharomyces cerevisiae ribosomal stalk: Binding of P1 proteins is required for the interaction of P2 proteins

The yeast ribosomal stalk is formed by a protein pentamer made of the 38 kD a PO and four 12 kDa acidic P1/P2. The interaction of recombinant acidic pr oteins P1 alpha and P2 beta with ribosomes from Saccharomyces cerevisiae D4 567, lacking all the 12 kDa stalk components, has been used to study the in vitro assembly of this important ribosomal structure. Stimulation of the r ibosome activity was obtained by incubating simultaneously the particles wi th both proteins, which were nonphosphorylated initially and remained unmod ified afterward. The N-terminus state, free or blocked, did not affect eith er the binding or reactivating activity of both proteins. Independent incub ation with each protein did not affect the activity of the particles, howev er, protein P2 beta alone was unable to bind the ribosome whereas P1 alpha could. The binding of P1 alpha alone is a saturable process in acidic-prote in-deficient ribosomes and does not take place in complete wild-type partic les. Binding of P1 proteins in the absence of P2 proteins takes also place in vivo, when protein P1 beta is overexpressed in S. cerevisiae. In contras t, protein P2 beta is not detected in the ribosome in the P1-deficient D67 strain despite being accumulated in the cytoplasm. The results confirm that neither phosphorylation nor N-terminal blocking of the 12 kDa acidic prote ins is required for the assembly and function of the yeast stalk. More impo rtantly, and regardless of the involvement of other elements, they indicate that stalk assembling is a coordinated process, in which P1 proteins would provide a ribosomal anchorage to P2 proteins, and P2 components would conf er functionality to the complex.