Asymmetric interactions between the acidic P1 and P2 proteins in the Saccharomyces cerevisiae ribosomal stalk

The Saccharomyces cerevisiae ribosomal stalk is made of five components, the 32-kDa PO and four 12-kDa acidic proteins, P1 alpha, P1 beta, P2 alpha, and P2 beta. The P0 carboxyl-terminal domain is involved in the interaction with the acidic proteins and resembles their structure. Protein chimeras were constructed in which the last 112 amino acids of PO were replaced by the sequence of each acidic protein, yielding four fusion proteins, PO-1 alpha, P0-1 beta, P0-2 alpha, and P0-2 beta. The chimeras were expressed in PO conditional null mutant strains in which wild-type PO is not present. In S. cerevisiae D4567, which is totally deprived of acidic proteins, the four fusion proteins can replace the wild-type PO with little effect on cell growth. In other genetic backgrounds, the chimeras either reduce or increase cell growth because of their effect on the ribosomal stalk composition. An analysis of the stalk proteins showed that each PO chimera is able to strongly interact with only one acidic protein. The following associations were found: P0-1 alpha -P2 beta, P0-1 beta -P2 alpha, P0-2 alpha -P1 beta, and P0-2 beta -P1 alpha. These results indicate that the four acidic proteins do not form dimers in the yeast ribosomal stalk but interact with each other forming two specific associations, P1 alpha -P2 beta and P1 beta -P2 alpha, which have different structural and functional roles.