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, th e 32-kDa PO and four 12-kDa acidic proteins, P1 alpha, P1 beta, P2 alpha, a nd P2 beta. The P0 carboxyl-terminal domain is involved in the interaction with the acidic proteins and resembles their structure. Protein chimeras we re 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 c onditional null mutant strains in which wild-type PO is not present. In S. cerevisiae D4567, which is totally deprived of acidic proteins, the four fu sion proteins can replace the wild-type PO with little effect on cell growt h. In other genetic backgrounds, the chimeras either reduce or increase cel l growth because of their effect on the ribosomal stalk composition. An ana lysis of the stalk proteins showed that each PO chimera is able to strongly interact with only one acidic protein. The following associations were fou nd: 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 no t form dimers in the yeast ribosomal stalk but interact with each other for ming two specific associations, P1 alpha -P2 beta and P1 beta -P2 alpha, wh ich have different structural and functional roles.