Structural and functional characterization of the amino terminal domain of the yeast ribosomal stalk P1 and P2 proteins

The essential ribosomal stalk is formed in eukaryotes by a pentamer of two P1–P2 protein heterodimers and the P0 rRNA binding protein. In contrast to the highly stable prokaryotic complex, the P1 and P2 proteins in the eukaryotic stalk undergo a cyclic process of assembly and disassembly during translation that seems to modulate the ribosome activity. To better understand this process, the regions of the Saccharomyces cerevisiae P1α and P2β proteins that are directly involved in heterodimer formation and ribosome binding have been characterized using a series of P1α/P2β chimeras. The region required for a stable interaction with the ribosome is formed by the first three predicted α-helices in the N-terminal domain of both proteins. The same region is required for heterodimer formation in P2β but the third helix is dispensable for this association in P1α. It seems, therefore, that stable ribosome binding is more structurally demanding than heterodimerization. A fourth predicted α-helix in the N-terminal domain of P1α and P2β appears not to be involved in the assembly process but rather, it contributes to the conformation of the proteins by apparently restricting the mobility of their C-terminal domain and paradoxically, by reducing their activity. In addition, the study of P1/P2 chimeras showed that the C-terminal domains of these two types of protein are functionally identical and that their protein specificity is exclusively determined by their N-terminal domains.