Role of the 40S beak ribosomal protein eS12 in ribosome biogenesis and function in<i>Saccharomyces cerevisiae</i>

Abstract

In eukaryotes, the beak structure of 40S subunits is formed by the protrusion of the 18S rRNA helix 33 and three ribosomal proteins: eS10, eS12 and eS31. The exact role of these proteins in ribosome biogenesis is not well understood. While eS10 is an essential protein encoded by two paralogous genes in <i>Saccharomyces cerevisiae</i>, eS12 and eS31 are not essential proteins encoded by the single-copy genes <i>RPS12</i> and <i>UBI3</i>, respectively. Here, we have analysed the contribution of yeast eS12 to ribosome biogenesis and compared it with that of eS31. Polysome analysis reveals that deletion of either <i>RPS12</i> or <i>UBI3</i> results in equivalent 40S deficits. Analysis of pre-rRNA processing indicates that eS12, akin to eS31, is required for efficient processing of 20S pre-rRNA to mature 18S rRNA. Moreover, we show that the 20S pre-rRNA accumulates within cytoplasmic pre-40S particles, as deduced from FISH experiments and the lack of nuclear retention of 40S subunit reporter proteins, in <i>rps12∆</i> and <i>ubi3∆</i> cells. However, these particles containing 20S pre-rRNA are not efficiently incorporated into polyribosomes. We also provide evidence for a genetic interaction between eS12 or eS31 and the late-acting 40S assembly factors Enp1 and Ltv1, which appears not to be linked to the dynamics of their association with or release from pre-40S particles in the absence of either eS12 or eS31. Finally, we show that eS12- and eS31-deficient ribosomes exhibit increased levels of translational misreading. Altogether, our data highlight distinct important roles of the beak region during ribosome assembly and function.