Las proteínas P, la esporulación y la respiración en Saccharomyces cerevisiae Hendricka

Resumo

The stalk is a lateral protuberance of the large subunit of the ribosome. In Saccharomyces cerevisiae the stalk is formed by a central protein called P0 and four accessory proteins, two of the P1 type (P1¿, P1ß) and two of the P2 type (P2¿ and P2ß). The P1 and P2 proteins bind as heterodimers P1ß/P2¿ and P1¿/P2ß to the P0 protein, which in turn interacts with the 25S rRNA through its amino terminal end. Although the basic function of the stalk is promoting the interaction of translation factors during the protein synthesis, a regulatory role has also been proposed for it, so that different populations of ribosomes, with different stalk composition, are responsible for the differential translation of some mRNAs in the cell. Among the stalk components, only P0 protein is essential. Mutants in P1 and P2 proteins are viable; however, there are conditions in which P1 and P2 proteins are essential for the yeast cells. As an example, mutants without P2 or P1 are unable to sporulate, being this phenotype reverted with the expression of any P1/P2 heterodimers. For further studying how the absence of the P1 and P2 proteins affects the process of sporulation, mutants without protein P1 (called D67) or P2 protein (mutant called D45) have been constructed in the SK1 genetic background. A common feature between D45 and D67 mutants is that they do not have associated P proteins to the stalk; differentially, ribosome unbound P2 proteins accumulate in D67 while non P protein is detected in D45. It has been confirmed that in the absence P proteins, these mutants do not develop the sporulation process. In addition, these mutants showed defects when grown in a media with non-fermentable carbon sources, which reflects a defect in mitochondrial function. D45 mutant was not able to grow at all under strict respiratory conditions, thus behaving as a petite strain; mitochondria characterization showed a severe alteration in the mitochondrial DNA and a remarkable intramitochondrial accumulation of P0 protein. On the contrary, it is possible to obtain a respiratory competent D67 mutant, although it is true that the frequency of spontaneous petite formation is much higher than in the wild type strain. Sporulation and respiration are two interlaced processes that are affected independently by the absence of P proteins free in the cytoplasm. These results suggest a potential extrarribosomal role of the P1 and P2 proteins in the cells.