Subcellular distribution of the acidic ribosomal P-proteins from Saccharomyces cerevisiae in various environmental conditions

The yeast ribosomal "stalk"-a lateral protuberance on the 60S subunit-consists of four acidic P-proteins, P1A, P1B, P2A and P2B, which play an important role during protein synthesis. Contrary to most ribosomal proteins, which are rapidly degraded in the cytoplasm, P-proteins are found as a cytoplasmic pool and are exchanged with the ribosome-bound proteins during translation. As yet, subcellular trafficking of P-proteins has not been extensively investigated. Therefore, we have characterized-using immunological approaches-the cellular distribution of P-proteins in several environmental conditions, characteristic of yeast cells, such as growth phases, and heat-, osmotic-, and oxygen-stress. Using the western blotting approach, we have shown P-proteins to be present in constant amounts on the ribosomes, despite their exchangeability with the cytoplasmic pool, and regardless of environmental conditions. On the other hand, P-protein level in the cytoplasm decreased sharply throughout the consecutive growth phases, but was not affected by several stress conditions. Applying the electron microscopic technique and immunogold labeling, we have found that P-proteins are located in two cell compartments. The first one is the cytoplasm and the second one-an unexpected place-the cell wall, where P-proteins are fully phosphorylated. Moreover, the existence of P-proteins on the cellular wall is not affected by various environmental conditions. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.