BROTEASOMES OF THE YEAST SACCHAROMYCES-CEREVISIAE - GENES, STRUCTURE AND FUNCTIONS

Proteasomes are large multicatalytic proteases complexes which fulfil central functions in major intracellular proteolytic pathways of the eukaryotic cell. 20S proteasomes are 700 kDa cylindrically shaped particles, found in the cytoplasm and the nucleus of all eukaryotes. They are composed of a pool of 14 different subunits (MW 22-25 kDa) arranged in a stack of 4 rings with 7-fold symmetry. In the yeast Saccharomyces cerevisiae a complete set of 14 genes coding for 20S proteasome subunits have been cloned and sequenced. 26S proteasomes are even larger proteinase complexes (about 1700 kDa) which degrade ubiquitinylated proteins in an ATP-dependent fashion in vitro. The 26S proteasome is build up from the 20S proteasome as core particle and two additional 19S complexes at both ends of the 20S cylinder. Recently existence of a 26S proteasome in yeast has been demonstrated. Several 26S proteasome specific genes have been cloned and sequenced. They share similarity with a novel defined family nf ATPases 20 and 26S proteasomes are essential fnr functioning of the eukaryotic cell. Chromosomal deletion of 20S and 26S proteasomal genes in the yeast S. cerevisiae caused lethality of the cell. The in vivo functions of proteasomes in major proteolytic pathways have been demonstrated by the use of 20S and 26S proteasomal mutants. Proteasomes are needed for stress dependent and ubiquitin mediated proteolysis. They are involved in the degradation nf short-lived,and regulatory proteins. Proteasomes are important for cell differentiation and adaptation to environmental changes. Proteasomes have also been shown to function in the control of the cell cycle.