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 e ukaryotic cell. 20S proteasomes are 700 kDa cylindrically shaped parti cles, found in the cytoplasm and the nucleus of all eukaryotes. They a re 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 Saccharomyce s cerevisiae a complete set of 14 genes coding for 20S proteasome subu nits have been cloned and sequenced. 26S proteasomes are even larger p roteinase complexes (about 1700 kDa) which degrade ubiquitinylated pro teins in an ATP-dependent fashion in vitro. The 26S proteasome is buil d up from the 20S proteasome as core particle and two additional 19S c omplexes at both ends of the 20S cylinder. Recently existence of a 26S proteasome in yeast has been demonstrated. Several 26S proteasome spe cific genes have been cloned and sequenced. They share similarity with a novel defined family nf ATPases 20 and 26S proteasomes are essentia l 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 me diated proteolysis. They are involved in the degradation nf short-live d,and regulatory proteins. Proteasomes are important for cell differen tiation and adaptation to environmental changes. Proteasomes have also been shown to function in the control of the cell cycle.