A new large proteolytic complex distinct from the proteasome is present in the cytosol of fission yeast

One eukaryotic proteolytic complex - the proteasome - is classed as the major nonlysosomal protease, by its known and suspected functions, its size and its complexity [1,2]. It seems improbable that other enzymes may be capable of substituting, even partially, for the potent proteasome, as this complex has a vital role in many cellular processes [1-3]. Nevertheless, it is possible to adapt cultured EL-4 mouse lymphoma cells to survive in the presence of a specific inhibitor of the proteasome [4]. The inhibition of the proteasome in these adapted EL-4 cells is accompanied by a dramatic increase in the activity of a new, as yet uncharacterized, large proteolytic complex [4]. Here, we have presented evidence that a similar proteolytic activity is constitutively present in fission yeast, Schizosaccharomyces pombe, and that the yeast and mouse enzymes share basic physicochemical properties. We have shown that the S. pombe protease is found in two stable oligomeric forms, both of which are peptidases, although only the larger form acts as a proteinase. The relative amounts of the large and the small forms of the protease in the complex depended on the growth phase of the yeast culture and affected enzyme activity, suggesting that the activity of the enzyme is regulated by its oligomerization status. We refer to the new proteolytic complex as the 'multicorn' to indicate its analogy to the archaebacterial tricorn protease [5].