Subcellular destination of mutant peroxisomal isocitrate lyase polypeptides of Candida tropicalis in Saccharomyces cerevisiae

The peroxisomal isocitrate lyase (ICL) of an n-alkane-utilizable yeast, Candida tropicalis, could be transported into the peroxisomes of Saccharomyces cerevisiae MT8-1 cells grown in an oleic acid medium, even if strains were different, To identify the important regions to direct C. tropicalis ICL (CT-ICL) to its proper subcellular location in S. cerevisiae, we observed the subcellular localization of the expressed products of mutant CT-ICL genes in S. cerevisiae by immunoelectron microscopy, Under the control of the UPR-ICL promoter, which enhanced gene expression by proliferation of the peroxisomes, the following mutant CT-ICL genes were constructed and expressed: (i) truncated or mutant genes in the C-terminal region which apparently contained peroxisomal targeting signals, Delta 550, Delta 549-550, Delta 548-550, Delta 548-550+SKL and Delta 340-550; and (ii) four internal deletion mutant genes, Delta 41-115, Delta 41-239, Delta 41-325 and Delta 237-339. The results showed that three C-terminal amino acid residues were not essential for targeting the peroxisomes in this enzyme, In cells harboring the deleted internal regions of mutant ICL genes, the expressed polypeptides did not target the peroxisomes and the polypeptides, except for Delta 41-115, were mistargeted to mitochondria. We observed an unknown structure which we called a ''protein aggregate body (PAB),'' in which various mutant CT-ICL polypeptides aggregated.