Secretion of a protoxin post-translationally controlled by NaCl in a halotolerant yeast, Pichia farinosa

Unusual properties of the halotolerant yeast Candida nodaensis Killer toxin, CnKT

CnKT, the Killer toxin from the extreme halotolerant yeast Candida nodaensis, presents a strong salt-stimulated phenotype and is a resilient toxin, able to cope with very diverse and aggressive environmental conditions. This zymocin is active in a broad range of pH and temperature and tolerates freezing and conservation for long periods of time. CnKT stability is increased under very high ionic strength and its activity is stimulated by sodium ions, which might interfere in the zymocin structure/stability. All these characteristics make CnKT a promising candidate for several biotechnological applications, e.g. in the high-salt food products preservation from spoilage by other yeasts.

Cloning and chromosomal mapping of URA3 genes of Pichia farinosa and P. sorbitophila encoding orotidine-5'-phosphate decarboxylase

The PfURA3 gene, which encodes orotidine-5'-phosphate decarboxylase, of osmotolerant yeast Pichia farinosa NFRI 3,621, was cloned by complementation of the ura3 mutation of Saccharomyces cerevisiae. The nucleotide sequence of the PfURA3 gene and its deduced amino acid sequence indicated that the gene encodes a protein (PfUra3p) of 267 amino acids. Pulsed-field gel electrophoresis and subsequent Southern blot analysis showed that the genome of P. farinosa NFRI 3621 consisted of seven chromosomes, each approximately 1.1-2.2 Mb in size (11.8 Mb in total) and that PfURA3 was located on chromosome V. Pichia sorbitophila is considered as a synonym of P. farinosa. The genome of P. sorbitophila IFO10021 may consist of 12 chromosomes, each approximately 1.2-2.2 Mb in size. P. sorbitophila has two copies of URA3 genes, termed PsURA3 and PsURA30, which were located on chromosome VIII and III, respectively. The difference between PfURA3 and PsURA3 was only two amino acid substitutions, whereas that between PsURA3 and PsURA30 was six amino acid substitutions and the deletion of the C-terminal amino acid by a stop codon insertion. The sequences of PfURA3, PsURA3 and PsURA30 have been deposited in the DDBJ data library under Accession Nos AB071417, AB109042 and AB109043, respectively.

Alternative CUG codon usage (Ser for Leu) in Pichia farinosa and the effect of a mutated killer gene in Saccharomyces cerevisiae

The halotolerant yeast Pichia farinosa KK1 strain produces a killer toxin termed SMKT (salt-mediated killer toxin). Mass spectrometry and Edman sequencing of peptides from the mature SMKT and secreted protoxin demonstrate that positions specified by the CUG codon contain unmodified serine (Ser) in P.farinosa. In order to express the authentic SMK1 product in Saccharomyces cerevisiae, which uses the universal genetic code, the three CUG codons corresponding to Ser87, Ser137 and Ser206 in the SMK1 gene were changed to universal Ser codons by site-directed mutagenesis. The expression of the modified SMK1 gene with universal Ser codons was lethal in S.cerevisiae, as well as that of the unmodified SMK1 gene with the CUG codons. The secretion of protoxin with the authentic amino acid sequence from the modified SMK1 was significantly increased, whereas the transcription level of SMK1 was not affected in the presence or absence of CUG codon. Our results provide the first in vivo evidence that non-universal decoding of CUG is used in a hemiascomycetous yeast, P.farinosa.

Interaction of SMKT, a killer toxin produced by Pichia farinosa, with the yeast cell membranes

SMKT (salt-mediated killer toxin), a killer toxin produced by the halotolerant yeast, Pichia farinosa, kills yeasts of several genera, including Saccharomyces cerevisiae. To elucidate the killing mechanism of SMKT, we examined the interaction of SMKT with membranes using liposomes. Leakage of calcein from calcein-entrapped liposomes was observed in the presence of SMKT. Destruction of liposomes was observed by dark-field microscopy. Comparison of intact S. cerevisiae cells with SMKT-treated cells by dark-field microscopy indicated that the spherical cell membrane is disrupted by SMKT. Using sodium carbonate extraction, we obtained direct evidence for the first time that SMKT is associated with the membrane of sensitive cells. Our results indicate that SMKT kills sensitive S. cerevisiae by interacting with the yeast cell membrane.

Lethal effect of the expression of a killer gene SMK1 in Saccharomyces cerevisiae

Expression of the SMK1 gene which encodes the yeast killer toxin SMKT is lethal in Saccharomyces cerevisiae. Effects of deletion and site-directed mutagenesis of SMK1 on the lethality and the secretion of the gene products were examined. Deletion of the interstitial gamma peptide or the C-terminal loop from Ala208 to the C-terminal Asp222 had no effect on the lethality. Those SMK1 products that lacked either the gamma peptide or the C-terminal loop were expressed in the cells but were not secreted into the culture medium, suggesting that these peptides may have a role in secretion or in protein stability. On the other hand, deletion of the signal sequence resulted in complete loss of the lethal activity. Entering the secretory pathway may be critical for the lethality. Further, deletion of the region from the C-terminus to Leu207 resulted in loss of the lethal activity. Leu207 is located at the C-terminus of the central strand of the beta-sheet structure of SMKT and its side chain is thrust into a hydrophobic environment between the beta-sheet and the alpha-helices. The result obtained upon substitutions of Ala, Ser or Glu for Leu207 suggested that the side chain of Leu207 stabilizes the hydrophobic environment that contributes to the overall structure of the SMK1 product.