Saccharomyces cerevisiae cell wall chitin, the Kluyveromyces lactis zymocin receptor

KTI11 and KTI13, Saccharomyces cerevisiae genes controlling sensitivity to G1 arrest induced by Kluyveromyces lactis zymocin

The Kluyveromyces lactis zymocin and its gamma-toxin subunit inhibit cell cycle progression of Saccharomyces cerevisiae. To identify S. cerevisiae genes conferring zymocin sensitivity, we complemented the unclassified zymocin-resistant kti11 and kti13 mutations using a single-copy yeast library. Thus, we identified yeast open reading frames (ORFs) YBL071w-A and YAL020c/ATS1 as KTI11 and KTI13 respectively. Disruption of KTI11 and KTI13 results in the complex tot phenotype observed for the gamma-toxin target site mutants, tot1-7, and includes zymocin resistance, thermosensitivity, hypersensitivity to drugs and slow growth. Both loci, KTI11 and KTI13, are actively transcribed protein-encoding genes as determined by reverse transcriptase-polymerase chain reaction (RT-PCR) and in vivo HA epitope tagging. Kti11p is highly conserved from yeast to man, and Kti13p/Ats1p is related to yeast Prp20p and mammalian RCC1, components of the Ran-GTP/GDP cycle. Combining disruptions in KTI11 or KTI13 with a deletion in TOT3/ELP3 coding for the RNA polymerase II (RNAPII) Elongator histone acetyltransferase (HAT) yielded synthetic effects on slow growth phenotype expression. This suggests genetic interaction and possibly links KTI11 and KTI13 to Elongator function.

Genome organization of the linear Pichia etchellsii plasmid pPE1A: evidence for expression of an extracellular chitin-binding protein homologous to the alpha-subunit of the Kluyveromyces lactis killer toxin

Pichia etchellsii CBS2011 (synonym Debaryomyces etchellsii) is a non-killer yeast harbouring two cryptic linear cytoplasmic DNA-elements, pPE1A (6.7 kb) and pPE1B (12.8 kb). Cloning and complete sequencing of pPE1A revealed a 6749-bp element with a remarkably high A+T content of 77.6%. The termini of pPE1A were found to consist of inversely orientated identical nucleotide repetitions of 178bp, to which proteins are linked at the 5'-ends. It is only the second small, non-autonomous cytoplasmic yeast linear plasmid for which the complete nucleotide sequence is known. Five open reading frames (ORFs) were identified preceded by upstream conserved sequence motifs (UCS) characteristic for cytoplasmic promoters and perfectly matching the UCS consensus (ATNTGA). As none of the putative genes encodes a DNA-polymerase, pPE1A is the first yeast linear plasmid known that does not possess its own element-specific replication machinery. No function could be attributed to ORF1, 3, 4, and 5; the predicted ORF2 gene product is similar to chitin-binding proteins and chitinases, highest homologies were found to the precursor of the alpha- and beta-subunits of the secreted Kluyveromyces lactis zymocin. Consistently, the Orf2p could be isolated from the culture fluid by chitin-Sepharose affinity chromatography and characterized by immuno-probing with an antibody specific for the K. lactis killer toxin alpha-subunit. Production of the protein was found to be plasmid-dependent. The sequence of pPE1A has been submitted to the EMBL data library, Accession No. AJ409097.

Molecular analysis of KTI12/TOT4, a Saccharomyces cerevisiae gene required for Kluyveromyces lactis zymocin action

TOT, the putative Kluyveromyces lactis zymocin target complex from Saccharomyces cerevisiae, is encoded by TOT1-7, six loci of which are isoallelic to RNA polymerase II (RNAPII) Elongator genes (ELP1-6). Unlike TOT1-3 (ELP1-3) and TOT5-7 (ELP5, ELP6 and ELP4 respectively), which display zymocin resistance when deleted, TOT4 (KTI12) also renders cells refractory to zymocin when maintained in multicopy or overexpressed from the GAL10 promoter. Elevated TOT4 copy number results in an intermediate tot phenotype, which includes mild sensitivities towards caffeine, Calcofluor white and elevated growth temperature, suggesting that TOT4 influences TOT/Elongator function. Tot4p interacts with Elongator, as shown by co-immunoprecipitation, and cell fractionation studies demonstrate partial co-migration with RNAPII and Elongator. As Elongator subunit interaction is not affected by either deletion of TOT4 or multicopy TOT4, Tot4p may not be a structural Elongator subunit but, rather, may regulate TOT/Elongator in a fashion that requires transient physical contact with TOT/Elongator. Consistent with a regulatory role, the presence of a potential P-loop motif conserved between yeast and human TOT4 homologues suggests capability of ATP or GTP binding and P-loop deletion renders Tot4p biologically inactive.

Sit4p protein phosphatase is required for sensitivity of Saccharomyces cerevisiae to Kluyveromyces lactis zymocin

We have identified two Saccharomyces cerevisiae genes that, in high copy, confer resistance to Kluyveromyces lactis zymocin, an inhibitor that blocks cells in the G(1) phase of the cell cycle prior to budding and DNA replication. One gene (GRX3) encodes a glutaredoxin and is likely to act at the level of zymocin entry into sensitive cells, while the other encodes Sap155p, one of a family of four related proteins that function positively and interdependently with the Sit4p protein phosphatase. Increased SAP155 dosage protects cells by influencing the sensitivity of the intracellular target and is unique among the four SAP genes in conferring zymocin resistance in high copy, but is antagonized by high-copy SAP185 or SAP190. Since cells lacking SIT4 or deleted for both SAP185 and SAP190 are also zymocin resistant, our data support a model whereby high-copy SAP155 promotes resistance by competition with the endogenous levels of SAP185 and SAP190 expression. Zymocin sensitivity therefore requires a Sap185p/Sap190p-dependent function of Sit4p protein phosphatase. Mutations affecting the RNA polymerase II Elongator complex also confer K. lactis zymocin resistance. Since sit4Delta and SAP-deficient strains share in common several other phenotypes associated with Elongator mutants, Elongator function may be a Sit4p-dependent process.

Kluyveromyces lactis zymocin mode of action is linked to RNA polymerase II function via Elongator

The putative Kluyveromyces lactis zymocin target complex, TOT, from Saccharomyces cerevisiae comprises five Tot proteins, four of which are RNA polymerase II (RNAP II) Elongator subunits. Recently, two more Elongator subunit genes, ELP6 (TOT6) and ELP4 (TOT7), have been identified. Deletions of both TOT6 and TOT7 result in the complex tot phenotype, including resistance to zymocin, thermosensitivity, slow growth and hypersensitivity towards drugs, thus reinforcing the notion that TOT/Elongator may be crucial in signalling zymocicity. Mutagenesis of ELP3/TOT3, the Elongator histone acetyltransferase (HAT) gene, revealed that zymocin sensitivity could be uncoupled from Elongator wild-type function, indicating that TOT interacts genetically with zymocin. To test the possibility that zymocin functions by affecting RNAP II activity in a TOT/Elongator-dependent manner, global poly(A)+ mRNA levels were found to decline drastically on zymocin treatment. Moreover, cells overexpressing Fcp1p, the RNAP II carboxy-terminal domain phosphatase, acquired partial zymocin resistance, whereas cells underproducing RNAP II became zymocin hypersensitive. This suggests that zymocin may convert TOT/Elongator into a cellular poison toxic for RNAP II function and eventually leading to the observed G1 cell cycle arrest.