Yeast surviving factor Svf1 as a new interacting partner, regulator and in vitro substrate of protein kinase CK2

Yeast Protein Asf1 Possesses Modulating Activity towards Protein Kinase CK2

Protein kinase CK2 plays an important role in cell survival and protects regulatory proteins from caspase-mediated degradation during apoptosis. The consensus sequence of proteins phosphorylated by CK2 contains a cluster of acidic amino acids around the phosphorylation site. The poly-acidic sequence in yeast protein Asf1 is similar to the acidic loop in CK2β, which possesses a regulatory function. We observed that the overexpression of Asf1 in yeast cells influences cell growth. Experiments performed in vitro and in vivo indicate that yeast protein Asf1 inhibits protein kinase CK2. Our data suggest that each CK2 isoform might be regulated in a different way. Deletion of the amino or carboxyl end of Asf1 reveals that the acidic cluster close to the C-terminus is responsible for the activation or inhibition of CK2 activity.

Casein Kinase 2 Mediates Degradation of Transcription Factor Pcf1 during Appressorium Formation in the Rice Blast Fungus

The appressorium is a specialized structure that is differentiated from Magnaporthe oryzae spores that can infect host cells. In the process of cellular transformation from spore to appressorium, the contents inside the spores are transferred into appressoria, accompanied by major differences in the gene expression model. In this study, we reported a transcription factor (TF), Pcf1, which was depressed at the transcription level and degraded at the protein level in nuclei of incipient appressoria at four hpi (hours post inoculation). To investigate its degradation mechanism, the interacting proteins of Pcf1 were identified using an immunoprecipitation-mass spectrometry (IP-MS) assay. Yeast two-hybrid (Y2H) and co-IP (co-immunoprecipitation) assays confirmed that Pcf1 interacted with the casein kinase 2 (CK2) holoenzyme through direct combination with the CKb2 subunit. Moreover, Pcf1 was ubiquitinated in the hyphae. These changes in Pcf1 protein levels in nuclei provide a new clue of how TFs are degraded during appressorium formation: temporarily unnecessary TFs in spores are phosphorylated through interacting with CK2 enzyme and are then ubiquitinated and digested by the ubiquitin proteasome system (UPS).

Survival factor SvfA plays multiple roles in differentiation and is essential for completion of sexual development in Aspergillus nidulans

The first member of the velvet family of proteins, VeA, regulates sexual development and secondary metabolism in the filamentous fungus Aspergillus nidulans. In our study, through comparative proteome analysis using wild type and veA-deletion strains, new putative regulators of sexual development were identified and functionally analyzed. Among these, SvfA, containing a yeast survival factor 1 domain, plays multiple roles in the growth and differentiation of A. nidulans. Deletion of the svfA gene resulted in increased sensitivity to oxidative and cold stress as in yeast. The svfA-deletion strain showed an increase in bi-polar germination and a decrease in radial growth rate. The deletion strain formed structurally abnormal conidiophores and thus produced lower amounts of conidiospores during asexual development. The svfA-deletion strain produced few Hülle cells and small cleistothecia with no ascospores, indicating the requirement of svfA for the completion of sexual development. Transcription and genetic analyses indicated that SvfA modulates the expression of key development regulatory genes. Western blot analysis revealed two forms of SvfA. The larger form showed sexual-specific and VeA-dependent production. Also, the deletion of svfA caused decreased ST (sterigmatocystin) production. We propose that SvfA is a novel central regulator of growth, differentiation and secondary metabolism in A. nidulans.

Interactions between subunits of protein kinase CK2 and their protein substrates influences its sensitivity to specific inhibitors

Five isoforms of CK2 may exist simultaneously in yeast cells: free catalytic subunits CK2α', CK2α and three holoenzymatic structures composed of αα'ββ', α(2)ββ' and α'(2)ββ'. Each isolated and purified form exhibits properties typical for CK2, but they differ in substrate specificity as well as in sensitivity to specific modulators. All five isoforms of protein kinase CK2 from Saccharomyces cerevisiae were examined for their binding capacity with ATP/GTP and two commonly used ATP-competitive inhibitors TBB and TBI. Enzymes were tested with protein substrates differently interacting with CK2 subunits: Elf1, Fip1, Svf1, P2B and synthetic peptide. Obtained results show that K(m) for ATP varies from 2.4-53 μM for Elf1/CK2α' and Svf1/CK2α, respectively. Similar differences can be seen in case when GTP was used as phosphate donor. The inhibitory effect depends on composition of CK2/substrate complexes. Highest sensitivity to TBB shows all complexes containing αα'ββ' isoform with K (i) values between 0.2 and 1.1 μM. The prospect that TBB and TBI could be utilized to discriminate between different molecular forms of CK2 in yeast cells was examined. Both inhibitors, TBB as well as TBI, decreases cell growth to extents devoting interactions with different CK2 isoforms present in the cell but the presence of β/β'-dimer has a high importance towards sensitivity. Conceivably, a given inhibitor concentration can inhibit only selected CK2-mediated processes in the cell.