Pre-fibrillar alpha-synuclein variants with impaired beta-structure increase neurotoxicity in Parkinson's disease models.

The relation of alpha-synuclein (alphaS) aggregation to Parkinson's disease (PD) has long been recognized, but the mechanism of toxicity, the pathogenic species and its molecular properties are yet to be identified. To obtain insight into the function different aggregated alphaS species have in neurotoxicity in vivo, we generated alphaS variants by a structure-based rational design. Biophysical analysis revealed that the alphaS mutants have a reduced fibrillization propensity, but form increased amounts of soluble oligomers. To assess their biological response in vivo, we studied the effects of the biophysically defined pre-fibrillar alphaS mutants after expression in tissue culture cells, in mammalian neurons and in PD model organisms, such as Caenorhabditis elegans and Drosophila melanogaster. The results show a striking correlation between alphaS aggregates with impaired beta-structure, neuronal toxicity and behavioural defects, and they establish a tight link between the biophysical properties of multimeric alphaS species and their in vivo function.

The yeast HtrA orthologue Ynm3 is a protease with chaperone activity that aids survival under heat stress.

Ynm3 is the only budding yeast protein possessing a combination of serine protease and postsynaptic density 95/disc-large/zona occludens domains, a defining feature of the high temperature requirement A (HtrA) protein family. The bacterial HtrA/DegP is involved in protective stress response to aid survival at higher temperatures. The role of mammalian mitochondrial HtrA2/Omi in protein quality control is unclear, although loss of its protease activity results in susceptibility toward Parkinson's disease, in which mitochondrial dysfunction and impairment of protein folding and degradation are key pathogenetic features. We studied the role of the budding yeast HtrA, Ynm3, with respect to unfolding stresses. Similar to Escherichia coli DegP, we find that Ynm3 is a dual chaperone-protease. Its proteolytic activity is crucial for cell survival at higher temperature. Ynm3 also exhibits strong general chaperone activity, a novel finding for a eukaryotic HtrA member. We propose that the chaperone activity of Ynm3 may be important to improve the efficiency of proteolysis of aberrant proteins by averting the formation of nonproductive toxic aggregates and presenting them in a soluble state to its protease domain. Suppression studies with Deltaynm3 led to the discovery of chaperone activity in a nucleolar peptidyl-prolyl cis-trans isomerase, Fpr3, which could partly relieve the heat sensitivity of Deltaynm3.

Differential Flo8p-dependent regulation of FLO1 and FLO11 for cell-cell and cell-substrate adherence of S. cerevisiae S288c.

Cell-cell and cell-surface adherence represents initial steps in forming multicellular aggregates or in establishing cell-surface interactions. The commonly used Saccharomyces cerevisiae laboratory strain S288c carries a flo8 mutation, and is only able to express the flocculin-encoding genes FLO1 and FLO11, when FLO8 is restored. We show here that the two flocculin genes exhibit differences in regulation to execute distinct functions under various environmental conditions. In contrast to the laboratory strain Sigma1278b, haploids of the S288c genetic background require FLO1 for cell-cell and cell-substrate adhesion, whereas FLO11 is required for pseudohyphae formation of diploids. In contrast to FLO11, FLO1 repression requires the Sin4p mediator tail component, but is independent of the repressor Sfl1p. FLO1 regulation also differs from FLO11, because it requires neither the KSS1 MAP kinase cascade nor the pathways which lead to the transcription factors Gcn4p or Msn1p. The protein kinase A pathway and the transcription factors Flo8p and Mss11p are the major regulators for FLO1 expression. Therefore, S. cerevisiae is prepared to simultaneously express two genes of its otherwise silenced FLO reservoir resulting in an appropriate cellular surface for different environments.

The yeast elongator histone acetylase requires Sit4-dependent dephosphorylation for toxin-target capacity.

Kluyveromyces lactis zymocin, a heterotrimeric toxin complex, imposes a G1 cell cycle block on Saccharomyces cerevisiae that requires the toxin-target (TOT) function of holo-Elongator, a six-subunit histone acetylase. Here, we demonstrate that Elongator is a phospho-complex. Phosphorylation of its largest subunit Tot1 (Elp1) is supported by Kti11, an Elongator-interactor essential for zymocin action. Tot1 dephosphorylation depends on the Sit4 phosphatase and its associators Sap185 and Sap190. Zymocin-resistant cells lacking or overproducing Elongator-associator Tot4 (Kti12), respectively, abolish or intensify Tot1 phosphorylation. Excess Sit4.Sap190 antagonizes the latter scenario to reinstate zymocin sensitivity in multicopy TOT4 cells, suggesting physical competition between Sit4 and Tot4. Consistently, Sit4 and Tot4 mutually oppose Tot1 de-/phosphorylation, which is dispensable for integrity of holo-Elongator but crucial for the TOT-dependent G1 block by zymocin. Moreover, Sit4, Tot4, and Tot1 cofractionate, Sit4 is nucleocytoplasmically localized, and sit4Delta-nuclei retain Tot4. Together with the findings that sit4Delta and totDelta cells phenocopy protection against zymocin and the ceramide-induced G1 block, Sit4 is functionally linked to Elongator in cell cycle events targetable by antizymotics.

Elongator's toxin-target (TOT) function is nuclear localization sequence dependent and suppressed by post-translational modification.

The toxin target (TOT) function of the Saccharomyces cerevisiae Elongator complex enables Kluyveromyces lactis zymocin to induce a G1 cell cycle arrest. Loss of a ubiquitin-related system (URM1-UBA4 ) and KTI11 enhances post-translational modification/proteolysis of Elongator subunit Tot1p (Elp1p) and abrogates its TOT function. Using TAP tagging, Kti11p contacts Elongator and translational proteins (Rps7Ap, Rps19Ap Eft2p, Yil103wp, Dph2p). Loss of YIL103w and DPH2 (involved in diphtheria toxicity) suppresses zymocicity implying that both toxins overlap in a manner mediated by Kti11p. Among the pool that co-fractionates with RNA polymerase II (pol II) and nucleolin, Nop1p, unmodified Tot1p dominates. Thus, modification/proteolysis may affect association of Elongator with pol II or its localization. Consistently, an Elongator-nuclear localization sequence (NLS) targets green fluorescent protein (GFP) to the nucleus, and its truncation yields TOT deficiency. Similarly, KAP120 deletion rescues cells from zymocin, suggesting that Elongator's TOT function requires NLS- and karyopherin-dependent nuclear import.

Subunit communications crucial for the functional integrity of the yeast RNA polymerase II elongator (gamma-toxin target (TOT)) complex.

In response to the Kluyveromyces lactis zymocin, the gamma-toxin target (TOT) function of the Saccharomyces cerevisiae RNA polymerase II (pol II) Elongator complex prevents sensitive strains from cell cycle progression. Studying Elongator subunit communications, Tot1p (Elp1p), the yeast homologue of human IKK-associated protein, was found to be essentially involved in maintaining the structural integrity of Elongator. Thus, the ability of Tot2p (Elp2p) to interact with the HAT subunit Tot3p (Elp3p) of Elongator and with subunit Tot5p (Elp5p) is dependent on Tot1p (Elp1p). Also, the association of core-Elongator (Tot1-3p/Elp1-3p) with HAP (Elp4-6p/Tot5-7p), the second three-subunit subcomplex of Elongator, was found to be sensitive to loss of TOT1 (ELP1) gene function. Structural integrity of the HAP complex itself requires the ELP4/TOT7, ELP5/TOT5, and ELP6/TOT6 genes, and elp6Delta/tot6Delta as well as elp4Delta/tot7Delta cells can no longer promote interaction between Tot5p (Elp5p) and Tot2p (Elp2p). The association between Elongator and Tot4p (Kti12p), a factor that may modulate the TOT activity of Elongator, requires Tot1-3p (Elp1-3p) and Tot5p (Elp5p), indicating that this contact requires a preassembled holo-Elongator complex. Tot4p also binds pol II hyperphosphorylated at its C-terminal domain Ser(5) raising the possibility that Tot4p bridges the contact between Elongator and pol II.

Protein interactions within Saccharomyces cerevisiae Elongator, a complex essential for Kluyveromyces lactis zymocicity.

mTn3-tagging identified Kluyveromyces lactis zymocin target genes from Saccharomyces cerevisiae as TOT1-3/ELP1-3 coding for the RNA polymerase II (pol II) Elongator histone acetyltransferase (HAT) complex. tot phenotypes resulting from mTn3 tagging were similar to totDelta null alleles, suggesting loss of Elongator's integrity. Consistently, the Tot1-3/Elp1-3 proteins expressed from the mTn3-tagged genes were all predicted to be C-terminally truncated, lacking approximately 80% of Tot1p, five WD40 Tot2p repeats and two HAT motifs of Tot3p. Besides its role as a HAT, Tot3p assists subunit communication within Elongator by mediating Tot2-Tot4, Tot2-Tot5, Tot2-Tot1 and Tot4-Tot5 protein-protein interactions. TOT1 and TOT2 are essential for Tot4-Tot2 and Tot4-Tot3 interactions respectively. The latter was lost with a C-terminal Tot2p truncation; the former was affected by progressively truncating TOT1. Despite being dispensable for Tot4-Tot2 interaction, the extreme C-terminus of Tot1p may play a role in TOT/Elongator function, as its truncation confers zymocin resistance. Tot4p/Kti12p, an Elongator-associated factor, also interacted with pol II and could be immunoprecipitated while being bound to the ADH1 promoter. Two-hybrid analysis showed that Tot4p also interacts with Cdc19p, suggesting that Tot4p plays an additional role in concert with Cdc19p, perhaps co-ordinating cell growth with carbon source metabolism.

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.

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.