Extracellular pH and high concentration of potassium regulate the primary necrosis in the yeast Saccharomyces cerevisiae.

Extracellular pH and concentration of K+ as well as their gradient across the plasma membrane have a significant impact on the physiology of the yeast cell, but their role in cell death has not been thoroughly investigated. Here we observed that increasing extracellular pH, as well as supplementing with K+ ions had a mitigating effect on cell death in yeast occurring under several conditions. The first is sugar induced cell death (SICD), and the second is death caused by several specific gene deletions, which have been recently identified in a systematic screen. It was shown that in both cases, primary necrosis is suppressed at neutral pH. SICD was also inhibited by the protonophore dinitrophenol (DNP) and 150 mM extracellular K+, with the latter condition also benefiting survival of cell dying due to gene mutations. In the case of SICD, these effects could not be mitigated by perturbing known pH-dependent signaling pathways, and thus are likely to be realized via direct effects on the plasma membrane potential. Thus, (a)-we show that stabilization of external pH at a neutral level can suppress different types of primary necrosis, and (b)-we suggest that changes to the cellular membrane potential can play a central role in yeast cell death caused by different factors.

Wild type huntingtin toxicity in yeast: Implications for the role of amyloid cross-seeding in polyQ diseases.

Proteins with expanded polyglutamine (polyQ) regions are prone to form amyloids, which can cause diseases in humans and toxicity in yeast. Recently, we showed that in yeast non-toxic amyloids of Q-rich proteins can induce aggregation and toxicity of wild type huntingtin (Htt) with a short non-pathogenic polyglutamine tract. Similarly to mutant Htt with an elongated N-terminal polyQ sequence, toxicity of its wild type counterpart was mediated by induced aggregation of the essential Sup35 protein, which contains a Q-rich region. Notably, polymerization of Sup35 was not caused by the initial benign amyloids and, therefore, aggregates of wild type Htt acted as intermediaries in seeding Sup35 polymerization. This exemplifies a protein polymerization cascade which can generate a network of interdependent polymers. Here we discuss cross-seeded protein polymerization as a possible mechanism underlying known interrelations between different polyQ diseases. We hypothesize that similar mechanisms may enable proteins, which possess expanded Q-rich tracts but are not associated with diseases, to promote the development of polyQ diseases.

Mechanisms of separation of the complementary strands of DNA during replication.

This article is a perspective on the separation of the complementary strands of DNA during replication. Given the challenges of DNA strand separation and its vital importance, it is not surprising that cells have developed many strategies for promoting unlinking. We summarize seven different factors that contribute to strand separation and chromosome segregation. These are: (1) supercoiling promotes unlinking by condensation of DNA; (2) unlinking takes place throughout a replicating domain by the complementary action of topoisomerases on precatenanes and supercoils; (3) topological domains isolate the events near the replication fork and permit the supercoiling-dependent condensation of partially replicated DNA; (4) type-II topoisomerases use ATP to actively unlink DNA past the equilibrium position; (5) the effective DNA concentration in vivo is less than the global DNA concentration; (6) mechanical forces help unlink chromosomes; and (7) site-specific recombination promotes unlinking at the termination of replication by resolving circular dimeric chromosomes.