The role of TMS1 in S. cerevisiae

The family of serine incorporators (SERINC) is present in all eukaryotes and contains one to five members predicted to have ten transmembrane domains. SERINC5, and to a lesser extent SERINC3, was reported as a novel HIV restriction factor counteracted by the lentiviral accessory protein Nef. SERINC5 is not subjected to positive selection and is well conserved throughout evolution suggesting an important physiological function in cells, which remains to be elucidated. Remarkably, looking at the evolution timeline, we can find a serinc ortholog even in simpler organisms such as C. Elegans, D. Melanogaster, and S. Cerevisiae. The yeast SERINC ortholog is known as TMS1 and is reported to localize on the vacuole membrane. The cellular function of TMS1 is currently unknown, but the protein topology is evolutionary conserved. In addition, TMS1 shares 28 % amino acid identity with human SERINC5. Here we show that the yeast TMS1 is not able to hamper HIV-1 infectivity, suggesting that the antiviral activity is associated with a primordial feature of this protein that originally evolved for another function. The SERINCs were proposed to have a role in serine-derived lipid biosynthesis and metabolism. However, recent evidence showed that SERINC ablation does not alter the composition of the cellular lipidome. Accordingly, our analysis done in collaboration with the Biological Research Centre in HungaryA shows that serine-containing lipids were not affected in an S. cerevisiae strain lacking TMS1. Nevertheless, we observed an increase in the percentage of phosphatidic acid and a decrease of phosphatidylinositol in the ΔTMS1 strain, but the biological meaning of these alterations remains unclear. We confimed that TMS1 localizes on the vacuole membrane. Interestingly, we observed that it follows a particular punctate pattern that suggests an involvement of the protein in vacuole dynamics. We also found that TMS1 is dispensable for the growth in culture and for the resistance to several environmental stress. However, the absence of TMS1 sensitizes the yeast to some of those conditions, among which are SDS, hydrogen peroxide, Hygromycin, and acetic acid treatment. Acetic acid is a by-product of alcoholic fermentation. Under physiological conditions, the acid is not toxic to S. cerevisiae but can be metabolized to produce energy. However, at low extracellular pH and in the presence of glucose, acetic acid becomes toxic. In this condition, the protonated form of acetic acid can passively diffuse through the plasma membrane and, once inside the cell dissociates generating protons and anions. The accumulation of these molecules leads to intracellular acidification, inhibition of metabolism, increase in oxidative stress, and eventually, cell death. From a yeast-two-hybrid analysis, we found that TMS1 interacts with proteins involved in the acetic acid stress response. Though the contribution of TMS1 to acetic acid stress adaptation remains to be elucidated, for the first time we identified a condition in which TMS1 is essential. This is a fundamental starting point for future investigation on TMS1 biological function.