Spetea, C. | Search bioRxiv

bioRxiv2022

Stable Potassium Isotopes (41K/39K) Track Transcellular and Paracellular Potassium Transport in Biological Systems

As the most abundant cation in archaeal, bacterial, and eukaryotic cells, potassium (K+) is an essential element for life. While much is known about the machinery of transcellular and paracellular K transport - channels, pumps, co-transporters, and tight-junction proteins - many quantitative aspects of K homeostasis in biological systems remain poorly constrained. Here we present measurements of the stable isotope ratios of potassium (41K/39K) in three biological systems (algae, fish, and mammals). When considered in the context of our current understanding of potential mechanisms of K isotope fractionation and K+ transport in these biological systems, our results provide evidence that the fractionation of K isotopes depends on transport pathway and transmembrane transport machinery. Specifically, we find that passive transport of K+ down its electrochemical potential through channels and pores in tight-junctions at favors 39K, a result which we attribute to a kinetic isotope effect associated with dehydration and/or size selectivity at the channel/pore entrance. In contrast, we find that transport of K+ against its electrochemical gradient via pumps and co-transporters is associated with less/no isotopic fractionation, a result that we attribute to small equilibrium isotope effects that are expressed in pumps/co-transporters due to their slower turnover rate and the relatively long residence time of K+ in the ion pocket. These results indicate that stable K isotopes may be able to provide quantitative constraints on transporter-specific K+ fluxes (e.g. the fraction of K efflux from a tissue by channels vs. co-transporters) and how these fluxes change under different physiological states. In addition, precise determination of K isotope effects associated with K+ transport through channels, pumps, and co-transporters may provide unique constraints on the mechanisms of K transport that could be tested with steered molecular dynamic simulations.

physiology↗

bioRxiv2020

TIP family aquaporins play role in chloroplast osmoregulation and photosynthesis

Photosynthetic oxygen evolution by photosystem II requires water supply into the chloroplast to reach the thylakoid lumen. A rapid water flow is also required into the chloroplast for optimal oxygen evolution and to overcome osmotic stress. The mechanisms governing water transport in chloroplasts are largely unexplored. Previous proteomics indicated the presence of three aquaporins from the tonoplast intrinsic protein (TIP) family, TIP1;1, TIP1;2 and TIP2;1, in chloroplast membranes of Arabidopsis thaliana. Here we revisited their location and studied their role in chloroplasts. Localization experiments indicated that TIP2;1 resides in the thylakoid, whereas TIP1;2 is present in both thylakoid and envelope membranes. Mutants lacking TIP1;2 and/or TIP2;1 did not display a macroscopic phenotype when grown under standard conditions. The mutant chloroplasts and thylakoids underwent less volume changes than the corresponding wild type preparations upon osmotic treatment and in the light. Significantly reduced rates of photosynthetic electron transport were obtained in the mutant leaves, with implications on the CO2 fixation rates. However, electron transport rates did not significantly differ between mutants and wild type when isolated thylakoids were examined. Less acidification of the thylakoid lumen was measured in mutants thylakoids, resulting in a slower induction of delta pH-dependent photoprotective mechanisms. These results identify TIP1;2 and TIP2;1 as chloroplast proteins and highlight their importance for osmoregulation and optimal photosynthesis. A third aquaporin, TIP1;1, is present in the chloroplast envelope, and may play role in photosynthesis under excessive light conditions, as based on the weak photosynthetic phenotype of its mutant.

plant biology↗