Characterization of cooperative phenomena in pH sensing of aquaporins

FLORENCIA GIORGETTO; GABRIELA SOTO; FLORENCIA SCOCHERA; CINTIA JOZEFKOWICZ; KARINA ALLEVA; VITALI, VICTORIA; AGUSTINA CANESSA FORTUNA; F. LUIS GONZÁLEZ FLECHA

La Plata

Congreso; XLVII Reunión Anual de la Sociedad Argentina de Biofísica; 2018

Sociedad Argentina de Biofísica

Cooperative regulation of protein function is a phenomenon of central importancein many cellular processes. PIP aquaporins are tetrameric pH dependent channelsand their dose-response profile of osmotic water permeability coefficient (Pf) vsintracellular proton concentration shows a sigmoidal shape, suggesting acooperative phenomenon. We consider two time-separated stages to analyze theimpact of proton concentration in water transport by PIP channels: a protonbinding event, and an open-closed conformational transition. In this regard, weproposed two models where the cooperative character was alternatively attributedto one of the two involved stages. By means of this model we studied the pHresponse of PIP homotetramers, PIP heterotetramers (where two differentparalogues PIP2 and PIP1 are assembled), and mutants PIP, expressed in theheterologous systems of Xenopus oocyte. Our results show that the cooperativityin PIP biological response cannot be directly attributed to a cooperative protonbinding as it is usually considered, since it could also be the consequence of acooperative conformation transition between open and closed channel states.Moreover, the decrease in the degree of positive cooperativity found when mixedpopulations of homo and heterotetrameric PIP channels are expressed in theplasma membrane of the same cell, would be the result of the simultaneouspresence of molecular species with different levels of proton sensing. Finally, thesemodels are also useful tools to estimate the conformational impact of mutationsthat alter PIP functioning. Despite the main goal of this approach is to offer a deepunderstanding on cooperative transport in PIP channels, we are also committed tostress that phenomenological modeling has an important epistemic value which isnot in opposition with mechanistic explanations.