Cooperativity and Flexible Domains Participation in PIP Aquaporin Gating

Abstract

From structural data two distinct mechanisms have been proposed to describe how aquaporins are gated (Frick et al. FEBSLett. 2013). Gating of some aquaporins -AQP0 or AQPZ- is mediated by the displacement of a single residue inside the pore (pinching), while for others -e.g. PIP- the blocking of the pore occurs as the result of loopD rearrangement (capping). Regarding PIP capping mechanism, it has been suggested that the open-closed transition not only involves loopD but also N and C-terminal movements. The PIP subfamily consists of two paralogs, PIP1 and PIP2, which can form heterotetramers. Heterotetramerization occurs with random stoichiometry and all formed heterotetramers have different water transport capability and pH0,5 than PIP2 homotetramers (Jozefkowicz et al. Biophys J. 2016). Since there is no structural information available for PIP1, many of the molecular events that explain changes in the pH sensing in heterotetramers are still unknown. We investigate the cooperative gating of homo and heterotetramers. In particular, we study the participation of flexible domains in this process. Our results show that all homo and heterotetrameric species share the same degree of cooperativity for proton sensing. However the cooperativity is apparently lower when PIP1-PIP2 heterotetramers and PIP2 homotetramers are both present in the membrane due to their different pH0,5. In addition, we detected loopD participation in PIP2 gating is more efficient than C-term, since mutation of Leu202 keep the channel open even at low pH, while truncation of C-term do not modify pH sensing. The elucidation of the peculiarities of PIP gating sheds light on the regulatory mechanisms that control water transport trough membranes and extends the knowledge about the participation of flexible domains in the control of the biological activity of oligomeric channels.