The molecular identity of the characean OH− transporter: a candidate related to the SLC4 family of animal pH regulators

Abstract

Characeae are closely related to the ancient algal ancestors of all land plants. The long characean cells display a pH banding pattern to facilitate inorganic carbon import in the acid zones for photosynthetic efficiency. The excess OH−, generated in the cytoplasm after CO2 is taken into the chloroplasts, is disposed of in the alkaline band. To identify the transporter responsible, we searched the Chara australis transcriptome for homologues of mouse Slc4a11, which functions as OH−/H+ transporter. We found a single Slc4-like sequence CL5060.2 (named CaSLOT). When CaSLOT was expressed in Xenopus oocytes, an increase in membrane conductance and hyperpolarization of resting potential difference (PD) was observed with external pH increase to 9.5. These features recall the behavior of Slc4a11 in oocytes and are consistent with the action of a pH-dependent OH−/H+ conductance. The large scatter in the data might reflect intrinsic variability of CaSLOT transporter activation, inefficient expression in the oocyte due to evolutionary distance between ancient algae and frogs, or absence of putative activating factor present in Chara cytoplasm. CaSLOT homologues were found in chlorophyte and charophyte algae, but surprisingly not in related charophytes Zygnematophyceae or Coleochaetophyceae.

The cell biology of charophytes: Exploring the past and models for the future

“…The internodal cells acidify their surface either directly or via carbonic anhydrase to help facilitate the conversion of to membrane-permeable CO 2 for increasing photosynthetic efficiency ( Eremin et al, 2019 ). In order to maintain pH homeostasis of the cytoplasm, the proton efflux is balanced by either influx of H + or efflux of OH − ( Quade et al, 2022 ). The resulting pattern of alternating acid and alkaline regions or “bands” helps maintain metabolism ( Beilby and Bisson, 2012 ).…”

The H+ Transporter SLC4A11: Roles in Metabolism, Oxidative Stress and Mitochondrial Uncoupling

“…As confirmed in two other studies, SLC4A11 does not transport borate [ 11 , 16 ]. In addition, there is an inconsistent demonstration of Na + -dependent and -independent electrogenic H + flux by SLC4A11 in corneal endothelial cells [ 10 ] and in SLC4A11 transfected cell lines [ 3 , 10 , 12 , 16 , 53 , 54 , 55 ]. Consistent with the inward H + flux from SLC4A11, mouse corneal endothelial cell lines from the SLC4A11 KO have a significantly higher pHi i than WT cells when perfused in a low buffering capacity bicarbonate-free ringer.…”

“…Myers et al [ 54 ], also using transfected oocytes, showed enhanced Na + -independent H + conduction, but concluded that SLC4A11 was activated by alkaline pH. Moreover, the H + conductance of SLC4A11 was found to be steepest at pH 8.5 and it was found that mutants can shift the optimum pKa [ 55 ]. In summary, from these studies, it appears that SLC4A11 provides H + conductance that can be either Na + dependent or independent, is sensitive to NH 4 Cl, and is stimulated by alkaline pH.…”

Chara canescenspart I: Oospore differentiation of parthenogenic and dioecious strains and salt-dependent oospore sizes

“…These macroscopic multicellular complex algae inhabit freshwater, as well as brackish and saline environments worldwide (Khan 1991). The relevance of Charophyceae for evolutionary and applied research areas, such as ecophysiology, paleoecology, ontogenesis, molecular and comparative genomics (Beilby 2019, Vicente et al 2019, Holzhausen et al 2022b, Quade et al 2022) peaked in the establishment of the freshwater alga Chara braunii C.C. Gmel.…”