Phosphorylation of the electrogenic sodium/bicarbonate co‐transporter 1 (NBCe1) at Ser²⁴⁵ and Ser^255‐257is linked to mTOR‐mediated transporter activity

Abstract

The electrogenic sodium bicarbonate co‐transporter 1 (NBCe1, Slc4a4) is a key pH‐regulating membrane protein and is widely expressed in kidney, pancreas, cornea, heart and brain. Phosphorylation in several residues can affect the surface expression and the activity of the transporter. NBCe1 has been found to be phosphorylated in several residues among them in Ser65 where SPAK inhibits its activity and in Thr49required for regulation of NBCe1 by IRBIT and SPAK. Moreover, IRBIT regulates the activity of NBCe1 by controlling the phosphorylation status of Ser232, Ser233 and Ser235. The aim of this study is to identify novel phosphorylation sites and examine their putative contribution in the transport activity of NBCe1. Phosphoproteomic analysis in mouse cortical astrocytes revealed constitutively phosphorylated NBCe1 at Ser245 and Ser255‐257. Mutational analysis in Hela cells and intracellular H+ recordings using pH‐sensitive fluorescent dye (BCECF) showed that the activity of NBCe1 is regulated by the phosphorylation of these residues, phospho‐Ser245 significantly reduced the activity of NBCe1 whereas phospho‐Ser255‐257led to increased NBCe1 activity. Additionally, in the presence of the mTOR inhibitor rapamycin or the mTOR activator 3BDO our data demonstrated that mTOR signaling pathway regulates the phosphorylation of the Ser245 and Ser255‐257. Our findings reveal phosphorylation at Ser245 and Ser255‐257as novel regulators of NBCe1 transport activity. We suggest that mTOR pathway is capable to fine tune NBCe1 activity, an event with putative implications during pathophysiological conditions.