Chaperone Stress 70 Protein (STCH) Binds and Regulates Two Acid/Base Transporters NBCe1-B and NHE1 *

Regulation of intracellular pH is critical for the maintenance of cell homeostasis in response to stress. We used yeast two-hybrid screening to identify novel interacting partners of the pH-regulating transporter NBCe1-B. We identified Hsp70-like stress 70 protein chaperone (STCH) as interacting with NBCe1-B at the N-terminal (amino acids 96-440) region. Co-injection of STCH and NBCe1-B cRNA into Xenopus oocytes significantly increased surface expression of NBCe1-B and enhanced bicarbonate conductance compared with NBCe1-B cRNA alone. STCH siRNA decreased the rate of Na+-dependent pH(i) recovery from NH4+ pulse-induced acidification in an HSG (human submandibular gland ductal) cell line. We observed that in addition to NBCe1-B, Na+/H+ exchanger (NHE)-dependent pHi recovery was also impaired by STCH siRNA and further confirmed the interaction of STCH with NHE1 but not plasma membrane Ca2+ ATPase. Both NBCe1-B and NHE1 interactions were dependent on a specific 45-amino acid region of STCH. In conclusion, we identify a novel role of STCH in the regulation of pHi through site-specific interactions with NBCe1-B and NHE1 and subsequent modulation of membrane transporter expression. We propose STCH may play a role in pHi regulation at times of cellular stress by enhancing the recovery from intracellular acidification.