Functional regulation of the SLC26-family protein prestin by calcium/calmodulin

The solute carrier gene family 26 (SLC26) encodes membrane proteins with diverse physiological roles but with the common feature of halide involvement. Here, we present bioinformatic and biochemical evidence that SLC26 proteins have intrinsically disordered regions (IDRs) in their C-terminal domains and that these regions contain calmodulin (CaM) binding sites. The veracity of these predictions and the functional consequences of CaM binding were examined in prestin, SLC26A5, as a model for the SLC26 family and as one of the most investigated and best understood members. We found that CaM binds directly to the IDR in the C-terminal domain of prestin in a calcium-obligate manner. Using both isolated murine outer hair cells (OHCs) and a heterologous expression system, we also found that this calcium-obligate CaM binding shifts the operating point of the protein to more hyperpolarized potentials with consequent alteration of the function of the prestin. Because calcium is the main intracellular second messenger used by the efferent medial olivocochlear (MOC) pathway of the auditory system and CaM is abundant in OHCs, the CaM-prestin interaction may be involved in the MOC-mediated modulation of cochlear amplification. However, this regulatory mechanism is not likely to be restricted to cochlear OHCs, in light of both clear bioinformatic evidence and the fact that calcium and CaM are ubiquitous intracellular second messengers used by virtually all cell types. Hence, the calcium/CaM-dependent regulatory mechanism described herein is likely applicable to most, if not all, SLC26 paralogs.