Functional basis for calmodulation of the TRPV5 calcium channel.

Within the transient receptor potential (TRP) superfamily of ion channels, TRPV5 is a highly Ca 2+ -selective channel important for active reabsorption of Ca 2+ in the kidney. Its channel activity is controlled by a negative feedback mechanism involving calmodulin (CaM) binding. Combining advanced microscopy techniques and biochemical assays, this study characterized the dynamic lobe-specific CaM regulation. We demonstrate for the first time that functional (full-length) TRPV5 interacts with CaM in the absence of Ca 2+ , and this interaction is intensified at increasing Ca 2+ concentrations sensed by the CaM C-lobe that achieves channel pore blocking. Channel inactivation occurs without requiring CaM N-lobe calcification. Moreover, we show a Ca 2+ -dependent binding stoichiometry at the single channel level. In conclusion, our study proposes a new model for CaM-dependent regulation - calmodulation - of this uniquely Ca 2+ -selective TRP channel TRPV5 that involves apoCaM interaction and lobe-specific actions, which may be of significant physiological relevance given its role as gatekeeper of Ca 2+ transport in the kidney. KEY POINTS: The renal Ca 2+ channel TRPV5 is an important player in maintenance of the body's Ca 2+ homeostasis. Activity of TRPV5 is controlled by a negative feedback loop that involves calmodulin (CaM), a protein with two Ca 2+ -binding lobes. We investigated the dynamics of the interaction between TRPV5 and CaM with advanced fluorescence microscopy techniques. Our data support a new model for CaM-dependent regulation of TRPV5 channel activity with CaM lobe-specific actions and demonstrates Ca 2+ -dependent binding stoichiometries. This study improves our understanding of the mechanism underlying fast channel inactivation, which is physiologically relevant given the gatekeeper function of TRPV5 in Ca 2+ reabsorption in the kidney.