Structural Studies of a Mammalian Epithelial Calcium Channel

Calcium plays an essential role in the physiology and biochemistry of many biological functions, including excitation-contraction coupling, neuronal signaling, and fertilization. In mammals, the calcium content in various tissues, organs, and cell types is tightly regulated to maintain homeostasis. A chief process controlling calcium levels is absorption of the ion from the lumen by epithelial cells that line organs including the intestines and kidney. Calcium entry at the apical membrane constitutes the first step of epithelial calcium absorption. Two highly calcium-selective transient receptor potential vanilloid (TRPV) channels, TRPV5 and TRPV6, are the pore-forming subunits responsible for epithelial calcium entry in kidney and intestine, respectively. Genetic knockout of TRPV5 or TRPV6 in animals leads to phenotypes related to defective calcium homeostasis, including lowered serum calcium levels, decreased calcium absorption, reduced bone density, impaired sperm motility, and decreased maternal-fetal calcium transfer. In humans, aberrant TRPV5/6 expression is associated with preeclampsia and calcium nephrolithiasis (kidney stones). Additionally, TRPV6 expression level is upregulated in carcinomas of prostate, colon, breast, thyroid, and ovary, suggesting a role for TRPV6 in cancer survival. A detailed understanding of epithelial calcium entry is hindered by a lack of high-resolution structural information on intact channels. This dissertation presents structural analyses of the epithelial calcium channel TRPV6. We applied modern membrane protein screening and expression techniques, including fluorescence-detection size exclusion chromatography (FSEC) and baculovirus mediated mammalian cell transduction (BacMam), to identify optimal TRPV6 constructs and purification schemes for crystallization. Using a surface mutagenesis approach guided by lower-resolution structural solutions, we engineered a rat TRPV6 mutant (TRPV6cryst) that permitted solving a 3.25 Å resolution crystal structure. We used fluorescent calcium indicator assays to show that TRPV6cryst retains the permeation and ionic block properties of the wild type channel. The tetrameric structure of TRPV6cryst reveals a transmembrane domain architecture similar to voltage gated ion channels, with the ion conducting pore coincident with the overall four-fold symmetry axis. A ring of aspartate (D541) residues, shown in previous studies as a critical determinant of calcium selectivity, forms a narrow constriction at the extracellular pore entrance, or selectivity filter. Methionine (M577) side chains in the lower portion of the channel pore plug the conduction pathway and define the closed state of the channel. The ankyrin repeat domain, linker domain, N-terminal helix, and C-terminal hook form an intracellular skirt surrounding a cavity that lies beneath the pore axis. Close interactions between these domains, in large part mediated by the N-terminal helix, suggest that they are involved in allosteric modulation or concerted movements associated with channel activation. To shed light on the structural bases of permeation and ionic block, we cocrystallized TRPV6cryst with the permeant cations Ca²⁺ and Ba²⁺, and the channel blocker Gd³⁺. We identified binding sites for these cations by exploiting their anomalous scattering properties. On the basis of the cation-binding sites, we propose a permeation mechanism in which cations are recruited toward the pore by electronegative side chains in the extracellular vestibule, followed by sequential binding at least three binding sites along the central pore axis. Ca²⁺ selectivity is apparently achieved by high-affinity binding to the ring of D541 side chains in the selectivity filter. Gd³⁺ blocks permeation by similarly binding to the D541 ring and outcompeting ions of lesser charge. The results described in this dissertation provide a structural framework to further study mechanisms of epithelial calcium entry in health and disease