The Transient Receptor Potential Superfamily of Ion Channels

Abstract

Abstract. The transient receptor potential (TRP) superfamily of proteins is cation-selective ion channels with six predicted transmembrane segments and intracellularly localized amino and carboxyl termini. Members of the TRP superfamily are identified on the basis of amino acid sequence and structural similarity and are classified into TRPC, TRPV, TRPM, TRPP, TRPN, and TRPML subfamilies. TRP channels are widespread and have diverse functions, ranging from thermal, tactile, taste, osmolar, and fluid flow sensing to transepithelial Ca 2ϩ and Mg 2ϩ transport. Mutations of TRP proteins produce many renal diseases, including Mg 2ϩ wasting, hypocalcemia, and polycystic kidney diseases. This review focuses on recent advances in the understanding of their functions.The first transient receptor potential (TRP) protein was discovered in studies that examined Drosophila phototransduction (1). The photoreceptor cells of Drosophila exhibit sustained receptor potentials in response to continuous light exposure. The ionic basis for the sustained receptor potentials is influx of Ca 2ϩ from the extracellular space. Cosens and Manning (1) reported in 1969 that one group of mutant flies exhibits TRP upon continuous light exposure and named it trp, for transient receptor potential. The trp gene was cloned in 1989 (2) and subsequently shown to encode a Ca 2ϩ -permeable cation channel (3). Since then, many channels that bear sequence and structural similarities to the Drosophila TRP have been cloned from flies, worms, and mammals. Together, they form the TRP superfamily.Ion channels (e.g., voltage-gated K ϩ channels) are typically identified by their modes of activation and/or ion selectivity. Each family or superfamily of ion channels of similar activation and selectivity consists of multiple members of channel proteins with amino acid sequence homology. Unlike most ion channel families, the TRP superfamily of ion channels are identified on the basis of homology only. The mode of activation and selectivity for TRP channels are disparate. Some TRP channels are activated by ligands, whereas others are regulated by physical stimuli (e.g., heat) or yet-unknown mechanisms. All TRP channels are cation selective, but the selectivity ratio for Ca 2ϩ versus the monovalent cation Na ϩ (P Ca /P Na ) varies widely, ranging from Ͼ100:1, to 1 to 10:1, to Ͻ0.05:1. The lack of common identifying features has in part contributed to the confusing nomenclature of TRP channels in the literature.A recent consensus report proposed a unified nomenclature for the TRP superfamily (4). It classified TRP channels into TRPC, TRPV, and TRPM subfamilies. More recent classification has expanded TRP superfamily to include three additional, more distantly related subfamilies, TRPP, TRPML, and TRPN ( Figure 1). Structurally, all of these TRP channels have six predicted transmembrane (TM) segments and N-terminal and C-terminal cytoplasmic tails similar to topologies of voltagegated K ϩ , Na ϩ , and Ca 2ϩ channels; cyclic nucleotide-gated channels; and hyperpolarizati...

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