Protein Cell 2010, 1(9): 802–810 Protein & Cell DOI 10.1007/s13238-010-0108-9 REVIEW Heteromerization of TRP channel subunits: extending functional diversity ✉ Wei Cheng1,2, Changsen Sun1, Jie Zheng2 1 Lab of Biomedical Optics, College of Physics and Optoelectronic Engineering, Dalian University of Technology, Dalian 116023, China 2 Department of Physiology and Membrane Biology, University of California School of Medicine, Davis CA 95616, USA ✉ Correspondence: [email protected] Received August 16, 2010 Accepted September 1, 2010 ABSTRACT similarity in their primary sequences, TRP channels are classified into seven subfamilies: TRPC (canonical), TRPV Transient receptor potential (TRP) channels are widely (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML found throughout the animal kingdom. By serving as (mucolipin), TRPA (ankyrin) and TRPN (NO mechanopoten- cellular sensors for a wide spectrum of physical and tial) (Clapham, 2003; Nilius, 2007b). As expected for such a chemical stimuli, they play crucial physiological roles large family of cation channels, TRP channels are widely ranging from sensory transduction to cell cycle modula- found throughout the biologic kingdom from yeast and flyto tion. TRP channels are tetrameric protein complexes. mouse and human. They are expressed in both sensory While most TRP subunits can form functional homomeric neurons and non-sensory cells (Stowers et al., 2002; Zhang channels, heteromerization of TRP channel subunits of et al., 2003; Zhou et al., 2003). either the same subfamily or different subfamilies has Structurally, TRPs are tetrameric cation channels (García- been widely observed. Heteromeric TRP channels exhibit Sanz et al., 2004). Each channel subunit contains six many novel properties compared to their homomeric transmembrane segments (S1–S6) and a pore-forming counterparts, indicating that co-assembly of TRP chan- region formed by a loop between the S5–S6 segments. nel subunits has an important contribution to the Both the amino (N) terminus and the carboxyl (C) terminus diversity of TRP channel functions. are intracellularly located. Most of the TRP channels have an N-terminal ankyrin repeat domain and a C-terminal TRP KEYWORDS co-assembly, molecular mechanism, domain. The long N and C termini of TRPs also contain diversification, nonselective cation channel, polymodal recep- several additional regulatory domains which are relatively tor, multi-subunit protein complex conserved in most of the TRPs (Fig. 1). For example, a coiled- coil domain present in TRPCs, TRPPs and TRPMs; TRPCs INTRODUCTION and TRPVs possess a PDZ-binding domain in their C-termini; TRPCs, TRPVs and TRPMs contain a calmodulin (CaM) The first member of transient receptor potential (TRP) binding domain; TRPMLs and TRPPs have a Ca2+-binding channels was identified in 1969 in Drosophila mutants that EF-hand motif in their C-termini. Most of these domains displayed a transient response to light (Cosens and Manning, mediate protein-protein interactions that allow intra- and inter- 1969). In the following 40 years, not only Drosophila TRP cellular associations. channels received intense attention but also their homologs in Functionally, TRP channels are activated by diverse stimuli other species were eagerly identified in realization of their that are physical (e.g., temperature, voltage or mechanical involvement of a wide spectrum of cellular sensing functions. stress) or chemical (e.g., pH, osmotic pressure, neurotrans- To date, 28 mammalian TRP channel genes are known. mitters, growth factors, environmental irritants) in nature. This Additional TRP channel genes are found in other species enables TRP channels to act as multifunctional cellular such as worm, fruit fly and zebra fish. Indeed, the TRP sensors. TRP channels responding to combinations of stimuli channel superfamily has bloomed to more than 50 members serve as polymodal signal detectors for noxious stimuli from a of mostly nonselective cation ion channels. According to cell’s ambient environment (Tominaga et al., 1998; Bautista et 802 © Higher Education Press and Springer-Verlag Berlin Heidelberg 2010 Heteromeric TRP channels Protein & Cell Figure 1. Topological arrangement of a TRPV channel subunit. (A) Each channel subunit contains six transmembrane segments (S1–S6), and a pore region formed by a loop between the S5–S6 segments. Long intracellular N and C termini contain several relatively conserved domains such as ankyrin repeat domain (ARD), TRP domain, PDZ domain, CaM binding site. (B) Topological plot of the TRPV ankyrin repeat domain. Red circle represents α-helices, and blue arrows and lines represent β loop fingers. al., 2006), or as coincidence detectors for simultaneous signal within the same subfamily or between different subfamilies inputs that contribute to learning and memory (Chuang et al., (Fig. 2). Compared with other channel families (e.g., the Kv 2004). As TRP channel subfamilies are defined by sequence voltage-gated potassium channel family and the ionotropic homology, channels belonging to the same subfamily may glutamate receptor family), co-assembly of TRP subunits exhibit distinct functional properties. For example, the TRPV exhibits substantial specificity. subfamily members TRPV1–4 possess exquisite temperature sensitivity important for nociception and thermo-sensing, TRPC whereas TRPV5–6 are involved in epithelium Ca2+ entry with ordinary sensitivity to temperature. Similarly, the cold The TRPC channels are classical or canonical TRPs which sensor TRPM8 belongs to the TRPM subfamily whose are ubiquitously expressed. Based on sequence homology members do not all exhibit high temperature sensitivity. and functional similarities, TRPCs can be further divided into The physiologic properties of most TRP channels are well four groups: TRPC1, TRPC2, TRPC3/6/7 and TRPC4/5. studied. Readers are referred to excellent recent reviews on Subunit heteromerization in the TRPC subfamily has been broad TRP channel subjects (Montell, 2005; Owsianik et al., extensively explored (Goel et al., 2002; Riccio et al., 2002; 2006; Nilius, 2007a; Reaves and Wolstenholme, 2007; Plant and Schaefer, 2003; Schaefer, 2005). A number of Gaudet, 2009; Moiseenkova-Bell and Wensel, 2009). In the independent studies confirmed that co-assembly of TRPC present review, we focus on studies regarding heteromeric subunits in both native cells and expression systems TRP channel formations, their functional diversity, as well as (Strübing et al., 2001; Goel et al., 2002; Hofmann et al., the molecular mechanism governing TRP channel subunits 2002; Strübing et al., 2003). Intensive interaction between co-assembly. TRPC subunits of three different subgroups (TRPC1, TRPC3/ 6/7 and TRPC4/5) were found (Lintschinger et al., 2000; Liu et TRP CHANNELS GENERATED BY SUBUNITS al., 2005b; Plant and Schaefer, 2005; Poteser et al., 2006). CO-ASSEMBLY However, TRPC2, which is thought to be a pseudogene in human, does not interact with any other known TRPC subunit. Subunits heteromerization in TRP channels has been widely This is probably due to its distant relationship with other reported. Heteromeric channels can form by subunits either members of the canonical TRP family. Co-assembly of © Higher Education Press and Springer-Verlag Berlin Heidelberg 2010 803 Protein & Cell Wei Cheng et al. Figure 2. Subunit association between TRP channel subunits. (A) Heteromerization of mammalian TRPs. Orange arrows indicate co-assembly between two channel subunits. Purple arrows indicate interaction among three different subunits of the same subfamily. Blue solid arrows indicate interaction between subunits of different subfamilies. Blue dotted arrows indicate functional interaction for which heteromeric channel has not been identified. (B) Co-assembly of Drosophila TRPs. TRPC1 and TRPC5 was shown in hippocampal neurons 2005). Moreover, heteromerization of TRPC3 and TRPC4 (Strübing et al., 2001; Goel et al., 2002). Similar associations seemed to produce channels with a distinct pore structure were reported between TRPC1 and TRPC3 via an N-termini clearly different from those of the homomeric TRPC3 and domain interaction in salivary gland cells lines (Liu et al., TRPC4 channels (Poteser et al., 2006). 2005b). In HEK293 cells co-transfection of TRPC1 and TRPC5 (Strübing et al., 2001) or TRPC1 and TRPC4 (Gudermann et TRPV al., 2004) yielded novel nonselective cation channels. Zagra- nichnaya et al. (2005) reported that TRPC1/3/7 can interact to The vanilloid TRPV channels include six members that can form a store-operated channel complex (Zagranichnaya et al., be divided into two groups: TRPV1/2/3/4 and TRPV5/6. The 804 © Higher Education Press and Springer-Verlag Berlin Heidelberg 2010 Heteromeric TRP channels Protein & Cell overall sequence similarity of this subfamily is more than Heteromeric interaction between PKD1 and TRPP2 has been 40%. TRPV1–4 subunits contain 3–6 ankyrin repeats in the identified by several groups throughout the past decade (Qian intracellular N-terminal region. Activation of TRPV1–4 chan- et al., 1997; Tsiokas et al., 1997; Hanaoka et al., 2000; Grimm nels is driven by diverse stimuli including heat, voltage, pH, as et al., 2003; Delmas et al., 2004; Sharif-Naeini et al., 2009). well as natural endogenous/extrogenous ligands and syn- PKD1 can also interact with PKD2L1 (Murakami et al., 2005); thetic compounds. Indeed, the very noticeable polymodal the interaction is essential for PKD2L1 trafficking and channel feature of TRPV1–4 channels indicates fascinating functional