Channel heteromeric

All Kir channels are tetrameric proteins (see Fig. 3) of one-pore/two-transmembrane (1P/2TM) domain subunits which equally contribute to the formation of highly selective K+ channels. Most Kir channels can be assembled in functional homotetramers while some require heteromeric assembly (see Fig. 3). For example, functional GIRK channels underlying DCAch (Acetyl-choline-activated) current in atria are heteromultimers of two members ofKir3 subfamily Kir3.1 andKir3.4. 

The ion channel receptors are multi-subunit proteins which may be either homomeric (made up of multiple copies of a single type of subunit) or heteromeric (composed of more than one subunit type). These subunits come together after synthesis in the endoplasmic reticulum to form the mature receptor. Notice that stoichiometry is denoted by a subscript number. A receptor composed of two a and three /I subunits is therefore denoted as having a stoichiometry of This can cause confusion when related subunits are given sequential numbers /II, j]2, 3, etc. The convention is therefore that subunits are numbered normally while stoichiometry is indicated by subscripts so that a pentamer of a4 and j33 subunits might have a stoichiometry of a42/133. 

Non-NMDA ionotropic glutamate receptors (the majority sodium channel containing) can be subdivided into a-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) (comprising cloned subunits GluRl ) and kainate (GluR5-7, KAl-2) preferring receptors, with native receptors most likely to comprise either homo- or heteromeric pentamers of these subunits. 

The kainate receptors are composed of subunits from the GluR5-GluR7 class and the KA1-KA2 class of subunits. Homomeric receptors of the former class generate functional receptors and bind kainate with an affinity of 50 to 100 nM. KA1 or KA2 do not generate functional channels, but the receptors bind kainate with an affinity of 5 to 10 nM. Homomeric GluR6 and KA2 receptors are neither activated by AMPA, nor do they bind AMPA. Interestingly, when they are co-expressed, heteromeric receptors respond to AMPA. 

Nelson, C. S., Marino, J. L. Allen, C. N. (1996). Melatonin receptors activate heteromeric G-protein coupled Kir3 channels. Neuroreport 7, 717-20. 

Nicotine is an agonist at the nicotinic acetylcholine receptor (nAChR). Activation of this receptor depolarizes target cells (see Ch. 11). nAChRs are composed of five subunits surrounding a central ion-channel pore. Twelve different nicotinic receptor subunits are expressed in the nervous system (a2-oclO and (32—134). Of these, a subset is expressed in the VTA (a3-a7 and P2—134). It is thought that a7 receptors form homomeric receptors a3, a4 and a6 form heteromeric channels with 02 or 04 and a5 and 03 can associate with other a/0 pairs. Studies in knockout mice implicate several subunits in the ability of nicotine to modulate dopamine neurons (a4, a6, a7, 02, 03) but... 

Keyser KT, Britto ERG, Schoepfer R, Whiting P Cooper J, Conroy W, Brozozowska-Prechtl A, Karten HJ, Lindstrom J (1993) Three subtypes of a-bungarotoxin-sensitive nicotinic acetylcholine receptors are expressed in chick brain. J Neurosci 13 442-452 Khiroug S, Harkness PC, Lamb PW, Sudweeks S, Khiroug L, Millar NS, Yakel JL (2002) Rat nicotinic ACh receptor al and fil subunits co-assemble to form functional heteromeric nicotinic receptor channels. J Physiol 540 425 34... 

The complex subsequently releases GDP, and guanosine 5 -triphosphate (GTP) binds to the a-subunit in its place (47,49,50). While there is evidence supporting a model that allows for the dissociation of both the active Ga-GTP and the noncova-lently bound Py-heteromeric complex from the receptor-effector complex, other models can also account for these data (51). Auxiliary proteins may regulate the potentiation of the GPCR-G protein effector complexes that generate second messengers or specific transmembrane proteins such as ion channels (44,52). These processes are illustrated schematically in Fig. 6.2. 

Figure 6.2. Diagrammatic representation of the 5-HT3 receptor. The 5-HT3 receptor is distinct from the other 5-HT receptor subtypes, in that it is a ligand gated ion channel that is permeable to sodium and potassium. The 5-HT3 receptor is structurally similar to the nicotinic acetylcholine receptor and is composed of five sub-units. Two sub-units have been cloned, 5-HT3A and 5-HT35, and homomeric (5-HT3A) and heteromeric (5-HT3a/5-HT3b) forms of the receptor have both been...

Megho M, lanelli A, Anile C, et al Interaction of epileptic activities of bilateral deep temporal origin an experimental study. Epilepsia 17 437-448, 1976 Meguro H, Mori H, Araki K, et al Functional characterization of a heteromeric NMDA receptor channel from cloned cDNAs. Nature 357 70-74, 1992 Meichenbaum D, Turk DC Facilitating treatment adherence. New York, Plenum, 1987... 

Among K+ channels, two candidates that meet these requirements (i.e. involvement in chronic pain as well as restricted expression) are now emerging the KCNQ2/3 heteromere and the Kv1.4 homomere. 

Meguro, H., Mori, H., Araki, K., Kushiya, E., Kutsuwada, T., Yamazaki, M., Kumanishi, T., Arakawa, M., Sakimura, K., Mishina, M. Functional characterization of a heteromeric NMDA receptor channel expressed from cloned cDNAs, Nature 1992, 357, 70-74. 

Yamakura, T., Sakimura, K., Shimoji, K. The stereoselective effects of ketamine isomers on heteromeric N-methyl-D-aspartate receptor channels, Anesth. Analg. 2000, 225-229. 

---