4TM receptors

The 4TM receptors are pentameric complexes composed of subunits of 420 to 550 amino acids. The subunits exhibit sequence identities from 25 to 75%, with a similar distribution of hydrophobic and hydrophilic domains (Table 3.1). The hydrophilic 210 to 230 amino-acid N-terminal domain is followed by three closely spaced hydrophobic and putative transmembrane domains, then a variable-length intracellular loop, and finally a fourth putative transmembrane region shortly before the C-terminus (Figure 3.1). Of the four candidate transmembrane regions, evidence suggests that TM2 forms an a-helix, while the other hydrophobic regions more likely are folded as (3-sheets. 

FIGURE 3.1 Schematic representation of the transmembrane topology of the 4TM receptor family. Only TM2 show an a-helical structure in electron microscopic studies the remaining TM regions may fold in (5-sheet structures. Both the N-terminus (indicated by NH2) and the C-terminus are located extracellularly. The cytoplasmic loops between TM3 and TM4 are variable in size and contain putative phosphorylation sites. 

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