Whereas the genes encoding the H1 and H2 receptors were cloned in 1991, the molecular architecture of the H3 receptor was not known until 1999 when Lovenberg et al. (1999) finally showed that the H3 receptor also belongs to the GPCR super-family. The cloning of the H3-receptor gene also shows the impact of genome projects on the life sciences. In a search for orphan GPCRs, a GPCR-related EST-sequence was identified in silico and used to clone a full-length human cDNA. The cDNA contained an open reading frame of 445 amino acids with an Asp residue in TM3 (Fig. 17.1), which suggested the protein to be a biogenic amine receptor. After expression of the cDNA, functional responses to HA were observed, and the protein was shown to be the H3 receptor. This protein shows very low homology with other GPCRs. Overall homology between the H3 receptor and the H1 and H2 receptor amounts only to 22 and 20 per cent, respectively. Within the TM domains the homology is still only 27 and 33 per cent, respectively. This remarkably low sequence homology explains why the H3 -receptor gene was not cloned by homology screening with H1 - or H2-receptor specific probes. With the information on the human H3-receptor cDNA, the rat, guinea-pig, and mouse cDNAs were cloned; all of these proteins have 445 amino acids (Drutel et al. 2001; Lovenberg etal. 2000; Tardivel-Lacombe etal. 2000).
The H3-receptor gene is located on chromosome 20 and contains at least three introns (Fig. 17.2). Consequently, various isoforms have been identified for the human, rat, and guinea-pig H3 receptor as a result of alternative splicing (Coge et al. 2001a; Drutel et al. 2001; Tardivel-Lacombe et al. 2000). For the rat H3 receptor, splicing of the first intron results in the introduction of a stop codon after TM2 and a truncated, non-functional receptor (not shown, see Drutel etal. 2001). Splicing of the third intron results in H3-receptor proteins, which lack 32 (H3b) or 48 amino acids (H3c) in the third intracellular loop (I3) (Drutel et al. 2001). One isoform with a deletion in I3 was also found for the guinea-pig H3 receptor (Tardivel-Lacombe et al. 2000). The I3 loop is known to be important for the GPCR signalling and indeed important differences have in this respect been reported for the three various rat isoforms. Finally, alternative splicing of a putative fourth intron eliminates the part encoding TM7, the C-terminal tail and the stop codon and introduces a new open reading frame with a putative new seventh TM domain. Combination of the splicing of this
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