MOLECULAR EVOLUTION OF GPCRS: GLP1/GLP1 receptors

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   Figure 1

   Conserved synteny for genome fragments containing the SCTR-like family genes. The genomic locations of the human SCTR-like family genes along with their neighbor genes were compared with those of orthologous genes in amphioxus (cephalochordate), tunicate (urochordate), and lamprey (basal vertebrate agnathan). Orthologs and paralogs are aligned on the same column with the same color. Chromosome numbers are indicated above the gene, and gene locations (megabase) are indicated below the gene in human and tunicate. In amphioxus and lamprey, scaffold numbers are shown below the genes. The absence of the gene in humans is indicated by white boxes with broken lines under which ‘X’ was labeled. Paralogous genes generated by 2R exist on human chromosomes 2, 7 (or 3), 17, and 12, which fall into reconstructed linkage groups GAC E0, E1, E2, and E3 respectively. The GCGR subfamily genes were aligned on different chromosomes as they are on different linkage groups other than GAC Es. Annotations for amphioxus, tunicate, and lamprey SCTR-like family genes are shown in Hwang et al. (2013). In the case of lamprey, chromosome duplication events for generation of paralogous genes cannot be excluded although genes are on the same row.
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   Figure 2

   Phylogeny and synteny for the GCGR subfamily genes. (A) Maximum likelihood phylogenetic tree for the GCGR subfamily of human (hu), mouse (mo), chicken (ch), anole lizard (an), Xenopus (xe), zebrafish (zf), medaka (md), fugu (fu), stickleback (sb), tetraodon (to), and lamprey (lam) along with human SCTR, GHRHR, VIPRs, and ADCYAP1R1. The receptor sequences were aligned by using MUSCLE and a tree was constructed with MEGA 5.05. Bootstrap number indicates 100 replicates. (B) Synteny for human genome fragments having the GCGR subfamily genes. Chromosome numbers are indicated above the gene, and gene locations (megabase) are indicated below the gene. Paralogous genes are aligned on the same column with the same color. (C) Proposed evolutionary history of the GCGR subfamily genes in vertebrates. GLP2R, GLP1R, and GCGR have emerged by local duplication before 2R while GIPR and GCRPR arose by 2R in an osteichthyan ancestor. After divergence of teleosts and tetrapods, GLP1R disappeared and GCGR were doubled through teleost-specific 3R in teleosts. The absence of the gene in each species is indicated by white boxes with broken lines.
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   Figure 3

   Phylogeny and exon structures of GCG and related peptide genes. (A) Exon structures of the GCG and related genes from various vertebrates and lamprey. In this diagram, as only the open reading frame-encoding exons are counted, the first exon contains the signal peptide (SP) sequence. The exons that do not encode the mature peptide sequence are also shown as simple lines. GCG, GLP1, and GLP2 are encoded by three different exons of the human GCG gene. The Xenopus GCG gene encodes three independent GLP1 peptides as well as GCG and GLP2. The second forms of medaka and zebrafish GCG genes lack GLP2. Location of the mature peptides on the exons of other peptide genes such as GIP, GCRP, ADCYAP1, VIP, SCT, and GHRH is also shown. Nonmammalian ADCYAP1 and VIP encode PACAP-related peptide (PRP) and VIP-related peptide (VRP), respectively, while those sequences in human genes are dysfunctional. The vestiges that remained in the human genes are indicated by spaces covered with gray dashed lines. (B) Maximum likelihood phylogenetic tree for GLP1-related peptides of human (hu), mouse (mo), chicken (ch), anole lizard (an), Xenopus (xe), zebrafish (zf), medaka (md), fugu (fu), stickleback (sb), tetraodon (to), and lamprey (lam) along with human SCT, GHRH, VIP, and PACAP are shown. The mature peptide sequences were aligned using MUSCLE, and a bootstrap consensus tree was constructed using MEGA 5.05. Bootstrap numbers represent 100 replicates.
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   Figure 4

   General structure of GLP1 and sequence alignment of GLP1 and related peptides. (A) General structure of GLP1 and functional residues. The biologically active GLP1 consists of a random coiled N-terminus with six residues followed by an α-helix starting from Thr-7 to Val-27. The residues, His-1, Gly-4, Phe-6, Thr-7, and Asp-9 are known to be important for receptor core domain binding and activation. Residues Phe-6, Thr-7, and Val-10 form a helix N-capping structure. The residues, Ala-18, Ala-19, Lys-20, Phe-22, Ile-23, and Leu-26 in the second half of the α-helix are demonstrated to have direct interaction with the N-terminal ECD of the receptor. Ala-2 is the target site of the dipeptidyl peptidase IV (DPPIV) protease and Gly-16 is involved in kinking of the α-helix in the ECD-bound structure. (B) Sequence logo of GLP1 and its related peptides. The amino acid sequences of the orthologous peptides were analyzed using a WebLogo program (http://weblogo.threeplusone.com/create.cgi). Sequences were retrieved from human, mouse, chicken, anole lizard, Xenopus, zebrafish, medaka, stickleback, tetraodon, and fugu.

• © 2014 Society for Endocrinology