Orexin-A is composed of a highly conserved C-terminal and a specific, hydrophilic N-terminal region, revealing the structural basis of specific recognition by the orexin-1 receptor.
Orexins-A and B, also called hypocretins-1 and 2, respectively, are neuropeptides that regulate feeding and sleep-wakefulness by binding to two orphan G protein-coupled receptors named orexin-1 (OX(1)R) and orexin-2 (OX(2)R). The sequences and functions of orexins-A and B are similar to each other, but the high sequence homology (68%) is limited in their C-terminal half regions (residues 15-33). The sequence of the N-terminal half region of orexin-A (residues 1-14), containing two disulfide bonds, is very different from that of orexin-B. The structure of orexin-A was determined using two-dimensional homonuclear and (15)N and (13)C natural abundance heteronuclear NMR experiments. Orexin-A had a compact conformation in the N-terminal half region, which contained a short helix (III:Cys6-Gln9) and was fixed by the two disulfide bonds, and a helix-turn-helix conformation (I:Leu16-Ala23 and II:Asn25-Thr32) in the remaining C-terminal half region. The C-terminal half region had both hydrophobic and hydrophilic residues, which existed on separate surfaces to provide an amphipathic character in helices I and II. The nine residues on the hydrophobic surface are also well conserved in orexin-B, and it was reported that the substitution of each of them with alanine resulted in a significant drop in the functional potency at the receptors. Therefore, we suggest that they form the surface responsible for the main hydrophobic interaction with the receptors. On the other hand, the residues on the hydrophilic surface, together with the hydrophilic residues in the N-terminal half region that form a cluster, are known to make only small contributions to the binding to the receptors through similar alanine-scan experiments. However, since our structure of orexin-A showed that large conformational and electrostatical differences between orexins-A and B were rather concentrated in the N-terminal half regions, we suggest that the region of orexin-A is important for the preference for orexin-A of OX(1)R.
Takai
Tomoyo
T
Institute for Protein Research, Osaka University, Yamadaoka 3-2, Osaka, 565-0871, Japan.
Takaya
Takao
T
Nakano
Mutsuko
M
Akutsu
Hideo
H
Nakagawa
Atsushi
A
Aimoto
Saburo
S
Nagai
Katsuya
K
Ikegami
Takahisa
T
D000595:Amino Acid Sequence; D001583:Benzoxazoles / Q000737:chemistry / Q000494:pharmacology; D001665:Binding Sites; D017124:Conserved Sequence; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D066298:In Vitro Techniques; D047908:Intracellular Signaling Peptides and Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008823:Micelles; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009287:Naphthyridines; D009479:Neuropeptides / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D019906:Nuclear Magnetic Resonance, Biomolecular; D064446:Orexin Receptors; D000068797:Orexins; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D043562:Receptors, G-Protein-Coupled; D018013:Receptors, Neuropeptide / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D014508:Urea / Q000031:analogs & derivatives / Q000737:chemistry / Q000494:pharmacology
C420062:1-(2-methylbenzoxazol-6-yl)-3-(1,5)naphthyridin-4-yl urea; D001583:Benzoxazoles; C000596229:HCRT protein, human; D047908:Intracellular Signaling Peptides and Proteins; D008823:Micelles; D009287:Naphthyridines; D009479:Neuropeptides; D064446:Orexin Receptors; D000068797:Orexins; D043562:Receptors, G-Protein-Coupled; D018013:Receptors, Neuropeptide; D014508:Urea