The Avian Proghrelin System

Table 1

Characteristics of chicken proghrelin system components.

Component^1	Size (aa)	Chromosome	Homology (%)^2
(1) Preproghrelin	116	12	36
(i) Signal Peptide	23		30
(ii) Ghrelin	26/28^3		54
(iii) C-Terminal Peptide	65		31
(iv) Obestatin	24^4		46
(2) Growth Hormone Secretagogue Receptor (GHS-R)		9
(i) GHS-R1a	347		76
(ii) GHS-R1aV	331		76
(iii) GHS-Rtv	220		78
(3) GPR39^5	462	7	61

¹Information for size and homology derived from: GenBank accession nos. NP_001001131, NP_989725, and NP_001073574 for preproghrelin, growth hormone secretagogue receptor (GHS-R), and GPR39, respectively.
²Based on amino acid identities compared to corresponding human sequence (NP_001128413, NP_004113, and NP_001499 for preproghrelin, growth hormone secretagogue receptor (GHS-R), and GPR39, resp.).
³The chicken ghrelin peptide contains two arginine residues (RR) at its C-terminal end that serve as a processing signal for proteolytic cleavage and are removed by the action of carboxypeptidase to give rise to the mature 26 amino acid peptide [2]. The human ghrelin peptide contains a proline and arginine pair (PR) at its C-terminal end and gives rise to a 28 amino acid mature peptide because these two amino acids are retained.
⁴The human obestatin peptide (23 amino acids) contains a glycine residue at its C-terminal end that is used for amidation. In contrast, chicken obestatin (24 amino acids) contains a glutamic acid residue at this site and is therefore one amino acid longer than the human peptide and is most likely not amidated.
⁵ Information for GPR39 is included because it is a G protein-coupled receptor related to GHS-R. However, it is now generally accepted that this orphan receptor is not the putative obestatin receptor in mammals and this is assumed to apply to birds as well.