Structure
| Species | 1 | 6 | 11 | 16 | 21 | 26 ! |
|---|---|---|---|---|---|---|
| Pig | G W T L N | S A G Y L | L G P H A | I D N H R | S F H D K | Y G L A * |
| Human | G W T L N | S A G Y L | L G P H A | V G N H R | S F S D K | N G L T S ** |
| Cow | G W T L N | S A G Y L | L G P H A | L D S H R | S F Q D K | H G L A * |
| Rat | G W T L N | S A G Y L | L G P H A | I D N H R | S F S D K | H G L T* |
Galanin is a peptide consisting of a chain of 29 amino acids (30 amino acids in humans) produced from the cleavage of a 123-amino acid protein known as preprogalanin, which is encoded by the GAL gene. The sequence of this gene is highly conserved among mammals, showing over 85% homology between rat, mouse, porcine, bovine, and human sequences. In these animal forms, the first 15 amino acids from the N-terminus are identical, but amino acids differ at several positions on the C-terminal end of the protein.
These slight differences in protein structure have far-reaching implications on their function. For example, porcine and rat galanin inhibit glucose-induced insulin secretion in rats and dogs but have no effect on insulin secretion in humans. This demonstrates that it is essential to study the effects of galanin, and other regulatory peptides, in their autologous species.
The galanin family of protein consists of four proteins, of which GAL was the first to be identified. The second was galanin message-associated protein (GMAP), a 59- or 60-amino acid peptide also formed from the cleavage of preprogalanin. The other two peptides, galanin-like peptide (GALP) and alarin, were identified relatively recently and are both encoded for in the same gene, the preproGALP gene. GALP and alarin are produced by different post-translational splicing of this gene.
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