BACKGROUND: The coral skeleton consists of CaCO3 deposited upon an organic matrix primarily
as aragonite. Currently galaxin, from Galaxea fascicularis, is the only soluble protein component of
the organic matrix that has been characterized from a coral. Three genes related to galaxin were
identified in the coral Acropora millepora.
RESULTS: One of the Acropora genes (Amgalaxin) encodes a clear galaxin ortholog, while the others
(Amgalaxin-like 1 and Amgalaxin-like 2) encode larger and more divergent proteins. All three
proteins are predicted to be extracellular and share common structural features, most notably the
presence of repetitive motifs containing dicysteine residues. In situ hybridization reveals distinct,
but partially overlapping, spatial expression of the genes in patterns consistent with distinct roles
in calcification. Both of the Amgalaxin-like genes are expressed exclusively in the early stages of
calcification, while Amgalaxin continues to be expressed in the adult, consistent with the situation
in the coral Galaxea.
CONCLUSION: Comparisons with molluscs suggest functional convergence in the two groups; lustrin
A/pearlin proteins may be the mollusc counterparts of galaxin, whereas the galaxin-like proteins
combine characteristics of two distinct proteins involved in mollusc calcification. Database searches
indicate that, although sequences with high similarity to the galaxins are restricted to the
Scleractinia, more divergent members of this protein family are present in other cnidarians and
some other metazoans. We suggest that ancestral galaxins may have been secondarily recruited to
roles in calcification in the Triassic, when the Scleractinia first appeared. Understanding the
evolution of the broader galaxin family will require wider sampling and expression analysis in a
range of cnidarians and other animals
