Primary structure of stanniocalcin in two basal Actinopterygii

The primary structure of stanniocalcin (STC), the principal product of the corpuscles of Stannius (CS) in ray-finned fishes, was deduced from STC cDNA clones for two species of holostean, the gar, Lepisosteus osseus and the bowfin, Amia calva. Overlapping partial cDNA clones were amplified by polymerase chain reaction (PCR) from single-strand cDNA of the CS. Excluding the poly(A) tail, the cDNAs of 1863 base pairs [bp] (gar) and 914 bp (bowfin) contained the 5' untranslated region followed by the coding region and the 3' untranslated region. Both the gar and bowfin STC cDNA encode a prehormone of 252 amino acids (aa) with a signal peptide of 32 aa and a mature protein of 220 aa. The deduced aa sequence of gar STC shows 87% identity with bowfin STC, 60-72% identity with most vertebrate STCs and 26% identity with mouse STC2. Phylogenetic analysis of the sequences support a view that the gar and bowfin form a monophyletic holostean clade. RT-PCR revealed in the gar and bowfin that, just as in mammals and rainbow trout, the expression of STC mRNA is widely spread in many tissues and organs. Since the gar and bowfin are representatives of the most ancient fishes known to possess CS, the corpuscular-derived STC molecule in fish has had a conserved evolution.