Purification and cloning of a corpuscles of Stannius protein from Anguilla australis

The kidneys of teleost fish are associated with tissues containing secretory granules--the corpuscles of Stannius (CS). Electron microscopy indicates that the granules are of a proteinaceous nature and may represent hormones or enzymes of unrecognized physiological and biochemical function. In the present study, two-dimensional gel electrophoresis and electroelution was used to purify the major protein to homogeneity; it is approximately 32,000 Da in the reduced form and glycosylated. From the partial NH2-terminal sequence, a 75-mer oligonucleotide probe was synthesized and used to isolate a cDNA clone from which the complete amino acid sequence of the major CS protein was deduced. Polyclonal antibodies raised against CS homogenates were specific for the CS proteins (confirmed by immunohistochemistry). Hybridization histochemistry was used to confirm the location of the mRNA encoding the isolated protein. Incubation of CS homogenate with eel plasma or ovine renin substrate did not result in any angiotensin-like peptides whereas kidney homogenate did.