New insights into the biological activity and secretion properties of a polypeptide derived from tilapia somatotropin

In a previous study, we unexpectedly found that tilapia growth hormone (tiGH) secreted to the culture media by transformed cells of the yeast Pichia pastoris lacks 46 amino acids from the C-terminal end. In the present study, we cloned the exact fragment that code for this truncated variant and demonstrated its growth promoting activity in goldfish when it's administered by immersion bath. Furthermore, a better characterization of this polypeptide was performed. Administration of the polypeptide derived from tiGH increased superoxide anion production and has a mitogenic effect on peripheral blood leukocytes. This molecule binds to liver membranes proteins in vitro in a saturable manner. Beside, we cloned and expressed tiGH and its truncated variant in mammalian cells using the signal peptide of this hormone and we observed that the secretion was drastically reduced in the truncated tiGH as compared to the intact molecule. Truncated tilapia growth hormone lacking the helix 4 and two disulfide loops is still a bioactive hormone, suggesting that the disulfide bonds and the helix 4 are not essential for the biological activities examined in this work. However, the growth hormone C-terminal portion seems to be essential for this hormone to be secreted by cultured cells in vitro.