Role of the C-termini of human and chicken plasma retinol-binding proteins.

The most prominent differences between mammalian and non-mammalian vertebrate retinol-binding proteins (RBP) are in the C-terminal sequences. We have cloned and sequenced the cDNA for chicken RBP. Transfected COS cells that transiently expressed mammalian (human) or non-mammalian (chicken) RBP were used to demonstrate that both proteins were able to bind retinol and human transthyretin. However, we observed an increased retinol-independent secretion in cells expressing chicken RBP and reduced ligand-dependent secretion compared to the human protein. It can therefore be concluded that the C-terminal amino acid tail which is missing in chicken RBP compared to human RBP might play a role in retention and ligand-induced secretion.