cDNA cloning of a putative protochordate FK506-binding protein

A tunicate (Botryllus schlossert) cDNA library was screened with a microsatellite probe. Five positive clones were sequenced, each with a 5' truncated microsatellite. One (Bs.6) revealed striking similarity to FK506 and rapamycin-binding proteins (FKBPs). Clone Bs.6 is 500 base pairs long and encodes for a putative protein of 134 amino acids. The predicted protein features the two FKBP-type peptidyl-prolyl cis-trans isomerase (PPIase) signatures and an endoplasmic reticulum retention signal. This protochordate protein is substantially similar to 12-13 kDa FKBPs, most remarkably to one of the receptors that had been proposed to mediate the immunosuppressive actions of FK506, the human FKBP-13 (62% amino acid identity and 74% similarity).