Comparative molecular modeling of Amphioxus calcium vector protein with calmodulin and troponin C

Calcium vector protein (CaVP), a new protein isolated from Amphioxus muscle, binds in a Ca2(+)-regulated manner to a 27 kd target protein, named CaVPT, whose function has not been elucidated yet. CaVP bears significant sequence homology to both calmodulin and skeletal muscle troponin C, especially in the C-terminal half of the molecule, which presumably contains the two functional Ca2(+)-binding sites. The N-terminal half contains two abortive EF-hands and is intramolecularly crosslinked with a disulfide bond. Using the crystallographic structures of calmodulin and striated muscle troponin C as a framework, we constructed two different three-dimensional models of CaVP and modeled the intramolecular disulfide bridge. The modeling based upon the coordinates of calmodulin yields a Ca2(+)-filled sites configuration in the N-terminal half of the molecule, even though no Ca2+ is bound in this half, whereas the troponin C-derived model generates a Ca2(+)-empty sites configuration. The models predict that neither is the Ca2(+)-filled nor in the Ca2(+)-empty sites conformation is there any steric and/or energetic obstacle for the formation of the disulfide bridge and that the disulfide bond is poorly accessible to reducing reagents. The optical properties of the Trp and Tyr residues of CaVP indicate that the calmodulin-derived model represents the most plausible prediction.