CALSENSIN - A NOVEL CALCIUM-BINDING PROTEIN EXPRESSED IN A SUBSET OF PERIPHERAL LEECH NEURONS FASCICULATING IN A SINGLE AXON TRACT

The mAb lan3-6 recognizes a cytosolic antigen which is selectively exp ressed in the growth cones and axons of a small subset of peripheral s ensory neurons fasciculating in a single tract common to all hirudinid leeches. We have used this antibody to clone a novel EF-hand calcium- binding protein, calsensin, by screening an expression vector library. A full-length clone of 1.1 kb identified by the antibody was isolated and sequenced. In situ hybridizations with calsensin probes and antib ody staining using new polyclonal anti-sera generated against calsensi n sequence demonstrate that calsensin indeed corresponds to the lan3-6 antigen. Calsensin consists of 83 residues with a calculated molecula r mass of 9.1 kD that contains two helix-loop-helix domains. The calci um-binding domains are likely to be functional in vivo since a fusion protein derived from the calsensin clone binds Ca-45(2+) in vitro. Imm unoaffinity purification experiments with the lan3-6 antibody shows th at a large 200,000 M(r) protein selectively copurifies with calsensin in two different leech species. These results suggest that calsensin m ay be functioning as a trigger protein which interacts with the larger protein. These data are consistent with the hypothesis that calsensin may mediate calcium-dependent signal transduction events in the growt h cones and axons of this small group of sensory neurons which fasicul ate in a single axon tract.