Larval release in brachyuran crustaceans Functional similarity of peptide pheromone receptor and catalytic site of trypsin.

Studies of crab egg hatching and larval release behavior in the crab,Rhithropanopeus harrisii, generated a model describing the process. In the model, carboxyl terminal arginine peptides serve as pheromones that synchronize larval release. In response to the peptides, the female performs Stereotypic larval release behavior and casts larvae into the water column. The peptides originate from trypsin-like enzymatic activity as part of the egghatching process. Hatching can be simulated experimentally by incubating ovigerous crabs in either bovine or porcine trypsin. The female performs the larval release behavior. Eggs detach from the female, and immobile larvae hatch prematurely. Preincubation of trypsin with trypsin inhibitors eliminates these effects. Approximately nanomolar concentrations of five different polypeptide trypsin inhibitors evoke the female's larval release behavior. Because both peptides and trypsin inhibitors evoke larval release behavior and because trypsin inhibitors bind to both the peptide receptor and the enzyme with high affinity, the receptor binding site and trypsin catalytic site must be very similar. A relationship between the binding site of a peptide receptor and the catalytic site of trypsin is postulated. The difference may be substitution by a basic amino acid for the catalytic site serine. Molecular graphics modeling indicates that all necessary conditions for receptor binding can be met by substitution with lysine for the active site serine in the trypsin catalytic site. This substitution eliminates catalytic activity, maintains the binding affinity for trypsin inhibitors, and increases binding strength for peptides.