Abstact

Molting in crustaceans is controlled by molt-inhibiting hormone (MIH) and ecdysteroids. It is presumed that MIH inhibits the synthesis and the secretion of ecdysteroids by the Y-organ, resulting in molt suppression. The amino acid sequence of MIH is similar to that of crustacean hyperglycemic hormone (CHH), and therefore, they form a peptide family referred to as the CHH family. Most of the CHH family peptides show no cross-activity, whereas a few peptides show multiple hormonal activities. To reveal the structural basis of this functional specificity, we determined the solution structure of MIH from the Kuruma prawn Marsupenaeus japonicus and compared the solution structure of MIH with a homology-modeled structure of M. japonicus CHH. The solution structure of MIH consisted of five alpha-helices and no beta-structures, constituting a novel structural motif. The homology-modeled structure of M. japonicus CHH was very similar to the solution structure of MIH with the exception of the absence of the N-terminal alpha-helix and the C-terminal tail, which were sterically close to each other. The surface properties of MIH around this region were quite different from those of CHH. These results strongly suggest that this region is a functionally important site for conferring molt-inhibiting activity.