Cuticular proteins from the lobster, Homarus americanus

The urea-extractable proteins from calcified regions of intermoult cuticle of the lobster, Homarus americanus, have been separated by two-dimensional electrophoresis, showing that the extracts contain a large number of proteins. The major proteins have isoelectric points between 4 and 9, and their apparent molecular weights are between 5 and 30 kDa. Two of the proteins have been purified by a combination of ion-exchange chromatography, gel-filtration and RP-HPLC, and their complete amino acid sequences were determined by a combination of mass spectrometry and automated Edman degradation. Although they were purified from a single animal, both proteins were obtained as two isoforms. The isoforms of the smaller protein (HaCP4.6) differed only in a single position (phenylalanine/isoleucine), and the isoforms of the larger protein (HaCP11.6) differed in two positions (valine/isoleucine and glutamine/lysine). HaCP11.6 is N-terminally blocked by a pyroglutamate residue. Variants of an 18-residue motif are a characteristic feature of both sequences: it occurs twice in HaCP4.6 and four times in HaCP11.6. Comparison of the sequence to sequences published for cuticular proteins from other arthropods shows that the repeated motif is also present in proteins from the exoskeleton of the Bermuda land crab, Gecarcinus lateralis, but not in the single shrimp protein (Pandalus borealis) sequenced so far. The amino acid compositions of the lobster proteins are similar to that of flexible cuticles in locusts, but no convincing sequence similarities were found between the lobster proteins and cuticular proteins from locusts or other insects.