Proteins of Crustacean Exoskeletons: I. Similarities and Differences among Proteins of the Four Exoskeletal Layers of Four Brachyurans

Most of the proteins extracted from exocuticle, endocuticle, and membranous layer of four species of anecdysial (intermolt) crabs (the Bermuda land crab Gecarcinus lateralis, the rock crab Cancer antennarius, the shield-backed kelp crab Pugettia producta, and the southern shield-backed kelp crab Taliepus nuttalli) were 31 kDa or smaller; proteins of similar M r were common to all three layers. Proteins from the membranous layer were qualitatively indistinguishable in all four species. More proteins 31 kDa or smaller were similar in size and pI to proteins from other exoskeletal layers than were proteins larger than 31 kDa. Proteins extracted from the epicuticle of G. lateralis included a group of five ranging from 54 to 42 kDa that bound 45 Ca++ in vitro. The group was not seen in other layers of the exoskeleton of G. lateralis or, with the exception of 44 and 42 kDa protein bands that were in the epicuticle of C. antennarius, in any layers of the exoskeletons of the other three species. During proecdysis, the membranous layer is completely degraded, and proteins 31 kDa or smaller are preferentially degraded from the exocuticle and endocuticle of the old exoskeleton of G. lateralis, which is cast as an exuvia at ecdysis. The relative amounts of proteins in extracts of epicuticle from (1) anecdysial exoskeletons and (2) exuviae were very similar, suggesting that there was little degradation of epicuticle during proecdysis. Some of the proteins of the three inner layers of the exoskeleton of G. lateralis have characteristics similar to those of flexible cuticles of insects; they have acidic pIs and they form "charge trains," i.e., proteins of the same size separated by differences in charge during isoelectric focusing.