Structural proteins of sarcophagid larval exoskeleton. Composition and distribution of radioactivity derived from [7-14C]dopamine

Proteins of crustacean exoskeleton: IV. Partial amino acid sequences of exoskeletal proteins from the Bermuda land crab, Gecarcinus lateralis, and comparisons to certain insect proteins

As in all decapod Crustacea, the exoskeleton of the land crab Gecarcinus lateralis consists of four layers. Prior electrophoretic analysis of proteins extracted from these layers revealed an abundance of small M(r) proteins with acidic pIs are found in insect cuticle (O'Brien et al. [1991 Biol. Bull., 181:427-441). Further, immunological cross-reactivity between crab exoskeletal proteins and insect cuticular proteins has been demonstrated (Kumari and Skinner [1993] J. Exp. Zool., 265:195-210). Partial amino acid sequences of a number of proteins from the four exoskeletal layers are described here. Proteins were electrophoresed on two-dimensional (2D) gels, transferred to polyvinylidene difluoride (PVDF) membranes, and stained; individual spots were recovered and their N-termini were sequenced. In addition, a 14-kDa protein (pI = 5.4) from membranous layer (ML14) was eluted from 2D gels and digested with endoproteinase Lys-C; N-termini of its constituent peptides were sequenced. The two epicuticular proteins differed from each other. Three proteins with identical electrophoretic mobility isolated from exocuticle, endocuticle, and membranous layer appeared to have identical N termini, while another electrophoretically identical set from the three layers appeared identical with each other but differed in three positions from the first set. Two proteins from the membranous layer both had a mass of 25 kDa but different isoelectric points. Their sequences were indistinguishable from each other but clearly distinct from another membranous layer protein. Another distinct sequence was found in a 14-kDa protein from endocuticle, while a less acidic pair of 14-kDa proteins from endocuticle and membranous layer were quite similar to one another. The three internal peptide fragments from ML14 were distinct, but one had regions similar to the ML14 N terminus. One crab exoskeletal protein sequence was similar to some structural proteins of vertebrates, whereas others had motifs found in insect cuticular proteins. The sequence similarities identified did not account for the antibody cross-reactivity.

Model sclerotization studies. 3. Cuticular enzyme catalyzed oxidation of peptidyl model tyrosine and dopa derivatives

Incubation of N-acetyltyrosine methyl ester with cuticular enzymes, isolated from the wandering stages of Calliphora sp larvae, resulted in the generation of N-acetyldopa methyl ester when the reaction was carried out in the presence of ascorbate which prevented further oxidation of the o-diphenolic product. Enzymatic oxidation of N-acetyldopa methyl ester ultimately generated dehydro N-acetyldopa methyl ester. The identity of enzymatically produced N-acetyldopa methyl ester and dehydro N-acetyldopa methyl ester has been confirmed by comparison of the ultraviolet and infrared spectral and chromatographic properties with those of authentic samples as well as by nuclear magnetic resonance studies. Since N-acetyldopaquinone methyl ester was also converted to dehydro N-acetyldopa methyl ester and tyrosinase was responsible for the oxidation of N-acetyldopa methyl ester, a scheme for the cuticular phenoloxidase catalyzed conversion of N-acetyltyrosine methyl ester to dehydro N-acetyldopa methyl ester involving the intermediary formation of the quinone and the quinone methide is proposed to account for the observed results. The conversion of N-acetyldopa methyl ester to dehydro derivative remarkably resembles the conversion of the sclerotizing precursor, N-acetyldopamine, to dehydro-N-acetyldopamine observed in the insect cuticle. Based on these comparative studies, it is proposed that peptidyl dopa derivatives could also serve as the sclerotizing precursors for the sclerotization of the insect cuticle.

Expression and hormonal control of a new larval cuticular multigene family at the onset of metamorphosis of the tobacco hornworm

The pattern of cuticular protein synthesis by the epidermis of the tobacco hornworm larva changes during the final day of feeding, leading to an alteration in cuticular structure and a stiffening of the cuticle. We have isolated a small multigene family which codes for at least three of the new cuticular proteins made at this time. The five genes which were isolated from this family map to two different genomic regions. Sequencing shows that one of the genes is 1.9 kb and consists of three exons coding for a 12.2-kDa acidic (pI = 5.26) protein that is predominantly hydrophilic. The deduced amino acid sequence shows regions of similarity to proteins from flexible lepidopteran cuticles and from Drosophila larval and pupal cuticles, but not to proteins found in highly sclerotized cuticles. This gene family is first expressed late on the penultimate day (Day 2) of feeding in the final larval instar and ceases expression 2 days later when metamorphosis begins. In situ hybridization shows that this gene family is expressed in all the epidermal cells of Day 3 larvae except the bristle cells and those at the muscle attachment site. Expression can be induced in Day 1 epidermis by exposure to 50 ng/ml 20-hydroxyecdysone in vitro, but only if juvenile hormone is absent. Its developmental expression, tissue specificity, and hormonal regulation strongly suggest that this multigene family is involved in the structural changes that occur in the larval cuticle just prior to the onset of metamorphosis.

Sclerotization of mosquito cuticle

The mode of sclerotization of Aedes aegypti pupal and adult cuticle was examined by employing biochemical and radioactive techniques. During larval-pupal metamorphosis, tyrosine is converted to tanning precursors and is incorporated into aryl-amino adducts and beta-crosslinks. The major hydrolysis product of beta-crosslinks in pupal cases is identified to be arterenone. Examination of tanning modes in five different mosquito species shows that the ratio of quinone to beta-sclerotization not only differs within the life stages of the insects, but also differs between species.

Isolation, characterization, and N-terminal sequence studies of cuticular proteins from the migratory locust, Locusta migratoria

The cuticle of the migratory locust, Locusta migratoria, contains more than a hundred different structural proteins, which can be extracted before but not after the cuticle is sclerotized. Fourteen of the proteins have been purified, covering a pI range of 6.4-10.6 and a molecular mass range of 15.2-36.8 kDa. The amino acid sequence from the N-terminal, ranging in length over 10-59 residues, have been obtained for eight of the proteins. A number of similarities, both in amino acid composition and in sequences, indicate that the proteins belong to a new protein family, characterized by an N-terminal part which is rich either in glycine, tyrosine and leucine or in hydrophilic amino acids, followed by a very alanine-rich portion. Similarities between this family of proteins and other structural proteins from insects are discussed.

Endogenous protease inhibitors prevent undesired activation of prophenolase in insect hemolymph

Phenoloxidase activation in the whole hemolymph of Sarcophaga bullata and Manduca sexta larvae is shown to be achieved by proteolytic cleavage of the proenzyme. This process is inhibited by the serine protease inactivator, Diisopropyl phosphofluoridate. Endogenous protease inhibitors isolated from the larvae inhibit alpha-chymotrypsin mediated prophenoloxidase activation in the hemolymph. These observations suggest that the endogenous protease inhibitors prevent undesired activation of prophenol oxidase in the hemolymph by inhibiting the serine protease involved in the activation process.

Sarcophagid larval proteins: partial sequence homologies among three cuticle proteins and related structures of drosophilids

Three structural proteins from the larval cuticle of Sarcophaga bullata have been sequenced at the amino terminus for 30-40 residues. We observed a high degree of homology with related proteins of Drosophila melanogaster, based on the previous findings of M. Snyder, J. Hirsh, and N. Davidson [(1981) Cell 25:165-177]. S. bullata protein SC1 had 65% homology with Drosophila isolate CP1, and SC6 showed 49% homology with CPX and 54% with CP2a. The three sarcophagid polypeptides also resembled each other with respect to mapped products of tryptic cleavage. The sites of posttranslational arylation required for puparium formation, namely histidyl and lysyl residues, were asymmetrically distributed in the sarcophagid samples. In SC1 the bulk of the loci of putative crosslinks lay beyond the 43-residue fragment. In SC6 half the histidines fell within the first 25% of the primary chain.