Catecholamine-containing proteins from the pharate pupal cuticle of the tobacco hornworm, Manduca sexta

Multifaceted biological insights from a draft genome sequence of the tobacco hornworm moth, Manduca sexta

Manduca sexta, known as the tobacco hornworm or Carolina sphinx moth, is a lepidopteran insect that is used extensively as a model system for research in insect biochemistry, physiology, neurobiology, development, and immunity. One important benefit of this species as an experimental model is its extremely large size, reaching more than 10 g in the larval stage. M. sexta larvae feed on solanaceous plants and thus must tolerate a substantial challenge from plant allelochemicals, including nicotine. We report the sequence and annotation of the M. sexta genome, and a survey of gene expression in various tissues and developmental stages. The Msex_1.0 genome assembly resulted in a total genome size of 419.4 Mbp. Repetitive sequences accounted for 25.8% of the assembled genome. The official gene set is comprised of 15,451 protein-coding genes, of which 2498 were manually curated. Extensive RNA-seq data from many tissues and developmental stages were used to improve gene models and for insights into gene expression patterns. Genome wide synteny analysis indicated a high level of macrosynteny in the Lepidoptera. Annotation and analyses were carried out for gene families involved in a wide spectrum of biological processes, including apoptosis, vacuole sorting, growth and development, structures of exoskeleton, egg shells, and muscle, vision, chemosensation, ion channels, signal transduction, neuropeptide signaling, neurotransmitter synthesis and transport, nicotine tolerance, lipid metabolism, and immunity. This genome sequence, annotation, and analysis provide an important new resource from a well-studied model insect species and will facilitate further biochemical and mechanistic experimental studies of many biological systems in insects.

Insect cuticular sclerotization: a review

Different regions of an insect cuticle have different mechanical properties, partly due to different degrees of stabilization and hardening occurring during the process of sclerotization, whereby phenolic material is incorporated into the cuticular proteins. Our understanding of the chemistry of cuticular sclerotization has increased considerably since Mark Pryor in 1940 suggested that enzymatically generated ortho-quinones react with free amino groups, thereby crosslinking the cuticular proteins. The results obtained since then have confirmed the essential features of Pryor's suggestion, and the many observations and experiments, which have been obtained, have led to a detailed and rather complex picture of the sclerotization process, as described in this review. However, many important questions still remain unanswered, especially regarding the precise regional and temporal regulation of the various steps in the process.

Model reactions for insect cuticle sclerotization: participation of amino groups in the cross-linking of Manduca sexta cuticle protein MsCP36

Current theories of sclerotization center on protein cross-linking and dehydration as major factors in the hardening and stability of the insect cuticle. Several studies have reported the identification of catechol-amino acid adducts from sclerotizing cuticle involving histidine, lysine, and tyrosine, though there have been no reports of a catechol linked between two amino acid residues. Previously, we reported an in vitro model system for sclerotization and observed that stable protein oligomers were formed, presumably through cross-links with oxidized catecholamines [Insect Biochem. Mol. Biol. (2006) 36, 353-365]. Using site-directed mutagenesis we created a mutant lacking histidine, rMsCP36(H65A/H178A), to investigate the possible involvement of the two histidine residues of MsCP36 in cross-linking. Surprisingly, this alteration had little or no effect on the formation of protein oligomers as determined by SDS-PAGE analysis. Blocking of the free amino groups in lysyl side chains and the amino-terminus by succinylation diminished, but did not eliminate, cross-linking of either rMsCP36 or rMsCP36(H65A/H178A). We also examined the possibility that cross-linking was due to intermolecular dityrosine linkages. Immunoblot analysis utilizing a monoclonal antibody known to recognize peptidyl dityrosine indicated that dityrosyl cross-links were present. Taken together, these results indicate that lysyl residues are important for the cross-linking of the cuticle protein rMsCP36, but that additional residues other than histidine can also contribute.

Enzyme-free quinone crosslinking reaction for proteins: a macromolecular characterization study using gelatin

To mimic the quinone hardening of extracellular proteins in invertebrates, we investigated an enzyme-free crosslinking of gelatin by HQ in a neutral aqueous phase. The mixture was rapidly transformed to a yellowish brown, thermally and mechanically stable hydrogel in the presence of a simple copper(II) salt. A dehydrated thin film made of the mixture was flexible, tough, and showed a large ultimate breaking force. Physicochemical examination of the gel suggested that the basic amino acid residues (lysine, hydroxylysine, and histidine) of the protein were modified by the quinone ring to form 2-6 crosslinks per protein. The enzyme-free crosslinking reaction is discussed with consideration of a copper(II) ion-catalyzed oxidation of HQ and the hydroquinone/protein adducts.

Characterization and cDNA cloning of three major proteins from pharate pupal cuticle of Manduca sexta

Three proteins, MsCP20, MsCP27 and MsCP36, that are secreted in greatest quantity into the pharate pupal cuticle of Manduca sexta ( Hopkins et al., 2000) were purified and their amino acid sequences determined by mass spectrometry and Edman degradation. Although these proteins become sclerotized and insoluble in the pupal exoskeleton, their sequences contain features characteristic for proteins occurring in less sclerotized pliable cuticles, such as arthrodial membranes and soft larval cuticles. These proteins carry a secondary modification attached to a threonine residue, presumably an O-linked sugar moiety. cDNA clones of the genes for MsCP20, MsCP27 and MsCP36 were constructed from pharate pupal integument RNA. Close agreement was found between the amino acid sequences determined by Edman degradation and sequences deduced from the cDNA clones. The molecular masses determined by protein sequencing for MsCP20, MsCP27, and MsCP36 were 17713, 17448, and 29582 Da, respectively, in close agreement with the masses deduced from the corresponding cDNA clones (17711, 17410, and 29638 Da). Temporal expression analysis indicates that MsCP20 and MsCP36 transcripts are present at low levels early in the fifth larval stadium, followed by a large increase in abundance prior to pupal ecdysis. MsCP27 was not detected during development of the fifth larval instar, but its transcript, like those of MsCP20 and MsCP36, increased to a peak level just before pupal ecdysis. Only the MsCP36 transcript was detected in adults. These results support the hypothesis that these proteins are synthesized by the epidermis and are subsequently deposited into the cuticle during the larval-pupal transformation of M. sexta where they become sclerotized in the formation of pupal exocuticle.

Pupal cuticle proteins of Manduca sexta: characterization and profiles during sclerotization

Proteins in pupal abdominal cuticle of the tobacco hornworm, Manduca sexta, were characterized during the pre-ecdysial and post-ecdysial periods of sclerotization and endocuticle formation. Protein extractability decreased dramatically as the cuticle became sclerotized through 6 h post-ecdysis, but increased rapidly from 9 to 48 h as endocuticular layers were secreted. Nearly 100 proteins that were extracted from pre-ecdysial cuticle became largely insoluble during sclerotization. Three major proteins in this group destined to become exocuticle had apparent molecular masses (Mapp) of 20, 27 and 36 kDa, and were designated MS-PCP20, MS-PCP27, and MS-PCP36. Amino acid analysis revealed glycine to predominate in all three proteins, and alanine, aspartate, glutamate, proline and serine were also relatively abundant. Histidine residues, which provide sites for adduct and cross-link formation with quinone metabolites of N-beta-alanyldopamine during sclerotization of pupal cuticle, ranged from 2 to 3 mol %. N-Terminal amino acid analysis of MSPC-20 and MSPC-36 also revealed some sequence similarities indicating they may be related. An almost entirely new group of proteins appeared by 9 h as endocuticule secretion began, and these increased in abundance through 48 h post-ecdysis. Two of these were major proteins with Mapps of 33 and 34 kDa, and they also had close similarities in their N-terminal amino acid sequences. This study showed that the large number of proteins secreted into the presumptive exocuticle of the pupa before ecdysis are involved in sclerotization reactions and as a consequence become largely insoluble. The epidermis then switches to the secretion of an entirely new group of proteins that are involved in formation of the endocuticle.

Mass spectrometric profiling of glucosamine, glucosamine polymers and their catecholamine adducts. Model reactions and cuticular hydrolysates of Toxorhynchites amboinensis (Culicidae) pupae

Glucosamine (Gln), glucosamine polymers, and their catecholamine adducts were characterized using positive ion electrospray mass spectrometry (ESMS) and tandem mass spectrometry (ESMS-MS). N-acetyldopamine (NADA), a catecholamine found in many insect cuticles, was oxidized using mushroom tyrosinase, and the resulting quinone derivatives were reacted with Gln, (Gln)3, and polymeric glucosamine (chitosan). Adducts of glucosamine and its trisaccharide with NADA were readily identified as [M + H]+ ions in ESMS spectra, and ESMS-MS of selected ions confirmed the condensation of 1-3 NADA residues with Gln. In addition to Gln modification by the quinone derivatives of NADA, other spectra were consistent with the formation of adducts with N-acetylnoradrenaline and moieties formed by intramolecular cyclization following oxidation. The primary amine of glucosamine was involved in initial adduct formation, but the sites for subsequent additions of oxidized NADA to glucosamine, presumably via hydroxyl groups, could not be identified by ESMS alone. The ESMS spectra of chitosan films infused into the spectrometer following solubilization in acidic methanol/water produced spectra similar to that of (Gln)3 up to m/z 502. Ions of gradually decreasing intensity consistent with (Gln)x, where x = 4-8, were observed. Modification of chitosan films following incubation with NADA plus tyrosinase rendered the films insoluble in dilute acid, simulating the cross-linking process proposed to occur during insect cuticle sclerotization. Acid hydrolysates of the pupal stage of the mosquito Toxorhynchites amboinensis, using only two pupal exuviae for the hydrolyses, were infused into the mass spectrometer without preliminary chromatography. Eight amino acids, glucosamine, N-acetylglucosamine, catecholamines, and a variety of polymers incorporating these compound classes were identified.

Mass spectrometric analysis of catechol-histidine adducts from insect cuticle

Adducts of catechols and histidine, which are produced by reactions of 1,2-quinones and p-quinone methides with histidyl residues in proteins incorporated into the insect exoskeleton, were characterized using electrospray ionization mass spectrometry (ESMS), tandem electrospray mass spectrometry (ESMS-MS, collision-induced dissociation), and ion trap mass spectrometry (ITMS). Compounds examined included adducts obtained from acid hydrolysates of Manduca sexta (tobacco hornworm) pupal cuticle exuviae and products obtained from model reactions under defined conditions. The ESMS and ITMS spectra of 6-(N-3')-histidyldopamine [6-(N-3')-His-DA, pi isomer] isolated from M. sexta cuticle were dominated by a [M + H]+ ion at m/z 308, rather than the expected m/z 307. High-resolution fast atom bombardment MS yielded an empirical formula of C14H18N3O5, which was consistent with this compound being 6-(N-1')-histidyl-2-(3, 4-dihydroxyphenyl)ethanol [6-(N-1')-His-DOPET] instead of a DA adduct. Similar results were obtained when histidyl-catechol compounds linked at C-7 of the catechol were examined; the (N-1') isomer was confirmed as a DA adduct, and the (N-3') isomer identified as an (N-1')-DOPET derivative. Direct MS analysis of unfractionated cuticle hydrolysate revealed intense parent and product ions characteristic of 6- and 7-linked adducts of histidine and DOPET. Mass spectrometric analysis of model adducts synthesized by electrochemical oxidative coupling of N-acetyldopamine (NADA) quinone and N-acetylhistidine (NAcH) identified the point of attachment in the two isomers. A prominent product ion corresponding to loss of CO2 from [M + H]+ of 2-NAcH-NADA confirmed this as being the (N-3') isomer. Loss of (H2O + CO) from 6-NAcH-NADA suggested that this adduct was the (N-1') isomer. The results support the hypothesis that insect cuticle sclerotization involves the formation of C-N cross-links between histidine residues in cuticular proteins, and both ring and side-chain carbons of three catechols: NADA, N-beta-alanyldopamine, and DOPET.

Catechol conjugation with hemolymph proteins and their incorporation into the cuticle of the American cockroach, Periplaneta americana

Newly ecdysed American cockroaches, Periplaneta americana (sixth to last instar) were injected with radioactive dopamine (DA) and hemolymph was collected at 10-60 min post-ecdysis. Size-exclusion chromatography established the presence of at least three proteins that serve as catecholamine carriers. Reinjection of the smaller radiolabeled phenol-bound proteins into newly ecdysed animals results in in vivo aggregation, with the radiolabel bound to large MW proteins (30- > 200 kDa). In addition, the reinjection of radiolabeled protein of any size resulted in the incorporation of the label into the newly sclerotized cuticle. Hemolymph proteins were synthesized in vivo using [14C]leucine and subsequently double labeled in vivo with [3H]dopamine. After sclerotization (7 h post-ecdysis) the cuticle was extirpated, hydrolyzed and counted. An identical ratio of 14C to 3H was found in cuticle extracts as in the double-labeled hemolymph proteins, suggesting that the phenol-bound protein was incorporated in the cuticle unchanged. It appears that the catechol bound to the proteins exists as a beta-glucoside.