The complete amino acid sequence of a structural protein, protein 8, isolated from the pharate cuticle of the locust Locusta migratoria was determined. Protein 8 contains 148 amino acid residues and has an Mr of 15,224. By the extensive use of information obtained by plasma-desorption mass spectrometry (p.d.m.s.) it was possible to reduce the need for conventional sequence determination and to improve the reliability of the results. On the basis of the determined Mr of the intact protein all the peptides that constitute the complete sequence could be isolated from a time-course enzymic digestion. The isolated peptides were sequenced by using a combination of Edman degradation and carboxypeptidase digestion monitored by p.d.m.s. The alignment of the peptides was established from the time-course digestion and further verified by a second enzymic digestion. The primary structure of the protein consists of two hydrophilic and two hydrophobic regions. The hydrophobic regions are enriched in alanine, valine and proline and dominated by a repetitive sequence Ala-Ala-Pro-(Ala/Val). The sequence strengthens the view that the cuticle proteins belong to a unique family of structural proteins.
The complete amino acid sequence of a structural protein isolated from pharate cuticle of the locust Locusta migratoria was determined. The protein has an unusual amino acid composition: 42% of the residues are alanine and only 14 of the 20 common amino acid residues are present. The primary structure consists of regions enriched in particular amino acid residues. The N-terminal region and a region close to the C-terminus are enriched in glycine. The rest of the protein is dominated by alanine, except for two short regions enriched in hydrophilic residues. Almost all the proline residues are situated in the alanine-rich regions in a conserved sequence ‘A-A-P-A/V’. An internal duplication has taken place covering most of the protein except for the glycine-rich regions. Owing to the unusual features of the protein a combination of automated Edman degradations and plasma-desorption m.s. was used to determine the complete sequence. The protein does not show sequence homology to other proteins, but proteins divided into regions enriched in the same kind of amino acid residues have been isolated from other insect structures.