Comparative Proteome Analysis Reveals that Cuticular Proteins Analogous to Peritrophin‐Motif Proteins are Involved in the Regeneration of Chitin Layer in the Silk Gland of Bombyx mori at the Molting Stage

The silk gland of silkworm produces silk proteins during larval development. Many studies have long focused on the silk gland of the fifth instar larvae, but few have investigated this gland at other larval stages. In the present study, the silk gland proteomes of the fourth instar and fourth molt are analyzed using liquid chromatography–tandem mass spectrometry. In total, 2654 proteins are identified from the silk gland. A high abundance of ribosomal proteins and RR‐motif chitin‐binding proteins is identified during day 2 of the fourth instar (IV‐2) larval developmental stage, and the expression of cuticular proteins analogous to peritrophin (CPAP)‐motif chitin‐binding proteins is higher during the fourth molt (IV‐M). In all, nine enzymes are found to be involved in the chitin regeneration pathway in the silk gland. Among them, two chitinase and two chitin deacetylases are identified as CPAP‐motif proteins. Furthermore, the expression of CPAP3‐G, the most abundant CPAP‐motif cuticular protein in the silk gland during the IV‐M stage, is investigated using western blot and immunofluorescence analyses; CPAP3‐G shows a reverse changing trend with chitin in the silk gland. The findings of this study suggest that CPAP‐motif chitin‐binding proteins are involved in the degradation of the chitin layer in the silk gland. The data have been deposited to the ProteomeXchange with identifier PXD008677.