Fig. 4. Schematic representation of the domain structures of insect enzymes involved in chitin metabolism. (A) Domain structure and membrane topology of the Manduca sexta chitin synthase 1 (accession no. AY062175). Transmembrane helices are depicted as vertical bars, cytoplasmic or extracellular regions are depicted as horizontal bars. The N-terminal, catalytic and C-terminal domains are marked with A, B and C, respectively, following a previously suggested nomenclature (Tellam et al., 2000). Highly conserved blocks are tagged with the respective consensus sequences, with EDR and QRRRW being the chitin synthase signature sequences. The gray-shaded box highlights the region that is affected by alternate exon usage of class A genes. The red, blue and yellow boxes indicate supposed sites of N-glycosylation, catalysis and coiled-coils, respectively. (B) Domain structure of the Manduca sexta chitinase (accession no. A56596). Highly conserved blocks are tagged with the respective consensus sequence, with YDFDGLDLDWEYP being the insect signature sequence, which is consistent with that of family 18 chitinases. The pink box represents the signal peptide preceding the amino acid sequence of mature chitinases. The blue, red and green boxes indicate the catalytic, serine/threonine-rich and chitin-binding domains, respectively. Note that the serine/threonine-rich region is extensively processed by O-glycosylation.