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First published online November 10, 2003

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Chitin metabolism in insects: structure, function and regulation of chitin synthases and chitinases

Hans Merzendorfer^* and Lars Zimoch

Department of Biology/Chemistry, University of Osnabrück, 49069 Osnabrück, Germany

View larger version (26K): [in a new window]   Fig. 1. Biosynthesis of chitin in insects. The pathway starts with trehalose, the main hemolymph sugar in most insects, and ends with the chitin polymer. The diagrammatic representation is based on previously published pathways (Kramer and Koga, 1986; Cohen, 2001).

View larger version (102K): [in a new window]   Fig. 2. Localization of chitin synthase in the epiproct of Periplaneta americana. (A) Cryosections of 10µm were stained with a polyclonal antiserum raised against a conserved region of the Manduca sexta chitin synthase as described previously (Zimoch and Merzendorfer, 2002). Visualization of primary antibodies was conducted with anti-rabbit antibodies (whole molecules) conjugated to alkaline phosphatase. Detection with 5-bromo-4-chloroindolylphosphate and nitroblue tetrazolium was carried out in the presence of 2 mmol l-1 levamisole to block endogenous alkaline phosphatase activity. (B) Control reaction performed in the absence of primary antibodies. Arrows in A and B mark the apical region of epidermal cells, which are in part detached from the endocuticle as a result of the sectioning procedure (asterisks). Scale bar, 50µm.

View larger version (28K): [in a new window]   Fig. 3. Alternative models of chitin formation in insects. (A) Chitin synthase-loaded vesicles are transported from the transgolgi network to the apical region of epithelial cells by a constitutive secretory pathway and subsequently fuse with the plasma membrane. Upon fusion, they may get activated by proteolytic enzymes present in the molting fluid or the gut lumen. Blue and orange stars indicate the catalytic site facing either the cytoplasm or the extraplasmic space, respectively. (B) In this more speculative model, chitin synthesis has already occurred before the vesicles have fused with the plasma membrane and may continue or cease upon fusion. If the catalytic domain faces the cytoplasm (blue star), nascent chitin polymers have to be transported across the vesicular membrane, presumably involving transmembrane segments of the chitin synthase. By contrast, intravesicular arrangement of the catalytic domain (orange star) would require some uptake mechanism for UDP-GlcNAc.

View larger version (15K): [in a new window]   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.