Fig. 8.

Chitin synthesis and orienting factors are conserved between insects (DmDrosophila) and crustaceans (DpDaphnia). Arthropod chitin is synthesised and extruded to the extracellular space by the large glycosyltransferase chitin synthase that resides in the apical plasma membrane of epidermal cells (chitin synthase-1) or in the epithelium of the midgut (chitin synthase-2). a In a similarity search with the amino acid sequence of Drosophila chitin synthase-1, we have found that the Daphnia genome encodes a third protein with a chitin synthase signature. b, c As illustrated by sequence comparison, crustaceans possess orthologues of the Drosophila chitin-orienting factors Rtv and Knk, respectively. b Retroactive proteins from Drosophila (DmRtv, 151 aa) and Daphnia pulex (DpRtv, 177 aa) share 63 from 138 aligned amino acids (46% identity). Both proteins have an N-terminal signal peptide (last residue marked by σ) and a C-terminal transmembrane domain (underlined). Those aromatic amino acids in DmRtv that are hypothesised to mediate association with chitin (Moussian et al. 2005b) are decorated by a star. Additional conserved aromatic amino acids are labelled by a question mark. No other sequence from Daphnia showed any similarity to DmRtv. cDrosophila Knk (DmKnk, 689 aa) and Daphnia Knk (DpKnk, 693 aa) are 52% identical (345/654 aa). Like DmKnk, DpKnk has an N-terminal signal peptide (last residue marked by σ) and is predicted to be inserted into the plasma membrane via a glycophosphatidyl-inositol (GPI)-anchor (predicted cleavage-site for GPI-modification marked by +)