Evolution of mammalian chitinase(-like) members of family 18 glycosyl hydrolases
Anton P. Bussink 1, Dave Speijer 1, Johannes M.F.G. Aerts 1 and Rolf G. Boot 1*
1 Academic Medical Center, University of Amsterdam
* To whom correspondence should be addressed. E-mail: r.g.boot{at}amc.uva.nl.
Submitted on May 11, 2007
Revised on June 9, 2007
Accepted on 2 August 2007
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Abstract |
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Family 18 of glycosyl hydrolases encompasses chitinases and so-called chi-lectins lacking enzymatic activity due to amino acid substitutions in their active site. Both types of proteins widely occur in mammals although these organisms lack endogenous chitin. Their physiological function(s) as well as evolutionary relationships are still largely enigmatic. An overview is presented of all family members and their relationships are described. Molecular phylogenetic analyses suggest that both active chitinases (chitotriosidase and AMCase) result from an early gene duplication event. Further duplication events, followed by mutations leading to loss of chitinase activity, allowed evolution of the chi-lectins. The homologous genes encoding chitinase(-like) proteins are clustered in two distinct loci that display a high degree of synteny among mammals. Despite the shared chromosomal location and high homology, individual genes have evolved independently. Orthologues are more closely related than paralogues, and calculated substitution rate ratios indicate that protein-coding sequences underwent purifying selection. Substantial gene specialization has occurred in time, allowing for tissue-specific expression of pH optimized chitinases and chi-lectins. Finally, several family 18 chitinase-like proteins are present only in certain lineages of mammals, exemplifying recent evolutionary events in the chitinase protein family.
Key Words:
Chi-lectins, Chitinase, Family 18 of Glycosyl hydrolases, Molecular Phylogeny, Synteny
