Evolution of mammalian chitinase(-like) members of family 18 glycosyl hydrolases

Abstract

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 of all family members is presented 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. Orthologs 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.

Figure 1.—

Protein alignment of the mature 39-kDa catalytic domain of mammalian chitinase(-like) proteins. Completely conserved amino acids are indicated a with solid baskground and those with a high similarity score (according to RISLER) are in a shaded background. The secondary structures (strands, arrows; helices and turns, T's) of human chitotriosidase, as published, are superimposed on the alignment. Cysteines involved in disulfide bridging are indicated by the numbers below the alignment. Chito, chitotriosidase; hcGP39, human cartilage glycoprotein 39; Ovi, oviductin; h, H. sapiens; m, M. musculus; r, R. norvegicus; b, B. taurus; c, Capra hircus; o, Ovis aries; s, Sus scrofa; cf, Canis familiaris; ma, Mesocricetus auratus. For further details, see text.

Figure 1.—

Protein alignment of the mature 39-kDa catalytic domain of mammalian chitinase(-like) proteins. Completely conserved amino acids are indicated a with solid baskground and those with a high similarity score (according to RISLER) are in a shaded background. The secondary structures (strands, arrows; helices and turns, T's) of human chitotriosidase, as published, are superimposed on the alignment. Cysteines involved in disulfide bridging are indicated by the numbers below the alignment. Chito, chitotriosidase; hcGP39, human cartilage glycoprotein 39; Ovi, oviductin; h, H. sapiens; m, M. musculus; r, R. norvegicus; b, B. taurus; c, Capra hircus; o, Ovis aries; s, Sus scrofa; cf, Canis familiaris; ma, Mesocricetus auratus. For further details, see text.

Figure 1.—

Protein alignment of the mature 39-kDa catalytic domain of mammalian chitinase(-like) proteins. Completely conserved amino acids are indicated a with solid baskground and those with a high similarity score (according to RISLER) are in a shaded background. The secondary structures (strands, arrows; helices and turns, T's) of human chitotriosidase, as published, are superimposed on the alignment. Cysteines involved in disulfide bridging are indicated by the numbers below the alignment. Chito, chitotriosidase; hcGP39, human cartilage glycoprotein 39; Ovi, oviductin; h, H. sapiens; m, M. musculus; r, R. norvegicus; b, B. taurus; c, Capra hircus; o, Ovis aries; s, Sus scrofa; cf, Canis familiaris; ma, Mesocricetus auratus. For further details, see text.

Figure 2.—

Maximum-likelihood tree of mammalian chitinase(-like) genes. The number on every node indicates the support bootstrap value for the likelihood analyses of a thousand replicate data sets. Only values other than 1000 are shown. The tree was rerooted by an outgroup (ceCht-1, Caenorhabditis elegans chitinase-1). Abbreviations as in Figure 1. For further details, see text.

Figure 3.—

Synteny of loci encoding chitinase(-like) proteins between mice and humans. Schematic overview of the synteny of mouse locus 1F4 with human 1q32 and mouse locus 3F3 with human 1p13. The orientation and position of the genes are indicated with arrows. The genes of members of the chitinase protein family are depicted by solid arrows, whereas the other genes in the loci are depicted by shaded arrows.

Figure 4.—

Overview of the evolution of chitinase(-like) genes. •, the “ancestral” gene duplications; ○, rodent-specific gene duplication; □, artiodactyle-specific gene duplication; a cross indicates the loss of catalytic activity mutations. “Chito-lectins” are chi-lectins evolved from the chitotriosidase gene (duplication).