Structural basis for carbohydrate binding properties of a plant chitinase-like agglutinin with conserved catalytic machinery

Abstract

A new chitinase-like agglutinin, RobpsCRA, related to family GH18 chitinases, has previously been identified in black locust (. Robinia pseudoacacia) bark. The crystal structure of RobpsCRA at 1.85. Å resolution reveals unusual molecular determinants responsible for the lack of its ancestral chitinase activity. Unlike other chitinase-like proteins, which lack chitinase catalytic residues, RobpsCRA has conserved its catalytic machinery. However, concerted rearrangements of loop regions coupled to non-conservative substitutions of aromatic residues central to the chitin-binding groove explain the lack of hydrolytic activity against chitin and the switch toward recognition of high-mannose type N-glycans. Identification of close homologs in flowering plants with conservation of sequence motifs associated to the structural adaptations seen in RobpsCRA defines an emerging class of agglutinins, as emphasized by a phylogenetic analysis, that are likely to share a similar carbohydrate binding specificity for high-mannose type N-glycans. This study illustrates the recent evolution and molecular adaptation of a versatile TIM-barrel scaffold within the ancestral GH18 family.