Structure of Human Stabilin-1 Interacting Chitinase-like Protein (SI-CLP) Reveals a Saccharide-binding Cleft with Lower Sugar-binding Selectivity

Geng Meng,Yanmei Zhao,Xiaoyun Bai,Yong Liu,Todd J Green,Ming Luo,Xiaofeng Zheng

doi:10.1074/jbc.m110.130781

Abstract

Human secreted protein stabilin-1 interacting chitinase-like protein (SI-CLP) has been identified as a novel member of Glyco_18 domain-containing proteins that is involved in host defense and inflammatory reactions. Efficient secretion of SI-CLP is mediated by its interaction with the endocytic/sorting receptor stabilin-1. SI-CLP is expressed abundantly in macrophages and neutrophils and is up-regulated by Th2 cytokine IL-4 and glucocorticoid, which suggest that SI-CLP could be a marker for adverse effects of glucocorticoid therapy. To gain insight into the biological function of SI-CLP, we determined the crystal structure of SI-CLP at 2.7 Å resolution by x-ray crystallography and found that it featured a typical triose-phosphate isomerase barrel fold with a putative saccharide-binding cleft. Comparison with other chitinase-like proteins showed the cleft to be atypically wide and open. The saccharide-binding capacity of SI-CLP was investigated, and its ligand-binding specificity was found to relate to the length of the oligosaccharides, with preference for chitotetraose. Further investigations reveal that SI-CLP could bind LPS in vitro and neutralize its endotoxin effect on macrophages. Our results demonstrate the saccharide-binding property of SI-CLP by structure and in vitro biochemical analyses and suggest the possible roles of SI-CLP in pathogen sensing and endotoxin neutralization.

Highlights

Isomerase (TIM)3 barrel fold that holds lectin properties with specific sugar-binding preference
YM1 reportedly binds GlcN oligomer and is regulated by Th2 cytokines; it is abundantly expressed in macrophages of Th2 chronic inflammation [21]
HCgp39 (YKL40), and YM1 show that they have a highly conserved TIM barrel fold with a saccharidebinding cleft [2,3,4,5, 17], suggesting that proteins containing this conserved fold could carry similar functions

Summary

Introduction

Isomerase (TIM) barrel fold that holds lectin properties with specific sugar-binding preference. True enzymes such as acidic mammalian chitinase and chitotriosidase each have an additional chitin-binding domain and are shown to hydrolyze chitin [1, 2]. The chitinase-like proteins have no enzyme activity, accumulated data suggest that these proteins possess the lectin property. The sequence identity between SI-CLP and any other Glyco_18 domain proteins is below 20%, and general sequence alignment between SI-CLP and other chitinase-like proteins could not provide any useful information. To gain insight into the biological function of SI-CLP, we determined the crystal structure of full-length SI-CLP and identified a putative saccharide-binding site in the Glyco_18 domain of SI-CLP. Crystal Structure of SI-CLP alignment suggested conserved aromatic residues in the saccharide-binding cleft. Our work provides structural basis for biological function of SI-CLP; subsequent study suggests the role of SI-CLP in microbe infection and inflammation

Methods

Results

Conclusion