Human CLEC18 Gene Cluster Contains C-type Lectins with Differential Glycan-binding Specificity

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• The human CLEC18 gene cluster on chromosome 16q22 comprises three highly similar loci: CLEC18A, CLEC18B, and CLEC18C.
• Despite sequence homology, distinct amino acid residues exist within the C-type lectin-like domain (CTLD) and SCP/TAPS/CAP domains of CLEC18A, CLEC18B, and CLEC18C.
• Multiple alleles of the CLEC18 gene cluster have been identified.

Purpose of the Study:

• To investigate the expression patterns of CLEC18 proteins in human peripheral blood cells.
• To characterize the binding properties of CLEC18 proteins to polysaccharides.
• To identify key amino acid residues responsible for differential polysaccharide binding.

Main Methods:

• Real-time PCR and sequencing for genotyping.
• Flow cytometry for protein expression analysis.
• Immunofluorescence staining for cellular localization.
• Polysaccharide binding assays and sugar competition assays.

Main Results:

• CLEC18 proteins are abundantly expressed in human peripheral blood cells, with increased expression upon monocyte differentiation into macrophages and dendritic cells.
• CLEC18 proteins are localized in the endoplasmic reticulum, Golgi apparatus, and endosomes, and are also found in human sera and cell culture supernatants.
• CLEC18 proteins bind polysaccharides in a calcium-independent manner, with specific residues (Ser/Arg(339) and Asp/Asn(421)) in the CTLD domain mediating differential binding affinities.
• A mutation at Ser(339) in CLEC18A abolished binding to Ganoderma lucidum polysaccharides (GLPS-F3), and CLEC18 preferentially binds fucoidan, β-glucans, and galactans.

Conclusions:

• CLEC18 proteins exhibit distinct expression profiles and polysaccharide-binding capabilities influenced by specific amino acid residues.
• The calcium-independent binding to various polysaccharides suggests a role for CLEC18 in modulating host immunity.
• Future research should explore CLEC18's involvement in glycolipid binding, transport, and potential roles in protein aggregation.