Four N-linked Glycosylation Sites in Human Toll-like Receptor 2 Cooperate to Direct Efficient Biosynthesis and Secretion

Abstract

Most higher organisms have a system of innate immune defense that is mediated by a group of evolutionarily related, germ line-encoded receptors, so-called Toll-like receptors. In mammals Toll-like receptors signal in response to pathogen-associated microbial structures. For example, Toll-like receptor 2 appears to mediate responses to bacterial peptidoglycan and acylated lipoproteins and Toll-like receptor 4 to bacterial lipopolysaccharide. However, the structural principles that underlie recognition of these structures are poorly understood. Toll-like receptors have leucine-rich repeats in their extracellular domains and are thus believed to adopt solenoid structures, similar to that found in platelet glycoprotein Ib. Additionally, all Toll-like receptors contain N-linked glycosylation consensus sites, and Toll-like receptor 4 requires glycosylation for function. Toll-like receptor glycosylation is also likely to influence receptor surface representation, trafficking, and pattern recognition. Using circular dichroism spectroscopy, we show here that purified human Toll-like receptor 2 and 4 proteins have secondary structure contents similar to glycoprotein Ib. We have also analyzed where consensus glycosylation sites are located in the extracellular domains of other human Toll-like receptors. We found that there are significant differences in the location and degree of conservation between sites in different Toll-like receptors. Using site-directed mutagenesis, we have found that in Toll-like receptor 2 extracellular domain all four predicted glycosylation sites are substituted, although one site is inefficiently core-glycosylated and its removal drastically affects secretion. The remaining Toll-like receptor 2 glycosylation sites also contribute to efficient protein secretion, albeit to a lesser degree.