Lubricin's Mucin Domain Has Strong Polyproline Type II Helical Character

Lubricin is a glycoprotein that is crucial for maintaining joint health by preventing joint wear through reducing joint friction in the boundary mode. Lubricin was recently observed to hinder the formation of uric acid crystals in the joint and prevent a form of gouty arthritis. However, despite lubricin's great physiological importance, our current understanding of the molecular origins of lubricin's beneficial properties is limited by a lack of detailed structural information regarding its central mucin domain: Lubricin's large size (227.5 kDa) and numerous glycosylations pose a substantial obstacle to conventional experimental methods for solving protein structures. In this work, we use a combination of physics-based replica exchange molecular dynamics simulations and circular dichroism experiments to shed light on the structure of lubricin's central mucin domain. Using replica exchange molecular dynamics, we model [KEPAPTTP]₂, an amino acid repeat found throughout the mucin domain, and find that the mucin domain is likely to exhibit polyproline type II helices, which are further stabilized by O-linked oligosaccharide chains. Motivated by these simulation results, we performed circular dichroism spectroscopy on fragments of the mucin domain that also show clear polyproline type II helical character, corroborating our computational findings. Altogether, this work provides strong evidence of a lubricin mucin domain with evidence of polyproline type II content. As polyproline helices are often also found in other glycoproteins with antifreeze properties, this work may also explain the atomistic underpinnings of their interfacial functions, including lubrication and competition with crystal formation.