Atomistic fingerprint of hyaluronan-CD44 binding.

Alexander Mackerell,Joni Vuorio,Hector Martinez-Seara,Ilpo Vattulainen

doi:10.1371/journal.pcbi.1005663

Abstract

Hyaluronan is a polyanionic, megadalton-scale polysaccharide, which initiates cell signaling by interacting with several receptor proteins including CD44 involved in cell-cell interactions and cell adhesion. Previous studies of the CD44 hyaluronan binding domain have identified multiple widespread residues to be responsible for its recognition capacity. In contrast, the X-ray structural characterization of CD44 has revealed a single binding mode associated with interactions that involve just a fraction of these residues. In this study, we show through atomistic molecular dynamics simulations that hyaluronan can bind CD44 with three topographically different binding modes that in unison define an interaction fingerprint, thus providing a plausible explanation for the disagreement between the earlier studies. Our results confirm that the known crystallographic mode is the strongest of the three binding modes. The other two modes represent metastable configurations that are readily available in the initial stages of the binding, and they are also the most frequently observed modes in our unbiased simulations. We further discuss how CD44, fostered by the weaker binding modes, diffuses along HA when attached. This 1D diffusion combined with the constrained relative orientation of the diffusing proteins is likely to influence the aggregation kinetics of CD44. Importantly, CD44 aggregation has been suggested to be a possible mechanism in CD44-mediated signaling.

Highlights

Hyaluronic acid (HA) known as hyaluronan is a natural carbohydrate polymer constituted by a repeating disaccharide of glucuronic acid (GlcUA) and N-acetylglucosamine (GlcNAc) ([-β(1,4)-GlcUA-β(1,3)-GlcNAc-]n) [1]
We show through atomistic molecular dynamics simulations that hyaluronan can bind CD44 with three topographically different binding modes that in unison define an interaction fingerprint, providing a plausible explanation for the disagreement between the earlier studies
The knowledge associated with hyaluronan and its receptors is constantly accumulating, the molecular-level insight is largely missing or incomplete due to the lack of techniques able to probe the dynamics of protein–carbohydrate interactions with sufficiently high resolution

Summary

Introduction

Hyaluronic acid (HA) known as hyaluronan is a natural carbohydrate polymer constituted by a repeating disaccharide of glucuronic acid (GlcUA) and N-acetylglucosamine (GlcNAc) ([-β(1,4)-GlcUA-β(1,3)-GlcNAc-]n) [1]. Reaching molecular weights of up to 106 Da (i.e., several thousand disaccharides), HA acts as a space-filling agent, molecular lubricant, and cell migration promoter in processes such as leukocyte trafficking, modulating embryonic morphogenesis and tumor metastasis It is an integral component of both the extracellular and pericellular matrices, where it interacts with cells through HA binding proteins [2], with CD44 [3]. The majority of the HA-binding capacity of CD44 stems from the globular HABD which can even be expressed as an individual soluble protein that retains its ability to bind HA [5] It is composed of a link module, which is extended by additional N-terminal and C-terminal flanking regions, that together form a globular HA-binding unit stabilized by three disulfide bridges [4]. The link module itself is a conserved α/β-fold shared by other similar HA binding proteins, such as TSG-6 [6] and LYVE-1 [7]

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