New insights into the structure of (1→3,1→6)-β-D-glucan side chains in the Candida glabrata cell wall.

Douglas W Lowman,Michael D Kruppa,Ken Haynes,Trevor D Power,Michael F Wempe,David L Williams,Harry E Ensley,Lara J West,Daniel W Bearden,Joy Sturtevant

doi:10.1371/journal.pone.0027614

Abstract

β-glucan is a (1→3)-β-linked glucose polymer with (1→6)-β-linked side chains and a major component of fungal cell walls. β-glucans provide structural integrity to the fungal cell wall. The nature of the (1–6)-β-linked side chain structure of fungal (1→3,1→6)-β-D-glucans has been very difficult to elucidate. Herein, we report the first detailed structural characterization of the (1→6)-β-linked side chains of Candida glabrata using high-field NMR. The (1→6)-β-linked side chains have an average length of 4 to 5 repeat units spaced every 21 repeat units along the (1→3)-linked polymer backbone. Computer modeling suggests that the side chains have a bent curve structure that allows for a flexible interconnection with parallel (1→3)-β-D-glucan polymers, and/or as a point of attachment for proteins. Based on these observations we propose new approaches to how (1→6)-β-linked side chains interconnect with neighboring glucan polymers in a manner that maximizes fungal cell wall strength, while also allowing for flexibility, or plasticity.

Highlights

Fungal cells are surrounded by a wall that consists of four main components: glucan, chitin, mannan and mannoprotein
Grifolan-LE is a purified (1R3)-b-D-glucan with single (1R6)-blinked glucosyl side chains every third backbone chain repeat unit on average. They assigned specific proton NMR resonances to the anomeric proton of the glucosyl side chain repeat unit and to one of the methylene protons of the branchpoint glucosyl repeat unit in the backbone chain connected by a (1R6)-b-linkage
The proton NMR spectrum of the glucan isolated from C. glabrata ace2 strain (Fig. 1) resembles the NMR spectrum of Grifolan-LE except that the integral areas of the anomeric proton and one of the methylene protons involved in the (1R6)-blinkages in the side chain are not of equal area

Summary

Introduction

Fungal cells are surrounded by a wall that consists of four main components: glucan, chitin, mannan and mannoprotein. These carbohydrates play an important role in maintaining the shape and viability of fungal cells in response to osmotic challenge, and they are the point of contact with the environment. A major carbohydrate component of the cell wall is (1R3)-b-D-glucan [1]. It is thought that the (1R6)-b-D-glucan serves as attachment sites for chitin, mannan, and mannoprotein [2,3] and they may serve to link or interconnect adjacent (1R3)-b-D-glucan polymer strands. The nature of the attachment to these carbohydrates has not been structurally characterized except that mannoprotein may be linked through a glycosylphosphatidylinositol anchor [9,10]. Since (1R6)-b-D-glucan appears to serve a major role for attachment of these polymers, it is important to understand the structural nature of (1R6)-b-D-glucan as it exists when attached to the (1R3)-b-D-glucan polymer

Methods

Results

Conclusion