Structural basis for chitin acquisition by marine Vibrio species

Anuwat Aunkham,Karunakar Reddy Pothula,Michael Zahn,Anusha Kesireddy,Bert Van Den Berg,Wipa Suginta,Arnaud Baslé,Ulrich Kleinekathöfer,Albert Schulte

doi:10.1038/s41467-017-02523-y

Abstract

Chitin, an insoluble polymer of N-acetylglucosamine, is one of the most abundant biopolymers on Earth. By degrading chitin, chitinolytic bacteria such as Vibrio harveyi are critical for chitin recycling and maintenance of carbon and nitrogen cycles in the world’s oceans. A decisive step in chitin degradation is the uptake of chito-oligosaccharides by an outer membrane protein channel named chitoporin (ChiP). Here, we report X-ray crystal structures of ChiP from V. harveyi in the presence and absence of chito-oligosaccharides. Structures without bound sugar reveal a trimeric assembly with an unprecedented closing of the transport pore by the N-terminus of a neighboring subunit. Substrate binding ejects the pore plug to open the transport channel. Together with molecular dynamics simulations, electrophysiology and in vitro transport assays our data provide an explanation for the exceptional affinity of ChiP for chito-oligosaccharides and point to an important role of the N-terminal gate in substrate transport.

Highlights

Chitin, an insoluble polymer of N-acetylglucosamine, is one of the most abundant biopolymers on Earth
Proteins involved in the initial processes include chemotaxis proteins responsive to chitin oligosaccharides and extracellular chitinases that degrade the insoluble polymer into water-soluble chitooligosaccharides that are imported across the outer membrane (OM) through a dedicated, chitooligosaccharide-specific channel[9,10,11]
Cloning of a number of chitin utilization genes has been reported from Vibrio furnissii, Vibrio cholera, and V. harveyi and the corresponding proteins have been characterized[9,10,12,15,16,17,18,19,20]

Summary

Introduction

An insoluble polymer of N-acetylglucosamine, is one of the most abundant biopolymers on Earth. While providing a constant rain of polysaccharides to the ocean floor (“marine snow”), no substantial accumulation of chitin in ocean sediments occurs due to the rapid recycling of chitin driven by chitinolytic bacteria, mainly from the family Vibrionaceae[5,6] These bacteria utilize only chitin as a sole source of cellular energy, and certain Vibrio species such as Vibrio harveyi and Vibrio parahaemolyticus are extremely fast growing. A key step in chitin utilization represents the cellular acquisition of soluble chitin oligosaccharides (GlcNAc2–6) produced by the action of extracellular chitinases[20] This uptake process is carried out by an OM diffusion channel, termed chitoporin (ChiP)[9,10,11]. The binding constant (K) for the most potent substrate chitohexaose is about 5 × 106 M−1, which is at least an a b

Methods

Results

Conclusion