Novel Cellulose-Binding-Domain Protein in Phytophthora Is Cell Wall Localized

Richard W. Jones,Ching-Hong Yang,Manuel Ospina-Giraldo

doi:10.1371/journal.pone.0023555

Abstract

Cellulose binding domains (CBD) in the carbohydrate binding module family 1 (CBM1) are structurally conserved regions generally linked to catalytic regions of cellulolytic enzymes. While widespread amongst saprophytic fungi that subsist on plant cell wall polysaccharides, they are absent amongst most plant pathogenic fungal cellulases. A genome wide survey for CBM1 was performed on the highly destructive plant pathogen Phytophthora infestans, a fungal-like Stramenopile, to determine if it harbored cellulolytic enzymes with CBM1. Only five genes were found to encode CBM1, and none were associated with catalytic domains. Surveys of other genomes indicated that the CBM1-containing proteins, lacking other domains, represent a unique group of proteins largely confined to the Stramenopiles. Immunolocalization of one of these proteins, CBD1, indicated that it is embedded in the hyphal cell wall. Proteins with CBM1 domains can have plant host elicitor activity, but tests with Agrobacterium-mediated in planta expression and synthetic peptide infiltration failed to identify plant hypersensitive elicitation with CBD1. A structural basis for differential elicitor activity is proposed.

Highlights

Cellulose binding domains (CBD), are highly conserved regions of family 1 carbohydrate binding modules (CBM), are generally associated with catalytic glycoside hydrolases, whose members include endoglucanases, exocellobiohydrolases and beta-glucosidases
We performed a genome-wide search of Phytophthora infestans genes encoding family 1 carbohydrate binding modules (CBM1) that are commonly found on cellulolytic enzymes from saprophytic fungi
Total extractable CBD1 was quite low indicating a resilient association with the cell wall. This genome wide screen provides the first comprehensive evidence that Phytophthora retains the structural paradigm first found for phytopathogenic fungi [3,4], where cellulolytic enzymes are devoid of carbohydrate binding module family 1 (CBM1)

Summary

Introduction

Cellulose binding domains (CBD), are highly conserved regions of family 1 carbohydrate binding modules (CBM), are generally associated with catalytic glycoside hydrolases, whose members include endoglucanases, exocellobiohydrolases and beta-glucosidases. The non-catalytic CBD aids in anchoring to polysaccharides, and is often separated from the catalytic region by a short, flexible linker region rich in serine, proline and threonine [1]. These carbohydrate binding modules aid in specific binding, but are required principally for binding to crystalline cellulose [2]. The gene products represent a novel group of Phytophthora proteins, with one or two cellulose binding domains. In our study we have focused on a previously unrecognized, 13 kD cellulose binding protein, determining cellular location and elicitor activity

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Conclusion