Structure of the exosporium and sublayers of spores of the <i>Bacillus cereus</i> family revealed by electron crystallography

Abstract

Summary We report on the first step in mapping out the spatial location of structural proteins within the exosporium, namely a description of its three‐dimensional architecture. Using electron microscopy and image analysis, we have characterized crystalline fragments from the exosporium of Bacillus cereus , B. thuringiensis and B. anthracis strains and identified up to three distinct crystal types. Type I and type II crystals were examined in three dimensions and shown to form arrays of interlinked crown‐like structures each enclosing a cavity ∼26–34 Å deep with threefold symmetry. The arrays appear to be permeated by tunnels allowing access from one surface to the other, possibly indicating that the exosporium forms a semi‐permeable barrier. The pore size of ∼23–34 Å would allow passage of the endospore germinants, alanine or inosine but not degradative enzymes or antibodies. Thus the structures appear compatible with a protective role for the exosporium. Furthermore the outermost crystalline layer must act as a scaffold for binding the BclA protein that contributes to the ‘hairy nap’ layer. The array of crowns may also act as a matrix for the binding or adsorption of other proteins that have been identified in the exosporium such as GroEL, immune inhibitor A and arginase.