CapP mediates the structural formation of biofilm-specific pili in the opportunistic human pathogen Bacillus cereus

Abstract

Polymeric proteinaceous filaments are structural scaffolds that diversify the functionality of the bacterial extracellular matrix. Here, we report a previously uncharacterized bacterial factor called bc1280 that is exclusive to B. cereus group and indispensable for the establishment of a biofilm lifestyle. We propose that BC1280 is an essential chaperone for the assembly of the filamentous platform that tightly controls the polymerization of heteropili containing CalY and TasA as major subunits in a concentration-dependent manner. Additionally, BC1280 modulates the expression of EPS via an uncharacterized pathway that is activated by a protease and an ECF-type sigma factor. The pilus biogenesis system described in this work highlights the complexity of extracellular matrix assembly in B. cereus and introduces a singular three-part structuration mechanism during biofilm formation and maturation. Graphical abstractDuring the process of biofilm formation in B. cereus, TasA, CapP, and CalY serve as the main components, in addition to other factors, ensuring that the extracellular matrix correctly assembles. The three proteins are produced and processed by the signal peptidase SipW and subsequently secreted through the Sec pathway in an unfolded conformation. At early biofilm formation stages, CapP is initially present at low levels, associates with the cell wall and is secreted into the ECM where it interacts with unfolded subunits of CalY, facilitating CalY folding and initiating fibril growth. CalY serves as a nucleator for incorporating TasA subunits into the pilus, forming TasA-CalY heteropolymers. Once the ECM scaffold is established, CapP levels increase, forming stable complexes with CalY that prevent its folding and arrest filament growth. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=142 SRC="FIGDIR/small/582368v2_ufig1.gif" ALT="Figure 1"> View larger version (55K): org.highwire.dtl.DTLVardef@df9d9dorg.highwire.dtl.DTLVardef@e4b712org.highwire.dtl.DTLVardef@1af2421org.highwire.dtl.DTLVardef@16d02c7_HPS_FORMAT_FIGEXP M_FIG C_FIG