Crystallography of gram-positive bacterial adhesins

Both Gram-negative and Gram-positive pathogens display a multitude of proteins and protein assemblies (pili or fimbriae) on their cell surfaces, which are often used for adherence and initiate colonization and pathogenesis. Adhesive proteins known as MSCRAMMs (microbial surface components recognizing adhesive matrix molecules), anchored by a specific enzyme called sortase in Gram-positive bacteria, target the host's extracellular matrix proteins (ECM) like collagen, fibrinogen and fibronectin. In the past decade, structural analysis by X-ray crystallography has enhanced our understanding of the interactions between MSCRAMMs and the host ECM by revealing several novel structural features that dictate surface protein assembly and the mode of their adhesion to host tissue. The latest focus is on the recently discovered Gram-positive bacterial pili, assembly of which is assisted by yet another specific sortase. Novel features like inter- and intra-molecular isopeptide bonds that facilitate the stability of the pilins, and intra-molecular donor strand complementation to stabilize the adhesin-target interactions are specific to Gram-positive bacteria. This chapter describes and discusses the common structural details between surface proteins and pilins of Gram-positive bacteria and biological implications emanating from these structures.