From fibril to framework: P. abyssi AbpX illuminates a calcium-responsive family of microbial biomatrix proteins that form thermostable hydrogels
Evolutionary pressure on microbial communities propagating under extreme environmental conditions often results in unique structural adaptations to promote cell survival. Here, we report an investigation of AbpX, a biomatrix protein identified in cultures of the hyperthermophilic archaeon Pyrodictium abyssi. Under ex vivo and in vitro conditions, AbpX assembles into a para-crystalline lattice composed of semiflexible fibrils. CryoEM analysis of recombinant AbpX fibrils reveals that the precursor protein polymerizes through donor strand complementation (DSC), a process previously reported for chaperone-usher fimbriae in Gram-negative bacteria. Unlike the latter DSC protein polymers, AbpX undergoes chaperone-free polymerization in the presence of calcium ions, which are sequestered at the donor strand-acceptor groove interface between protomers in the fibril. Using a combination of cryoEM and crystallographic structural information, an atomic model is proposed for the AbpX lattice that provides insight into its potential role in biofilm formation. These findings suggest that calcium ion coordination triggers fibril assembly and pre-organizes the fibrils for incorporation into the protein lattice. Bioinformatic analysis indicates that AbpX exemplifies a distinct and broadly distributed clade of calcium ion responsive biomatrix proteins within the TasA superfamily that can be fabricated into hydrogel biomaterials in vitro under environmentally benign conditions. SignificanceBiofilms provide a protective environment for microbes that enhances resilience against environmental stressors. Secreted protein filaments constitute a major structural component of these extracellular matrices, however limited information is available on the mechanism of biofilm formation and structure of the resultant protein assemblies. Here, we report a class of biomatrix proteins that are widely distributed in bacteria and archaea. We demonstrate that one such protein, P. abyssi AbpX, self-assembles into fibrils and subsequently into para-crystalline lattices in response to the presence of calcium ion. We describe a mechanistic model for the structural evolution of the fibrils into an ordered protein framework that mimics the lattice structure of the ex vivo assembly observed during cell culture.