Functional specialization of the subdomains of a bactofilin driving stalk morphogenesis in Asticcacaulis biprosthecum

Bactofilins are a recently discovered class of cytoskeletal protein, widely implicated in subcellular organization and morphogenesis in bacteria and archaea. Several lines of evidence suggest that bactofilins polymerize into filaments using a central {beta}-helical core domain, flanked by variable N- and C-terminal domains that may be important for scaffolding and other functions. However, a systematic exploration of the characteristics of these domains has yet to be performed. In Asticcacaulis biprosthecum, the bactofilin BacA serves as a topological organizer of stalk synthesis, localizing to the stalk base and coordinating the synthesis of these long, thin extensions of the cell envelope. The easily distinguishable phenotypes of wild-type A. biprosthecum stalks and{Delta} bacA "pseudostalks" make this an ideal system for investigating how mutations in BacA affect its functions in morphogenesis. Here, we redefine the core domain of A. biprosthecum BacA using various bioinformatics and biochemical approaches to precisely delimit the N- and C-terminal domains. We then show that loss of these terminal domains leads to cells with severe morphological abnormalities, typically presenting a pseudostalk phenotype. BacA mutants lacking the N- and C-terminal domains also exhibit localization defects, implying that the terminal domains of BacA may be involved in its subcellular positioning, whether through membrane interactions through the N-terminal domain or through interactions with the stalk-specific morphological regulator SpmX through the C-terminal domain. We further show that point mutations that render BacA defective for polymerization lead to stalk synthesis defects. Overall, our study suggests that BacAs polymerization, membrane association, and interactions with other morphological factors all play a crucial role in the proteins function as a morphogenic regulator. The specialization and modularity of the terminal domains may underlie the remarkable functional versatility of the bactofilins in different species. Author summaryBacteria exhibit a wide variety of shapes and structures, many of which are crucial for their cellular functions. Among these structures is the stalk--a thin, tubular extension of the cell envelope formed by bacteria such as Asticcacaulis biprosthecum. Stalk synthesis in Asticcacaulis biprosthecum relies on the bactofilin BacA, a self-polymerizing cytoskeletal protein, whose deletion results in the dysregulation of stalk synthesis, and the formation of short, stubby "pseudostalks". We use this unique phenotype to characterize the subdomains of BacA, and find that BacAs ability to coordinate stalk synthesis depends on its conserved polymerization domain as well as its flanking N- and C-terminal domains, which are essential for proper localization and interactions. Our findings highlight how bactofilins combine conserved and variable regions to generate complex structures that serve as a platform for evolving new functions.