Periodontitis, commonly known as gum disease, is a chronic inflammatory disease characterized by dysregulation of the oral microbiota in the subgingival space below the gum line (Hajishengallis, 2015). Healthy gums are compromised by a proliferation of pathogenic anaerobic bacteria, most commonly the triad of Porphyromonas gingivalis, Treponema denticola and Tannerella forsythia. These pathogens release 'virulence factors', molecules that aid their colonization and survival in the subgingival niche and their evasion of the host immune response. Synergistic interactions of the pathogenic microbes with normal oral microbiota lead to a sustained host inflammatory response, degrading periodontal tissues, eroding bone, and ultimately leading to tooth loss. Severe periodontitis affects around 10% of the human population, and health implications are not limited to oral health; the chronic inflammatory conditions precipitated by the disease can extend to systemic manifestations, contributing to risk of cardiovascular and respiratory diseases, diabetes, rheumatoid arthritis, and cancer (Hajishengallis, 2015; Peters et al., 2017). Proteases are amongst the most abundant virulence factors produced by periodontal pathogens. These enzymes degrade host proteins to feed bacterial growth, and also serve as key mediators of immune-subversive mechanisms, for example by degrading or inactivating endogenous antimicrobial peptides and components of the host complement system (Hajishengallis, 2015). One of the proteases implicated in these activities is mirolysin, a metalloprotease secreted by T. forsythia. Mirolysin cleaves multiple components of the complement pathway to protect the pathogen against complementmediated killing, and degrades the antimicrobial peptide LL-37, a human host defense peptide secreted by epithelial and immune cells at sites of infection (Jusko et al., 2015; Koneru et al., 2017). As a recently discovered mediator of T. forsythia survival and immune evasion, mirolysin offers a potential therapeutic target, yet until now the structural basis for its latency, activation and substrate specificity have not been defined. In this issue of IUCrJ, Guevara, Rodriguez-Banqueri et al. report high-resolution crystal structures of promirolysin, defining the mechanism of latency of the precursor protein (Guevara et al., 2020). Like nearly all proteases, mirolysin is first produced as a zymogen, maintained in an inactive state by the presence of an N-terminal extension known as a pro-peptide, pro-segment or pro-domain (Fig. 1). Upon proteolytic cleavage of the pro-segment, the active site of a protease becomes exposed and accessible to substrates. While unified by this common theme, different clans and families of metalloproteases show myriad variations in the details of how latency is maintained and activation subsequently achieved. These variations have been carefully cataloged in over 60 crystal structures of metalloprotease zymogens, including many from the Gomis-Rü th research group (Arolas et al., 2018). Here, the grou...