Host cyclophilin-mediated maturation of an obligate intracellular bacterial surface virulence factor

Obligate intracellular bacterial pathogens cannot grow extracellularly, a trait that renders them highly understudied, but also endows them with unexpected host dependencies. Here, using the emerging obligate intracellular tickborne pathogen Rickettsia parkeri, we conducted a genome-scale CRISPR/Cas12a knockout infection screen to identify human cell determinants of rickettsial intracellular fitness. We discovered that the host peptidylprolyl isomerase cyclophilin A (PPIA/CypA), was essential for the formation of the R. parkeri actin tails that enable pathogen motility. PPIA localized to actin-associated bacterial poles and directly interacted with Sca2, the R. parkeri surface-exposed autotransporter chiefly responsible for actin tail nucleation. Sca2 bound PPIA through a domain implicated in surface translocation, and Sca2 failed to reach the R. parkeri surface in PPIA-deficient host cells. We propose that host PPIA enables Sca2 surface exposure during R. parkeri infection through a direct interkingdom protein maturation event, which represents an unexplored axis of the intracellular host-bacterial interface.