Legionella pneumophila is an intracellular bacterial pathogen that delivers over 330 effector proteins into host cells via its Dot/Icm type IV secretion system (Qiu and Luo, 2017, Lockwood et al., 2022). While several Legionella effectors are known to localise to host mitochondria, the molecular basis and functional significance of these interactions remain poorly defined (Escoll et al., 2017). It is unclear whether mitochondrial-targeted effectors primarily facilitate protein localisation or directly modulate host enzymatic activity to promote bacterial replication and survival (Escoll et al., 2016). Given the rising incidence of Legionnaires' disease in Australia, elucidating the bacterial strategies that manipulate host mitochondrial functions during infection is crucial.
This study focuses on the interaction between the Legionella effector LpPIP and the human catalytic subunit of protein phosphatase 1 (PPP1CA). PPP1CA was identified as an LpPIP interactor by our collaborators through proteomic analysis (Yek et al., 2025). AlphaFold modelling predicts that LpPIP is largely intrinsically disordered, suggesting that association with a host binding partner may be required for structural stabilisation. To investigate this interaction, LpPIP was co-expressed with human PPP1CA to promote complex formation and stability.
Progress towards characterising PPP1CA phosphatase activity in the presence and absence of LpPIP will be presented using colourimetric enzyme assays. In parallel, X-ray crystallography is being employed to characterise the LpPIP-PPP1CA complex at atomic resolution. Together, these approaches aim to identify the molecular interface, assess potential conformational changes, and determine whether LpPIP directly modulates PPP1CA activity. This work seeks to provide mechanistic insight into how Legionella effectors manipulate host phosphatase function at mitochondria-associated pathways during infection