Cheating Death: A Coxiella Effector Prevents Apoptosis

dria to provide a stable intracellular niche for replication (Luhrmann and Roy, 2007). The pathogen also activates Akt and Erk1/2 signaling to promote survival (Voth and Heinzen, 2009). Each of these events relies on Coxiella protein synthesis, suggesting the organism secretes a distinct effector(s) to regulate apoptosis. Unfortunately, effector identification and characterization has been hampered by a lack of methods for Coxiella genetic manipulation. As an alternative, Luhrmann et al. (2010) investigated the mechanistic activity of a Coxiella Ank through exploration of its anti-apoptotic function in Legionella. Luhrmann et al. (2010) tested the ability of four Anks (AnkA, B, F, and G) to inhibit intrinsic apoptosis. Eukaryotic cells ectopically expressing individual Anks were treated with the apoptosis-inducing agent staurosporine. Only AnkG prevented apoptosis with ∼65% of AnkG-expressing cells maintaining viability. Ankyrin repeat domains mediate eukaryotic protein– protein interactions (Mosavi et al., 2004) and Dot/Icm substrates predictably bind to, and manipulate, a specific host protein(s). Therefore, Luhrmann et al. (2010) used GST pulldown and immunoprecipitation approaches to identify host-binding partners for AnkG. Interestingly, mass spectrometry analysis and confirmatory immunoprecipitation studies identified the mitochondrial inner matrix protein p32 as a specific AnkG interacting protein. p32 is a pro-apoptotic protein that binds to Hrk to promote cytochrome c release (Sunayama et al., 2004) and also interacts with ARF, a pro-apoptotic p53 regulatory protein (Itahana and Zhang, 2008). Thus, AnkG interaction with p32 likely precludes interaction with other pro-apoptotic mitochondrial proteins, resulting in decreased cytochrome c release. This is also the first known example of a bacterial protein targeting p32 to manipulate host cell survival. Next, the authors performed a set of interesting gain of function experiments. In contrast to Coxiella, Legionella induces Cheating death: a Coxiella effector prevents apoptosis