The cyclomodulin Cif of Photorhabdus luminescens inhibits insect cell proliferation and triggers host cell death by apoptosis

Cycle inhibiting factors (Cif) constitute a broad family of cyclomodulins present in bacterial pathogens of invertebrates and mammals. Cif proteins are thought to be type III effectors capable of arresting the cell cycle at G2/M phase transition in human cell lines. We report here the first direct functional analysis of CifPl, from the entomopathogenic bacterium Photorhabdus luminescens, in its insect host. The cifPl gene was expressed in P. luminescens cultures in vitro. The resulting protein was released into the culture medium, unlike the well characterized type III effector LopT. During locust infection, cifPl was expressed in both the hemolymph and the hematopoietic organ, but was not essential for P. luminescens virulence. CifPl inhibited proliferation of the insect cell line Sf9, by blocking the cell cycle at the G2/M phase transition. It also triggered host cell death by apoptosis. The integrity of the CifPl catalytic triad is essential for the cell cycle arrest and pro-apoptotic activities of this protein. These results highlight, for the first time, the dual role of Cif in the control of host cell proliferation and apoptotic death in a non-mammalian cell line.