A novel kinase function of a nucleoside‐diphosphate‐kinase homologue in Porphyromonas gingivalis is critical in subversion of host cell apoptosis by targeting heat‐shock protein 27

We have previously shown that a homologue of a conserved nucleoside‐diphosphate‐kinase (Ndk) family of multifunctional enzymes and secreted molecule in Porphyromonas gingivalis can modulate select host molecular pathways including downregulation of reactive‐oxygen‐species generation to promote bacterial survival in human gingival epithelial cells (GECs). In this study, we describe a novel kinase function for bacterial effector, P. gingivalis‐Ndk, in abrogating epithelial cell death by phosphorylating heat‐shock protein 27 (HSP27) in GECs. Infection by P. gingivalis was recently suggested to increase phosphorylation of HSP27 in cancer‐epithelial cells; however, the mechanism and biological significance of antiapoptotic phospho‐HSP27 during infection has never been characterised. Interestingly, using glutathione S‐transferase‐rNdk pull‐down analysed by mass spectrometry, we identified HSP27 in GECs as a strong binder of P. gingivalis‐Ndk and further verified using confocal microscopy and ELISA. Therefore, we hypothesised P. gingivalis‐Ndk can phosphorylate HSP27 for inhibition of apoptosis in GECs. We further employed P. gingivalis‐Ndk protein constructs and an isogenic P. gingivalis‐ndk‐deficient‐mutant strain for functional examination. P. gingivalis‐infected GECs displayed significantly increased phospho‐HSP27 compared with ndk‐deficient‐strain during 24 hr infection. Phospho‐HSP27 was significantly increased by transfection of GFP‐tagged‐Ndk into uninfected‐GECs, and in vitro phosphorylation assays revealed direct phosphorylation of HSP27 at serines 78 and 82 by P. gingivalis‐Ndk. Depletion of HSP27 via siRNA significantly reversed resistance against staurosporine‐mediated‐apoptosis during infection. Transfection of recombinant P. gingivalis‐Ndk protein into GECs substantially decreased staurosporine‐induced‐apoptosis. Finally, ndk‐deficient‐mutant strain was unable to inhibit staurosporine‐induced Cytochrome C release/Caspase‐9 activation. Thus, we show for the first time the phosphorylation of HSP27 by a bacterial effector—P. gingivalis‐Ndk—and a novel function of Ndks that is directly involved in inhibition of host cell apoptosis and the subsequent bacterial survival.