CYCLOPS, a mediator of symbiotic intracellular accommodation.

The initiation of intracellular infection of legume roots by symbiotic rhizobia bacteria and arbuscular mycorrhiza (AM) fungi is preceded by the induction of calcium signatures in and around the nucleus of root epidermal cells. Although a calcium and calmodulin-dependent kinase (CCaMK) is a key mediator of symbiotic root responses, the decoding of the calcium signal and the molecular events downstream are only poorly understood. Here, we characterize Lotus japonicus cyclops mutants on which microbial infection was severely inhibited. In contrast, nodule organogenesis was initiated in response to rhizobia, but arrested prematurely. This arrest was overcome when a deregulated CCaMK mutant version was introduced into cyclops mutants, conferring the development of full-sized, spontaneous nodules. Because cyclops mutants block symbiotic infection but are competent for nodule development, they reveal a bifurcation of signal transduction downstream of CCaMK. We identified CYCLOPS by positional cloning. CYCLOPS carries a functional nuclear localization signal and a predicted coiled-coil domain. We observed colocalization and physical interaction between CCaMK and CYCLOPS in plant and yeast cell nuclei in the absence of symbiotic stimulation. Importantly, CYCLOPS is a phosphorylation substrate of CCaMK in vitro. Cyclops mutants of rice were impaired in AM, and rice CYCLOPS could restore symbiosis in Lotus cyclops mutants, indicating a functional conservation across angiosperms. Our results suggest that CYCLOPS forms an ancient, preassembled signal transduction complex with CCaMK that is specifically required for infection, whereas organogenesis likely requires additional yet-to-be identified CCaMK interactors or substrates.

Activation of the enhancer of zeste homologue 2 gene by the human papillomavirus E7 oncoprotein.

The malignant phenotype of human papillomavirus (HPV)-positive cancer cells is maintained by the activity of the viral E6 and E7 genes. Here, we identified the polycomb group gene enhancer of zeste homologue 2 (EZH2) as a novel downstream target for the viral oncogenes in HPV-transformed cells. EZH2 expression was activated by HPV16 E7 at the transcriptional level via E7-mediated release of E2F from pocket proteins. RNA interference analyses showed that continuous EZH2 expression is required for the proliferation of HPV-positive tumor cells by stimulating cell cycle progression at the G1-S boundary. In addition to its growth-promoting activity, EZH2 also contributed to the apoptotic resistance of cervical cancer cells. Furthermore, we found that HPV-positive dysplastic and tumorigenic cervical lesions were characterized by high levels of EZH2 protein in vivo. We conclude that the E7 target gene EZH2 is a major determinant for the proliferation of HPV-positive cancer cells and contributes to their apoptotic resistance. Moreover, EZH2 may serve as a novel therapeutic target for the treatment of cervical cancer.