The subnuclear localizations of redox regulators
The subnuclear localizations of redox regulators is also largely unknown. The potential role of thiol reductases as transcription regulators or DNA repair molecules may suggest an association to DNA. In this way, the presence of a zinc finger domain potentially involved in protein/DNA interactions on plant NRXs is intriguing . The discovery that plant nuclei are an active source of production of H2O2 also triggers questions regarding the sites of ROS generation in the nucleus. The nucleolus accumulates tremendous amounts of iron, either free or complexed with proteins . Free iron might constitute a source of ROS by reacting with H2O2 and generating highly reactive hydroxyl radicals in Fenton reactions. Whether antioxidant molecules accumulate in the nucleolus is currently not known (Fig. 3).
Acknowledgements This work was supported by the Centre National de la Recherche Scientifique, the Agence Nationale de la Recherche (ANR-Blanc Cynthiol 12- BSV6-0011). AJM is grateful to the Deutsche Forschungsgemeinschaft (DFG) for continues grant support and to the Ministry of Innovation, Science and Research for support within the framework of the NRW Strategieprojekt BioSC No. 313/323-400-002 13. SAKB is funded through a scholarship from the Higher Education Commission (HEC) of Pakistan and the Agricultural University Peshawar (grant number 360/SIBGE).
Introduction Individual protein kinase C (PKC) isoenzymes are involved in specific signal transduction pathways , , ,  as well as in the regulation of multiple basal cell functions, including the control of intracellular pH, maintenance of the cytoskeleton and progression of the 5 03 mg , , , . The different functions attributed to single members of the PKC family expressed in eukaryotic cells cannot be explained only on the basis of differences observed in their catalytic properties, such as substrate specificity or sensitivity to activators , , , . It has been hypothesized that the subcellular localization, the presence of characteristic domains, capable to promote or prevent association of PKCs with specific sites and interaction with activating or inhibiting protein factors could play a relevant role for the intracellular modulation of PKC isoenzyme activities. In fact, it has been recently reported that isoenzymes belonging to the conventional, novel or atypical PKC subfamilies  interact with specific proteins both under in vivo and in vitro conditions , , . Several RACK proteins have been yet identified as sites of binding for activated PKC isoenzymes . Moreover, it has been proposed that protein receptors specific for the inactive form of the kinases (RICKs) are capable to anchor single PKC isoenzymes to different subcellular sites. A number of molecular motifs, identified both in the regulatory and catalytic domain of PKC isoenzymes, seem to be involved in these associations . We reported previously that PKC plays a fundamental role in murine erythroleukaemia (MEL) cell differentiation . However, the level of PKC-α, that in these cells is present in two forms, is positively correlated with the sensitivity to the chemical inducer and the decrease in the latent period preceding cell commitment . An opposite role is played by PKC-δ, an isoenzyme highly expressed in inducer-resistant MEL cell clones and rapidly down-regulated following induction . More recently, we observed that PKC-θ, an almost completely nuclear PKC isoenzyme in MEL cells, seems not to be involved in MEL cell commitment and disappears late during the multistep differentiation process, at a time corresponding to the cell acquirement of the ungrowing phenotype . PKC-θ, a member of the novel PKC subfamily, has been indicated as a mediator of many specific functions in different cell types. This PKC isoenzyme is required in the process of apoptosis in thymocytes , during mitosis and formation of F-actin stress fibers in endothelial cells  and in the process of activation of T-cells . Moreover, the 14-3-3 tau protein has been proposed as a specific modulator of PKC-θ functions preventing its translocation to the plasma membrane in Jurkat T-cells . A specific PKC-θ protein substrate, moesin, has also been identified in human leucocytes, among the membrane/cytoskeletal linkage proteins . In MEL cells, as well as in other murine and human cells, PKC-θ associates to centrosomes and kinetochores during mitosis , suggesting an active role for this kinase in the control of protein functions linked to the cell cycle progression. To establish the biochemical relevance of the interaction of PKC-θ to mitotic spindle and chromosomal structures, here, we have investigated the expression of phosphorylating activity of the kinase recovered from mitotic MEL cells, searching also for protein substrates localized in the same structures.