Further we postulate a dynamic exchange of hAgo2 between both compartments

Further we postulate a dynamic exchange of hAgo2 between both compartments. in SGs and the loss of distinctly composed complexes containing hAgo2 or their sub-cellular context. Resatorvid Transfection of cells with PS-ON induces cell stress that is phenotypically similar to the established inducers heat shock and NaAsO2. The intracellular re-distribution of hAgo2 is related to its increased metabolic stability and to decreased RNAi directed by Resatorvid microRNA or by short interfering RNA. Here, we propose a functional model of the relationship between cell stress, translocation of hAgo2 to SGs providing a depot function, and loss of RNAi activity. == INTRODUCTION == The Argonaute protein family constitute a highly conserved family of nucleic acid-binding proteins whose members have been implicated in RNA interference (RNAi) and related phenomena in several organisms (15). In humans eight Argonaute proteins have been identified, which can be subdivided into the Ago subfamily and the Piwi (P-element-induced wimpy testis) subfamily (6,7). The expression of Piwi proteins (HIWI1, HIWI2, HIWI3 and HILI) is mostly restricted to the germ line where they associate with piRNAs to facilitate silencing of mobile genetic elements (6,810). The Ago subfamily consists of four ubiquitously expressed members, hAgo1-4. Despite their high sequence similarity endonuclease activity is restricted to hAgo2 (5,11,12). Human Ago2 can bind short interfering RNA (siRNA) as well as microRNA (miRNA). As the effector molecule of the RNA induced silencing complex (RISC) it represses target RNA either by site-specific cleavage or by inhibition of translation. In addition, hAgo2 seems to be involved in distinct steps of small RNA maturation (13). To accomplish gene regulatory processes hAgo2 needs to interact with diverse proteins and protein complexes. A recent study showed that most of these proteins are RNA-binding proteins that are involved in distinct steps of RNA processing, maturation, transport and the regulation of RNA stability and translation (14). Some of these interactions are likely to be mediated by RNA whereas some proteins may bind directly to hAgo2 or associate with it through other protein components (14,15). It seems to be reasonable to speculate on a dynamic network of hAgo2-complexes, which vary in composition and localization at distinct cellular sites of action. In addition, Ntn2l miRNA components of the RNAi machinery are thought to be involved in the control of gene expression of up to 30% of all human genes (16,17), which regulate essential developmental processes such as embryogenesis and cell differentiation as well as cell proliferation and programmed cell death (1820). Further, miRNAs are thought to play an essential role Resatorvid in human diseases, in particular in malignant cell proliferation (20). Because of its central role in gene regulation processes the RNAi machinery itself needs to be regulated under certain cellular conditions by post-translational modifications (21,22). For instance, a very recent study described that phosphorylation of TRBP at four serine residues (serine-142, -152, -283 and -286) is mediated by the mitogen-activated protein kinase (MAPK) Erk leading to enhanced miRNA production by increasing the stability of the miRNA-generating complex. In addition this post-translational modification was shown to be important in effecting the mitogenic signalling (23). In mouse andDrosophilait was shown that their respective Piwi proteins underlie post-translational modifications, more precisely arginine methylations, which have an impact on the sub-cellular localization and stability of these proteins Resatorvid (2426). A post-translational modification of hAgo2 was reported Resatorvid by Qiet al.(27) who described that hydroxylation of hAgo2 at proline-700 mediated by type I collagen prolyl-4-hydroxylase [C-P4H(I)] is important for hAgo2 stability and effective siRNA-mediated RNAi. In addition this hydroxylation had an impact on hAgo2 PB-localization. Putative hydroxylation sites were also found in other human Ago proteins as well as in mouse andDrosophilaAgo2 proteins (21). In line with those findings it was reported by two independent laboratories that hAgo2-mediated gene silencing might be linked to MAPK signalling pathways that are activated in response to cellular stress (28,29). Zenget al.(29) showed that hAgo2 is post-translationally modified by phosphorylation of serine-387 through the p38/MAPK signalling pathway. Mutating serine-387 to alanine decreased the localization of hAgo2 to PBs. Adamset al.showed that the hAgo2 protein stability, i.e. the hAgo2 expression level, is regulated by the epidermal growth factor receptor (EGFR)/MAPK signalling pathway. Over-expression of EGFR increased cellular hAgo2 protein levels, which were correlated with enhanced miRNA activity in malignant cell growth of breast cancer cells. MAPKs are members of signalling pathways that transfer extracellular signals from the cell membrane to the nucleus allowing the cell to respond to extracellular stimuli. Thus induction of post-translational modifications of hAgo2 and/or other proteins of the RNAi machinery may represent mechanisms how cellular systems regulate RNAi, thereby co-ordinating gene expression in context of the cellular environment. The sub-cellular localization of hAgo2 is thought to occur mainly cytoplasmatically visualized by.