assessment-selective-mrna-translation-mammalian-cells-polysome

Assessment of Selective mRNA Translation in Mammalian Cells by Polysome Profiling

Regulation of protein synthesis represents a key control point in cellular response to stress. In particular, discreet RNA regulatory elements were shown ...

University of Ottawa

The vast majority of cellular mRNAs initiate their translations through a well-defined mechanism of ribosome recruitment that occurs at the 5'-terminal 7-methylguanosine cap with the help of several canonical protein factors. A subset of cellular and viral mRNAs contain regulatory motifs in their 5' untranslated regions (UTRs), termed internal ribosome entry sites (IRES), that sidestep this canonical mode of initiation. On cellular mRNAs, this mechanism requires IRES trans-acting protein factors (ITAFs) that facilitate ribosome recruitment downstream of the cap. While several ITAFs and their target mRNAs have been empirically identified, the in silico prediction of targets has proved difficult. Here, we report that a high AU content (>60%) of the IRES-containing 5' UTRs serves as an excellent predictor of dependence on NF45, a recently identified ITAF. Moreover, we provide evidence that cells deficient in NF45 ITAF activity exhibit reduced IRES-mediated translation of X-linked inhibitor of apoptosis protein (XIAP) and cellular inhibitor of apoptosis protein 1 (cIAP1) mRNAs that, in turn, leads to dysregulated expression of their respective targets, survivin and cyclin E. This specific defect in IRES translation explains in part the cytokinesis impairment and senescence-like phenotype observed in HeLa cells expressing NF45 RNA interference (RNAi). This study uncovers a novel role for NF45 in regulating ploidy and highlights the importance of IRES-mediated translation in cellular homeostasis.

Nucleotide composition of cellular internal ribosome entry sites defines dependence on NF45 and predicts a posttranscriptional mitotic regulon.

Regulation of protein expression through RNA metabolism is a key aspect of cellular homeostasis. Upon specific cellular stresses, distinct transcripts are selectively controlled to modify protein output in order to quickly and appropriately respond to stress. Reprogramming of the translation machinery is one node of this strict control that typically consists of an attenuation of the global, cap-dependent translation and accompanying switch to alternative mechanisms of translation initiation, such as internal ribosome entry site (IRES)-mediated initiation. In cancer, many aspects of the RNA metabolism are frequently misregulated to provide cancer cells with a growth and survival advantage. This includes changes in the expression and function of RNA binding proteins termed IRES trans-acting factors (ITAFs) that are central to IRES translation. In this review, we will examine select emerging, as well as established, ITAFs with important roles in cancer initiation and progression, and in particular their role in IRES-mediated translation. This article is part of a Special Issue entitled: Translation and Cancer.

Mame Daro Faye

Apoptosis Research Centre,
Children's Hospital of Eastern Ontario Research Institute and Department of Biochemistry,
Microbiology and Immunology,
Apoptosis Research Centre, Children's Hospital of Eastern Ontario Research Institute and Department of Biochemistry, Microbiology and Immunology

University of Ottawa

Assessment of Selective mRNA Translation in Mammalian Cells by Polysome Profiling