The investigation of internal ribosome entry in the c-myc and c-myb genes
The c-myc gene contains an internal ribosome entry site (IRES) within its 5' untranslated region. The IRES was shown to have different activities between cell lines suggesting a requirement for protein trans-acting factors that are present in these cell lines in varying amounts. In addition a number of proteins have been shown to interact with the IRES by north-western and UV cross-linking analysis. Investigation of the protein factors involved in c-myc IRES translation identified PCBP1 (Poly (rC) binding protein 1), PCBP2, HnRNPK (heterogeneous nuclear ribonucleoprotein K), UNR (upstream of N-ras) and UNRIP (unr interacting protein) as having a role in c-myc IRES translation, PCBP1, PCBP2, HnRNPK and UNR were found to directly interact with the IRES RNA by UV cross-linking and electrophoretic mobility shift assays (EMSAs). Investigation of the proteins effect on c-myc IRES activity showed stimulation of IRES activity in HeLa cells by PCBP1 and PCBP2. The factor HnRNPK was found to have a slight stimulatory effect in vivo. In addition PCBP1 and PCBP2 were found to stimulate IRES activity in vitro in combination with UNR and UNRIP. Using the yeast three-hybrid system a number of additional proteins were found to interact with the c-myc IRES RNA. A novel Fibrillarin-like protein was identified and shown to strongly interact with the IRES by EMSA. Studies to determine a direct role of this factor in c-myc IRES translation were inconclusive. The study of translation of the c-myc gene identified an IRES within its 5'UTR. Investigation of the role of trans-acting factors in its translation showed a possible role of the factors PCBP2, HnRNPk and ITAF45 (IRES trans-acting factor 45).