Differentiation-induced internal translation of c-sis mRNA: Analysis of the cis elements and their differentiation-linked binding to the hnRNP C protein

In previous reports we showed that the long 5' untranslated region (5' UTR) of c-sis, the gene encoding the B chain of platelet-derived growth factor, has translational modulating activity due to its differentiation-activated internal ribosomal entry site (D-IRES). Here we show that the 5' UTR conta ins three regions with a computer-predicted Y-shaped structure upstream of an AUG codon, each of which can confer some degree of internal translation by itself In nondifferentiated cells, the entire 5' UTR is required for max imal basal IRES activity. The elements required for the differentiation-sen sing ability (i.e., D-IRES) were mapped to a 630-nucleotide fragment within the central portion of the 5' UTR. Even though the region responsible for IRES activation is smaller, the full-length 5' UTR is capable of mediating the maximal translation efficiency in differentiated cells, since only the entire 5' UTR is able to confer the maximal basal IRES activity. Interestin gly, a 43-kDa protein, identified as hnRNP C, binds in a differentiation-in duced manner to the differentiation-sensing region. Using UV cross-linking experiments, we show that while hnRNP C is mainly a nuclear protein, its bi nding activity to the D-IRES is mostly nuclear in nondifferentiated cells, whereas in differentiated cells such binding activity is associated with th e ribosomal fraction. Since the c-sis 5' UTR is a translational modulator i n response to cellular changes, it seems that the large number of cross-tal king structural entities and the interactions,vith regulated trans-acting f actors are important for the strength of modulation in response to cellular changes. These characteristics may constitute the major difference between strong IRESs, such as those seen in some viruses, and IRESs that serve as translational modulators in response to developmental signals, such as that of c-sis.