Heterogeneous Nuclear Ribonucleoprotein C Modulates Translation of c- myc mRNA in a Cell Cycle Phase-Dependent Manner

Abstract

ABSTRACT The c- myc proto-oncogene plays a key role in the proliferation, differentiation, apoptosis, and regulation of the cell cycle. Recently, it was demonstrated that the 5′ nontranslated region (5′ NTR) of human c- myc mRNA contains an internal ribosomal entry site (IRES). In this study, we investigated cellular proteins interacting with the IRES element of c- myc mRNA. Heterogeneous nuclear ribonucleoprotein C (hnRNP C) was identified as a cellular protein that interacts specifically with a heptameric U sequence in the c- myc IRES located between two alternative translation initiation codons CUG and AUG. Moreover, the addition of hnRNP C1 in an in vitro translation system enhanced translation of c- myc mRNA. Interestingly, hnRNP C was partially relocalized from the nucleus, where most of the hnRNP C resides at interphase, to the cytoplasm at the G 2 /M phase of the cell cycle. Coincidently, translation mediated through the c- myc IRES was increased at the G 2 /M phase when cap-dependent translation was partially inhibited. On the other hand, a mutant c- myc mRNA lacking the hnRNP C-binding site, showed a decreased level of translation at the G 2 /M phase compared to that of the wild-type message. Taken together, these findings suggest that hnRNP C, via IRES binding, modulates translation of c- myc mRNA in a cell cycle phase-dependent manner.