Bumsuk Hahm

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

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 G2/M phase of the cell cycle. Coincidently, translation mediated through the c-myc IRES was increased at the G2/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 G2/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.

Polypyrimidine tract-binding protein interacts with HnRNP L

Polypyrimidine tract‐binding protein (PTB) is involved in pre‐mRNA splicing and internal ribosomal entry site (IRES)‐dependent translation. In order to identify cellular protein(s) interacting with PTB, we performed a yeast two‐hybrid screening. Heterogeneous nuclear ribonucleoprotein L (hnRNP L) was identified as a PTB‐binding protein. The interaction between PTB and hnRNP L was confirmed in an in vitro binding assay. Both PTB and hnRNP L were found to localize in the nucleoplasm, excepting the nucleoli, in HeLa cells by the green fluorescent protein (GFP)‐fused protein detection method. The N‐terminal half of PTB (aa 1–329) and most of hnRNP L (aa 141–558) is required for the interaction between PTB and hnRNP L.