INITIATION FACTOR eIF2-INDEPENDENT MODE OF c-Src mRNA TRANSLATION OCCURS VIA AN INTERNAL RIBOSOME ENTRY SITE (IRES)

Overexpression and activation of c-Src protein have been linked to the development of a wide variety of cancers. The molecular mechanism(s) of c-Src overexpression in cancer cells is not clear. We report here an internal ribosome entry site (IRES) in the c-Src mRNA that is constituted by both 5’ noncoding and coding regions. The inhibition of capdependent translation by m 7 GDP in cell-free translation system or induction of ER stress in hepatoma-derived cells resulted in stimulation of the c-Src IRES activities. Sucrose density gradient analyses revealed formation of a stable binary complex between the c-Src IRES and purified HeLa 40S ribosomal subunit in the absence of initiation factors. We further demonstrate eIF2-independent assembly of 80S initiation complex on the c-Src IRES. These features of the c-Src IRES appear to be reminiscent to that of hepatitis C virus-like IRESs and translation initiation in prokaryotes. Transfection studies and genetic analysis revealed that the c-Src IRES permitted initiation at the authentic AUG351 which is also used for conventional translation initiation of the c-Src mRNA. Our studies unveiled a novel regulatory mechanism of c-Src synthesis mediated by an IRES element, which exhibits enhanced activity during cellular stress, and is likely to cause c-Src overexpression during oncogenesis and metastasis.