La protein has a positive effect on the translation of TOP mRNAs in vivo

In vertebrates, the mRNAs encoding ribosomal proteins, as well as other pro teins implicated in translation, are characterized by a 5'-untranslated reg ion (5'-UTR), including a stretch of pyrimidines at the 5'-end. The 5'-term inal oligopyrimidine (5'-TOP) sequence, which is involved in the growth-dep endent translational regulation characteristic of this class of genes (so-c alled TOP genes), has been shown to specifically bind the La protein in vit ro, suggesting that La might be implicated in translational regulation in v ivo. In order to substantiate this hypothesis, we have examined the effect of La on TOP mRNA translational control in both stable and transient transf ection experiments. In particular we have constructed and analyzed three st ably transfected Xenopus cell lines inducible for overexpression of wild-ty pe La or of putative dominant negative mutated forms. Moreover, La-expressi ng plasmids have been transiently co-transfected together with a plasmid ex pressing a reporter TOP mRNA in a human cell line. Our results suggest that in vivo La protein plays a positive role in the translation of TOP mRNA, T hey also suggest that the function of La is to counteract translational rep ression exerted by a negative factor, possibly cellular nucleic acid bindin g protein (CNBP), which has been previously shown to bind the 5'-UTR downst ream from the 5'-TOP sequence.