Involvement of the Xenopus laevis Ro60 autoantigen in the alternative interaction of La and CNBP proteins with the 5'UTR of L4 ribosomal protein mRNA

In vertebrates the synthesis of ribosomal proteins is co-ordinately regulated at the translational level. The 5'-untranslated region (5'UTR) of this class of mRNAs contains conserved regions that are necessary and sufficient for translational regulation. Recently, we found that two proteins, the Xenopus laevis La autoantigen and the cellular nucleic acid binding protein (CNBP), are able to bind in vitro a pyrimidine tract at the 5' end and a downstream region, respectively. These regions are considered the common cis-acting elements of translational regulation. It was previously observed that the binding of both these putative trans-acting factors to their RNA sequences is assisted by a protease-sensitive factor(s) that dissociates from the complex after its formation. Here we provide evidence that the requirement for an ancillary factor assisting La binding to the pyrimidine tract of ribosomal protein mRNAs is typical of this RNA, and secondly that it may involve an RNA recognition motif of the La protein not clearly characterized previously. We also show that the Ro60 autoantigen is involved in the common factor activity necessary for the binding of La and CNBP proteins to their respective sequences. In addition, our findings suggest that an RNA also participates in this process. We show that CNBP can multimerise and that it binds to the 5'UTR as a dimer. Both La and CNBP compete for the interaction with the factor, and their binding to the 5'UTR is mutually exclusive. Our results from the binding analysis of mutations in the 5'UTR, which are known to disrupt the translational control in vivo, suggest a model in which the protein interactions and the 5'UTR RNA structure may co-operate in regulating the translational fate of ribosomal protein mRNAs.