Abstract
La is an abundant, mostly nuclear, RNA-binding protein that interacts with regions rich in pyrimidines. In the nucleus it has a role in the metabolism of several small RNAs. A number of studies, however, indicate that La protein is also implicated in cytoplasmic functions such as translation. The association of La in vivo with endogenous mRNAs engaged with polysomes would support this role, but this point has never been addressed yet. Terminal oligopyrimidine (TOP) mRNAs, which code for ribosomal proteins and other components of the translational apparatus, bear a TOP stretch at the 5' end, which is necessary for the regulation of their translation. La protein can bind the TOP sequence in vitro and activates TOP mRNA translation in vivo. Here we have quantified La protein in the cytoplasm of Xenopus oocytes and embryo cells and have shown in embryo cells that it is associated with actively translating polysomes. Disruption of polysomes by EDTA treatment displaces La in messenger ribonucleoprotein complexes sedimenting at 40-60 S. The results of polysome treatment with either low concentrations of micrococcal nuclease or with high concentrations of salt indicate, respectively, that La association with polysomes is mediated by mRNA and that it is not an integral component of ribosomes. Moreover, the analysis of messenger ribonucleoprotein complexes dissociated from translating polysomes shows that La protein associates with TOP mRNAs in vivo when they are translated, in line with a positive role of La in the translation of this class of mRNAs previously observed in cultured cells.
Highlights
Associated with newly synthesized RNA polymerase III transcripts and is implicated in the termination and initiation of transcription by this polymerase [11,12,13,14,15], in tRNA processing [16, 17] and in transport and nuclear retention of some polymerase III transcripts (18 –20)
Evaluation of Cytoplasmic La in Xenopus Cells—Since we were interested in a cytoplasmic function of La, we wanted first to have an idea of the amount of La in the cytoplasm and determine whether this was sufficient for binding Terminal oligopyrimidine (TOP) mRNAs
The amount of La protein found in a total oocyte extract corresponds to that present in the equivalent amount of nuclear plus cytoplasmic extract (Fig. 1B, lanes 1, 3, and 4), confirming that most of the protein is localized in the nucleus
Summary
An extensive in vitro binding analysis was carried out using normal and mutated forms of the 5Ј-UTR and of the TOP sequence [6, 7, 34, 35], some of which are known to disrupt the control in vivo [32]. The outcome of these studies lead to the identification of two proteins: La and the cellular nucleic acidbinding protein (CNBP). We have confirmed an association of La protein with TOP mRNAs in vivo, in agreement with the previous in vitro binding data and with the observed positive role in their translation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
