Interaction of the universal mRNA-binding protein, p50, with actin: a possible link between mRNA and microfilaments

We have shown previously that p50 is the most abundant protein associated w ith a variety of eukaryotic mRNAs and exhibits about 98% amino acid sequenc e identity to mammalian Y-box binding transcription factors. The dual funct ion of p50 in the cell as a regulator of both transcription and translation has been suggested. To gain insight into the role of p50 in these processe s, we performed the yeast two-hybrid screen to identify p50 molecular partn ers, Here we report the identification of actin as a p50-interacting protei n. Coimmunoprecipitation of p50 and actin from HeLa extracts as well as in vitro binding studies indicate specificity and a high affinity for the inte raction between p50 and actin, Interestingly, p50 binding to actin is affec ted by mRNA; binding was observed at a low p50/mRNA ratio and was greatly r educed at higher ratios. Since the p50/mRNA ratio appears to be important f or mRNA translatability, we speculate that p50 can regulate the attachment of mRNA to the actin network depending on its translational activity, Using immunofluorescence, we show that p50 binds to actin filaments in permeabil ized cells and causes actin fibers to bundle in vitro. Together, these find ings support the view that p50 may play an important role in mRNA transport , anchoring, and localization on actin filaments in the cell.