In Vivo Association of the Stability Control Protein αCP with Actively Translating mRNAs

Abstract

Posttranscriptional controls play a major role in eucaryotic gene expression. These controls are mediated by sequence-specific interactions of cis-acting determinants in target mRNAs with one or more protein factors. The positioning of a subset of these mRNA-protein (RNP) complexes within the 3′ untranslated region (3′ UTR) may allow them to remain associated with the mRNA during active translation. Robust expression of human α-globin mRNA during erythroid differentiation has been linked to formation of a binary complex between a KH-domain protein, αCP, and a 3′ UTR C-rich motif. Detection of this “α-complex” has been limited to in vitro studies, and the functional state of the α-globin mRNA targeted by αCP has not been defined. In the present study we demonstrate that a significant fraction of αCP is associated with polysomal mRNA. Targeted analysis of the polysomal RNP complexes revealed that αCP is specifically bound to actively translating α-globin mRNA. The bound αCP is restricted to the poly(C)-rich 3′ UTR motif and is dislodged when ribosomes are allowed to enter this region. These data validate the general importance of the 3′ UTR as a sheltered site for RNP complexes and support a specific model in which the stabilizing function of αCP is mediated on actively translating target mRNAs.

ACKNOWLEDGMENTS

This work was supported by NIH grants HL 65449 and CA72765 to S.A.L. and by the generous support of the Doris Duke Foundation.