Assembly of the ribonucleoprotein complex containing the mRNA of the β-subunit of the mitochondrial H+-ATP synthase requires the participation of two distal cis-acting elements and a complex set of cellular trans-acting proteins

The mRNA encoding the β-subunit of the mitochondrial H⁺-ATP synthase (β-F1-ATPase) is localized in an approx. 150nm structure of the hepatocyte of mammals. In the present study, we have investigated the cis- and trans-acting factors involved in the generation of the ribonucleoprotein complex containing β-F1-ATPase mRNA. Two cis-acting elements (β1.2 and 3′β) have been identified. The β1.2 element is placed in the open reading frame, downstream of the region encoding the mitochondrial pre-sequence of the protein. The 3′β element is the 3′ non-translated region of the mRNA. Complex sets of proteins from the soluble and non-soluble fractions of the liver interact with the β1.2 and 3′β elements. A soluble p88, present also in reticulocyte lysate, displays binding specificity for both the cis-acting elements. Sedimentation and high-resolution in situ hybridization experiments showed that the structure containing the rat liver β-F1-ATPase mRNA is found in fractions of high sucrose concentration, where large polysomes sediment. Treatment of liver extracts with EDTA promoted the mobilization of β-F1-ATPase mRNA to fractions of lower sucrose concentration, suggesting that the structure containing β-F1-ATPase mRNA is a large polysome. Finally, in vitro reconstitution experiments with reticulocyte lysate, using either the full-length, mutant or chimaeric versions of β-F1-ATPase mRNA, reveal that the assembly of the β-F1-ATPase mRNA polysome requires the co-operation of both the cis-acting mRNA determinants. The present study illustrates the existence of an intramolecular RNA cross-talking required for the association of the mRNA with the translational machinery.