Four hypothetical ways to establish end-to-end interactions in an mRNA molecule. (A) Interaction “through solution”: The two ends are tethered to each other through the mRNA body, but otherwise move independently in solution. EIF4F is the complex containing the cap-binding protein eIF4E and the PABPC-interacting protein eIF4G. (B) Interaction by threading: A protein complex stably associated with the 3′-UTR contains a helicase that threads the RNA through itself until the 5′ end is reached. Arrows indicate the direction of RNA threading (blue) and helicase movement (brown). (C) Interaction by polymerization: A protein complex stably associated with the 3′-UTR nucleates the assembly of a protein coat along the mRNA. The arrow indicates the direction of polymerization. (D) Interaction by noncontinuous binding: A protein complex primarily associated with the 3′-UTR can grab neighboring segments of the same RNA without moving strictly along the backbone. Incorporation of additional subunits into the protein complex permits the engagement of additional RNA segments.
