Translocation portals for the substrates and products of a viral transcription complex: the bluetongue virus core

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Abstract
The bluetongue virus core is a molecular machine that simultaneously and repeatedly transcribes mRNA from 10 segments of viral double‐stranded RNA, packaged in a liquid crystalline array. To determine how the logistical problems of transcription within a sealed shell are solved, core crystals were soaked with various ligands and analysed by X‐ray crystallography. Mg2+ ions produce a slight expansion of the capsid around the 5‐fold axes. Oligonucleotide soaks demonstrate that the 5‐fold pore, opened up by this expansion, is the exit site for mRNA, whilst nucleotide soaks pinpoint a separate binding site that appears to be a selective channel for the entry and exit of substrates and by‐products. Finally, nucleotides also bind to the outer core layer, providing a substrate sink.