Musing on the structural organization of the exosome complex.

The exosome complex of 3'-->5' exoribonucleases functions in both the precise processing of 3' extended precursor molecules to mature stable RNAs and the complete degradation of other RNAs. Both processing and degradative activities of the exosome depend on additional cofactors, notably the putative RNA helicases Mtr4p and Ski2p. It is not known how these factors regulate exosome function or how the exosome distinguishes RNAs destined for processing events from substrates that are to be completely degraded. Here we review the available data concerning the modes of action of the exosome and relate these to possible structural arrangements for the complex. As no detailed structural data are yet available for the exosome complex, or any of its constituent enzymes, this discussion will rely heavily on rather speculative models.