In eukaryotes, the covalent attachment of ubiquitin chains directs substrates to the proteasome for degradation. Recently, ubiquitin-like modifications have also been described in the archaeal domain of life. It has subsequently been hypothesized that ubiquitin-like proteasomal degradation might also operate in these microbes, since all archaeal species utilize homologues of the eukaryotic proteasome. Here we perform a structural and biochemical analysis of a ubiquitin-like modification pathway in the archaeon Sulfolobus acidocaldarius. We reveal that this modifier is homologous to the eukaryotic ubiquitin-related modifier Urm1, considered to be a close evolutionary relative of the progenitor of all ubiquitin-like proteins. Furthermore we demonstrate that urmylated substrates are recognized and processed by the archaeal proteasome, by virtue of a direct interaction with the modifier. Thus, the regulation of protein stability by Urm1 and the proteasome in archaea is likely representative of an ancient pathway from which eukaryotic ubiquitin-mediated proteolysis has evolved.N.P.R. is grateful to the Department of Biochemistry and the Isaac Newton Trust (Trinity College, Cambridge) for supporting this study. The N.P.R. laboratory is funded by the Medical Research Council [Career Development Award G0701443], and also by an Isaac Newton Trust Research Grant. S.M.B. is supported by a BBSRC Doctoral Training Grant [RG53842]. L.P. and M.L.K were funded by a Wellcome Trust Senior Fellowship Award in Basic Biomedical Sciences [grant no. 08279/Z/07/Z]. S.V.A. was supported by intramural funds of the Max Planck Society.This is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/ncomms916
