title: Sequence and structure evolved separately in a ribosomal ubiquitin variant.
abstract: Encoded by a multigene family, ubiquitin is expressed in the form of three precursor proteins, two of which are fusions to the ribosomal subunits S27a and L40. Ubiquitin assists in ribosome biogenesis and also functions as a post-translational modifier after its release from S27a or L40. However, several species do not conserve the ribosomal ubiquitin domains. We report here the solution structure of a distant variant of ubiquitin, found at the N-terminus of S27a in Giardia lamblia, referred to as GlUb(S27a). Despite the considerable evolutionary distance that separates ubiquitin from GlUb(S27a), the structure of GlUb(S27a) is largely identical to that of ubiquitin. The variant domain remains attached to S27a and is part of the assembled holoribosome. Thus, conservation of tertiary structure suggests a role of this variant as a chaperone, while conservation of the primary structure--necessary for ubiquitin's function as a post-translational modifier--is no longer required. Based on these observations, we propose a model to explain the origin of the widespread ubiquitin superfold in eukaryotes.
authors.affiliation: Department of Biology, Whitehead Institute for Biomedical Research and Massachusetts Institute of Technology, Cambridge, MA 02142, USA.
authors.forename: André
authors.initials: A
authors.lastname: Catic
authors.forename: Zhen-Yu J
authors.initials: ZY
authors.lastname: Sun
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Ratner
authors.forename: Shahram
authors.initials: S
authors.lastname: Misaghi
authors.forename: Eric
authors.initials: E
authors.lastname: Spooner
authors.forename: John
authors.initials: J
authors.lastname: Samuelson
authors.forename: Gerhard
authors.initials: G
authors.lastname: Wagner
authors.forename: Hidde L
authors.initials: HL
authors.lastname: Ploegh
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D019143:Evolution, Molecular; D016829:Giardia lamblia / Q000737:chemistry*; D006801:Humans; D013058:Mass Spectrometry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D010447:Peptide Hydrolases / Q000378:metabolism; D017510:Protein Folding; D017433:Protein Structure, Secondary; D012269:Ribosomal Proteins / Q000737:chemistry*; D016415:Sequence Alignment; D025801:Ubiquitin / Q000737:chemistry*
chemical_list: D012269:Ribosomal Proteins; D025801:Ubiquitin; C096227:ribosomal protein S27a; D010447:Peptide Hydrolases