| Abstract:Cullin-Ring-Ligases mediate protein polyubiquitination, a signal for degradation in the 26S-proteasome. The CRL1 class consists of Skp1/cullin-1/F-box protein/Rbx1 (SCF) complexes that cyclically associate with ubiquitin-E2 to build the polyubiquitin chain. Within the SCF complex, the 162-amino acid DdSkp1 from Dictyostelium bridges cullin-1 with an F-box protein (FBP), the specificity factor for substrate selection. The hydroxylation-dependent glycosylation of Pro143 of DdSkp1 by a pentasaccharide forms the basis of a novel O2-sensing mechanism in the social amoeba Dictyostelium and other protists. Previous evidence indicated that glycosylation promotes increased a-helical content correlating with enhanced interaction with three F-box proteins. To localize these differences, we used NMR methods to compare non-glycosylated DdSkp1 and a glycoform with a single GlcNAc sugar (Gn-DdSkp1). We report NMR resonance assignments of backbone 1HN, 15N, 13C? and 13CO nuclei, as well as side-chain 13C and methyl 13C/1H nuclei of Ile(d1), Leu, and Val in both unmodified DdSkp1 and Gn-DdSkp1. Random Coil Index (RCI) and 15N{1H}-HNOE indicate that the C-terminal region, which forms a helix-loop-helix centered on Pro143 at the crystallographically-defined binding interface with F-box domains, remains dynamic in both DdSkp1 and Gn-DdSkp1. Chemical shifts indicate that conformation variation in Gn-DdSkp1, relative to DdSkp1, is limited to this region, and characterized by increased helical fold. Extension of the glycan chain results in further changes, also limited to this region. Thus, glycosylation may control F-box protein interactions via a local effect on DdSkp1 conformation, by a mechanism that may be general to many unicellular eukaryotes.
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