THE CYTOPLASMIC F-BOX BINDING-PROTEIN SKP1 CONTAINS A NOVEL PENTASACCHARIDE LINKED TO HYDROXYPROLINE IN DICTYOSTELIUM

SKP1 is involved in the ubiquitination of certain cell cycle and nutri tional regulatory proteins for rapid turnover. SKP1 from Dictyostelium has been known to be modified by an oligosaccharide containing Fuc an d Gal, which is unusual for a cytoplasmic or nuclear protein. To estab lish how it is glycosylated, SKP1 labeled with [H-3]FUC was purified t o homogeneity and digested with endo-Lys-C. A single radioactive pepti de was found after two-dimensional high performance Liquid chromatogra phy, Analysis in a quadrupole time-of-flight mass spectrometer reveale d a predominant ion with a novel mass. Tandem mass spectrometry analys is yielded a set of daughter ions which identified the peptide and sho wed that it was modified at Pro-143, A second series of daughter ions showed that Pro-143 was hydroxylated and derivatized with a potentiall y Linear pentasaccharide, Hex-->Hex-->Fuc-->Hex-->HexNAc-->(HyPro). Th e attachment site was confirmed by Edman degradation. Gas chromatograp hy-mass spectrometry analysis of trimethylsilyl-derivatives of overexp ressed SKP1 after methanolysis showed the HexNAc to be GlcNAc. Exoglyc osidase digestions of the glycopeptide from normal SKP1 and from a fuc osylation mutant, followed by matrix-assisted laser desorption time-of -flight mass spectrometry analysis, showed that the sugar chain consis ted of D-Galp alpha 1-->6-D-Galp alpha 1-->L-FUCp alpha 1-->2-D-Galp b eta 1-->3GlcNAc. Matrix-assisted laser-desorption time-of-flight mass spectrometry analysis of all SKP1 peptides resolved by reversed phase- high performance liquid chromatography showed that SKP1 was only parti ally hydroxylated at Pro-143 and that all hydroxylated SKP1 was comple tely glycosylated, Thus SKP1 is variably modified by an unusual linear pentasaccharide, suggesting the localization of a novel glycosylation pathway in the cytoplasm.