| Popis: |
Although we have known for many years that most cell surface and extracellular proteins are glycosylated, only recently have we come to appreciate that most proteins within the nucleus and cytoplasm are also dynamically modified by the addition and removal of saccharides [1]. Indeed, in eukaryotes most polypeptides are glycosylated. Extracellular protein-bound glycans are generally complex and large, whereas cytosolic and nuclear glycans are often modified by simple monosaccharides [2]. Each unique type of protein glycosylation presents special challenges to the structural analyses or identification of glycoproteins by mass spectrometry (MS)[3–9]. MS analyses of extracellular or cell-surface glycoproteins are complicated by the enormous structural diversity of the glycan side chains, by their large size, by the astonishing site-specific structural variability of glycans, and by the fact that many component monosaccharides have the same mass. Mass spectrometric analysis of O-G1cNAc-bearing nuclear and cytoplasmic glycoproteins is confounded by the highly-dynamic nature of the modification [10], causing sub-stoichiometric levels at single sites, by the inherent insensitivity of the method to glycopeptides as compared to unmodified peptides, and most importantly, by the lability of the saccharide linkage under most MS analytical conditions [11, 12]. Nonetheless, mass spectrometry of all types is the most powerful tool currently available to the glycoscientist interested in the structure/functions of posttranslationally modified proteins as they actually occur in biological systems. |
