Recently, reports on the synthesis of homogeneous glycoproteins are increasing. These achievements are based on the progress in the preparation method of carbohydrate chains and of polypeptides. Here we discuss about the results of these glycoprotein synthesis, mainly based on the peptide science point of view.
Invariant natural killer (NK) T cells bridge innate and acquired immunity and play an important role in both protective and regulatory responses. NKT cell responses are induced by recognition of a complex of glycolipid antigens with CD1d, an antigen presenting protein of the immune system, with its invariant T cell receptors. KRN7000, an α-galactosyl ceramide developed by KIRIN Brewery Co. in 1995, stimulates NKT cells strongly to induce the production of a large amount of cytokines, and is used as a standard reagent for a positive control of NKT cell studies. At the beginning, KRN7000 was developed as an anticancer drug candidate. However, it induces both helper T (Th)1-type cytokines, which induce immunostimulatory activity, and Th2-type ones, which induce immunosuppressive activities, in large quantities at the same time. Its therapeutical use, therefore, is limited. Fifteen years has passed since the discovery of KRN7000, and a number of analogues of KRN7000 have been developed worldwide to solve the above problem. Many researchers are also trying to understand how NKT cells recognize the structure of glycolipids and induce cytokines. In this minireview, we discuss the structure-activity relationship studies on novel glycolipids which stimulate NKT cells efficiently.
Nascent polypeptides emerging into the lumen of the endoplasmic reticulum (ER) undergo high-mannose-type glycan modifications to the asparagines in their Asn-Xaa-Ser/Thr motifs. The processing of these oligosaccharides has been shown to play important roles in protein quality control. This process is highly complex, involving a number of lectins, chaperones, and glycan-processing enzymes. For example, calnexin (CNX) and calreticulin (CRT) are molecular chaperones that recognize monoglucosylated high-mannose-type glycans. Glucosidase II has two independent Glc-trimming activities, which participate in glycoprotein delivery to CNX/CRT to modify their conformations and glycoprotein recovery from the molecular chaperones. The protein folding sensor enzyme UDP-glucose : glycoprotein glucosyltransferase (UGGT) specifically glucosylates high-mannose-type glycans attached to partially folded proteins to deliver the candidate glycoprotein to CNX/CRT. Although recent studies have clarified the properties of these proteins, most of them used oligosaccharides derived from natural sources, which show structural heterogeneity. In order to gain a more precise understanding of this process, we attempted to synthesize high-mannose-type glycans associated with the protein quality control system. Additionally, an investigation using artificial glycoproteins led us to discover the first non-peptidic substrate of UGGT. These synthetic oligosaccharide probes have allowed us to conduct quantitative evaluations of the activity and specificity of UGGT, glucosidase II, and CRT.