Yeast provide a useful model system to study the biosynthesis and processing of N-and O-linked glycoprotein glycans. The synthesis of Glc3Man9GlcNAc2-PP-dolichol, initial transfer of the tetradecasaccharides to asparagine residues in nascent proteins, and trimming of the three glucose residues are highly conserved events in yeast and mammalian cells. Whereas mammalian cells generally trim the original oligosaccharide to a trimannosyl core, which in the Golgi apparatus is elongated with varying amounts of GlcNAc, Gal, sialic acid, sulfate and fucose to form multiantennary “complex” glycans, yeast may trim only one mannose from the original oligosaccharide in the endoplasmic reticulum, then elongate the resultant high mannose oligosaccharide in the Golgi to “mannan”, a poly α1, 6-linked mannose backbone with α1, 2-and α1, 3-linked mannose side chains that may consist of 50 to 100 residues. In a significant departure from mammalian cells, where O-glycosylation of serine/threonine residues occurs wholly in the Golgi, in yeast this event is initiated in the endoplasmic reticulum with the transfer of a single mannose to acceptor amino acids. Subsequently, the O-linked mannose can be elongated in the yeast Golgi with one to four additional mannose residues. Although yeast and mammalian cells reveal significant differences in the latter stages of N-and O-linked glycan processing, the ability to utilize the genetic and biochemical approach in yeast has provided a paradigm for understanding both the specificity and complexity involved in glycoprotein metabolism.
Mucins are highly O-glycosylated glycoproteins implicated in the protection of cells from extracellular agents. Two general classes of mucins have been described: secreted and membrane. Most, but not all, mucin structures contain a central tandem repeat region which is rich in serine and threonine and is highly variable in size between different mucins; many also contain a cysteine-rich domain. Repeat sequences are often conserved within a single molecule, such as human MUC1 protein, but are poorly conserved between species, e.g. mouse and human MUC1 protein repeats. Many mucins are polymorphic due to variable numbers of repeats, and mucin transcripts are often heterogeneous. Although mucin expression is relatively tissue specific, some mucins, such as the MUC1 and MUC2 proteins, are found in multiple tissues. Moreover, a single tissue may express more than one mucin. Limited studies suggest that regulation of mucin expression is complex. Membrane mucins have been implicated in development and tumor progression, possibly by modulating cell-cell interactions. Some mucin cysteine-rich domains may also play a role in regulatingcell proliferation. Undoubtedly, future studies using recombinant DNA probes will greatly expand our understanding of these complex molecules.
CD44 is a widely distributed cell surface glycoprotein that has been shown to play an important role in a large number of adhesion-dependent cellular processes including lymphopoiesis and myelopoiesis, lymphocyte recirculation, tumour metastasis, and monocyte/macrophage activation. At present, however, the molecular mechanisms that regulate the functional activity and ligand binding specificity of this important molecule remain poorly defined. In particular, although CD44 has been shown to function as a major cell surface receptor for the glycosaminoglycan hyaluronan, not all CD44-positive cell lines are able to bind this particular ligand. As discussed in this review, several non-mutually exclusive regulatory mechanisms may be involved. Thus alternative splicing may generate a large number of differentially expressed CD44 isoforms with distinct ligand binding specificities. In addition, the functional activity of these various molecules may be further regulated via changes in their conformation and/or glycosylation, or by altering their distribution in the plane of the membrane.
Lignin-carbohydrate complexes (LCCs) are glycoconjugates in which hydrophobic lignin is chemically bound to hydrophilic polysaccharides in wood cell walls. Alkali-stable linkages in the amphipathic substances are one of the major origins of chromophoric substances remaining in kraft pulp. Because decolorization of the chromophore with chlorinated chemicals results in the production of toxic substances, microbial conversion of the chromogens has gathered much interest in this decade. However, the nature of lignin-carbohydrate complexes in the pulp has not been systematized and reactivities of LCC during the biobleaching have not been analyzed of a molecular level. Chemical analysis and microbial cleavage of the alkali-stable lignin-carbohydrate bonds are indispensable for environmentally safe paper making.