Ep-CAM is a recently identified Ca2+-independent, homophilic cell-cell adhesion molecule of epithelial cells. This transmembrane glycoprotein has a molecular weight of only 40kD, which is unusually small for an adhesion protein. The extracellular domain of Ep-CAM contains two EGF-like domains followed by a cysteine-poor region- structural features that set Ep-CAM apart from the four main groups of cell adhesion molecules- cadherins, integrins, selectins and adhesion molecules of the immunoglobulin superfamily. Ep-CAM is abundantly expressed in simple, transitional and some pseudostratified epithelia, but not in squamous epithelia, where the de novo expression of this molecule is often associated with proliferative/neoplastic changes. Ep-CAM is not expressed in cell types of nonepithelial origin. The exact biological role of the Ep-CAM molecule is still unknown, but the evidence so far suggests involvement in regulation of cell-cell interactions, segregation of cell types, tissue pattern formation, and support of tissue architecture.
Ryudocan is a type I integral membrane heparan sulfate proteoglycan, which was originally cloned from rat microvascular endothelial cells as an important molecule maintaining blood fluidity. We purified rat ryudocan as well as rat syndecan, which had an apparent molecular sizes of 30kDa and 50kDa of the core proteins established by SDS gel electrophoresis, respectively. We cloned the cDNAs encoded the core proteins for rat ryudocan and rat syndecan. Both of the deduced amino acids residues of rat ryudocan and syndecan had homologous transmembrane and intracellular domains, but very distinct extracellular regions. Now, ryudocan is known to be one of the syndecan family members. We also cloned the human ryudocan cDNA, of which the gene localizes on the chromosome 20q 12. Purified endogenous ryudocan as well as expressed epitope-tagged ryudocan bore anticoagulantly active heparan sulfate (HSact) and/or inactive heparan sulfate (HSinact). HSact and HSinact have molecular sizes of about 25-30kDa with the same overall composition of monosaccharides except that HSact exhibits more glucuronsyl 3-O-sulfated glucosamines than HSinact. Increased intracellular levels of stably expressing epitope-tagged ryudocan probably act by saturating the capacity of components which regulate HSact production by coordinating the function of biosynthetic enzymes. Moreover, stably expressing epitope-tagged ryudocan showed the production of the following multiple isoforms: pure HS-ryudocan, various HS/CS-hybrids, and pure CS-ryudocan. The production of multiple isoforms of ryudocan may serve to expand the functional versatility of this cell surface component and allow it to participate in many different biological processes.
Plant starch can be distinguished from bacterial, fungal and animal glycogen by the simultaneous presence of at least 2 distinct fractions (amylose and amylopectin) whose relative arrangement inside a huge insoluble granule remains to be determined. Amylopectin, the major branched fraction is itself a highly organized molecule displaying a succession of clusters of glucans packed in crystal arrays. The unit cluster size (9nm) is a remarkably constant feature of all plants examined which in turn suggests the existence of a highly conserved and ordered biosynthetic pathway. This review after giving a brief overview on starch structure and metabolism will focuss on the building of the granule's architecture. The genetic and biochemical evidence summarized here all point to the presence of multiple elongation and branching enzymes who are only partly redundant in function for starch biosynthesis. We suggest that each enzyme is responsible for the building of specific granule substructures.
Integrins are a large family of transmembrane glycoproteins consisting of αβ heterodimers. Although the first members of this family were identified based on their ability to mediate cellular adhesion to components of the extracellular matrix, it is now clear that integrins can recognize a broad array of ligands including immunoglobulin family members, at least one member of the cadherin family, and probably other integrins. The functional significance of integrins binding to their ligands has now been extended well beyond simple cell adhesion with the recognition that integrins are true signaling receptors, inducing signals that can lead to numerous changes in cell behavior, including migration, proliferation, and induction of expression of a wide array of other cellular genes. At least 5 different integrins bind to the extracellular matrix protein, tenascin, at 3 distinct sites. Although the specific roles played by each of these integrins in cellular responses to tenascin have not been thoroughly investigated, preliminary evidence suggests that this structural diversity has functional significance. Because of the central role played by this protein family in development, inflammation, thrombosis, and tumorigenesis, integrins are among the most intensely studied proteins in biology.
The synthesis of the characteristic chondroitin sulfate glycosaminoglycan chains begins with the transfer of xylose from UDP-xylose to certain serine hydroxyl groups of the core protein acceptor, in a reaction catalyzed by xylosyltransferase. Several lines of evidence suggest that xylose addition may be a point of specific regulation of proteoglycan biosynthesis: the position of the reaction as the initial step in modification of the core protein; coordinate expression of core protein and xylosyltransferase; specific interaction of xylosyltransferase and galactosyltransferase I, the next enzyme in the pathway; feedback inhibition of UDP-xylose production; and necessity for traversing intracellular compartments between the synthesis of core protein and the addition of the rest of the glycosaminoglycan chain. This review focuses on the properties, behavior, and subcellular localization of xylosyltransferase and the reaction it catalyzes.