Extracellular matrix macromolecules regulate cell shape through specific interactions with receptors: thus cell-matrix interactions have a significant role in directing cell growth, gene expression, and cell differentiation. Matrix proteins typically promote cell attachment, spreading, and cytoskeletal organization. In addition, there is a group of extracellular matrix proteins that are primarily anti-adhesive, rather than adhesive. These proteins, thrombospondin (TSP), tenascin, and SPARC, induce a loss of focal adhesions from spread endothelial cells with accompanying cytoskeletal changes characterized by a cortical redistribution of stress fibers and a dispersal of vinculin from adhesion plaques. The temporally- and spatially-restricted expression of these proteins in injured and developing tissues is consistent with their anti-adhesive properties. The interactions of these proteins with cells has potentially important consequences for cell migration, wound healing, and tumor cell metastasis. The anti-adhesive matrix proteins are thought to mediate these effects via interactions with receptors and generation of intracellular messengers rather than by physical disruption of cell-matrix interactions, as evidenced by the retention of the αvβ3 in adhesion plaques. In this review, the structural and functional characteristics of the anti-adhesive matrix proteins, their effects on adhesion and cytoskeletal organization, the active sites of these molecules and their putative receptors, and the influence of glycosaminoglycans on their activity will be discussed.
Shortly after their discovery in 1984, O-linked N-acetylglucosamine-modified proteins (O-GlcNAc glycoproteins) were shown to be important components of the nuclear pore complex (nucleoporins). This review discusses the isolation, characterization, and molecular cloning of these and related proteins from both vertebrates and yeast. Apart from the modification by O-GlcNAc, several nuclear pore complex proteins share (at least) two unique, repetitive sequence motifs, XFXFG and GLFG, which comprise the epitopes of some antinucleoporin antibodies. A brief overview covers the biosynthesis of O-GlcNAc glycoproteins including the characterization of two candidate enzymes which are responsible for O-GlcNAc addition and turnover. A summary of the data relevant to the biological significance of glycosylated nucleoporins is then given. Nuclear-reformation experiments, in vitro, have been used to study the roles of O-GlcNAc glycoproteins in nuclear pore structure and function. Although definitive data have not been obtained, requirements for particular glycoproteins (NUP62) in both nuclear pore structure and nucleocytoplasmic transport have been observed. This discussion is prefaced by a description of nuclear pore ultrastructure with particular emphasis on the localization of O-GlcNAc glycoproteins.
The FACIT family of collagen genes includes genes encoding the polypeptide subunits of types IX, XII, XIV, XVI, and XIX collagen. Studies of collagens IX, XII, and XIV indicate that they are associated with collagen fibrils and suggest that they modulate the biomechanical properties of tissues. For type IX collagen a direct linkage to type II-containing fibrils has been demonstrated, and studies of transgenic mice demonstrate that mutations in type IX collagen lead to degenerative changes in articular cartilage resembling osteoarthritis in humans. Type IX collagen genes are therefore candidates for genetic skeletal disorders that include early-onset osteoarthritis. Interestingly, one locus for multiple epiphyseal dysplasia has been linked to the α2(IX) collagen gene locus. The alternative use of promoters and alternative splicing provides a basis for considerable diversity among many members of the FACIT family. Some of these variants contain glycosaminoglycan side-chains and are therefore part-time proteoglycans.