In the chicken, three tenascin variants have been characterized that are generated by alternative splicing of 3 of its 11 fibronectin type III repeats. Using monoclonal antibodies that react with common regions versus extra repeats of tenascin, we could distinguish and separate tenascin variants and investigate their interaction with fibronectin using multiple experimental procedures. Interestingly, in all assays used the smallest tenascin variant bound more strongly to fibronectin than the larger ones. These biochemical data were paralleled by the observation that in chick embryo fibroblast cultures only the smallest form of tenascin could be detected in the fibronectin-rich extracellular matrix network laid down by the cells. Furthermore, each tissue present in adult chicken gizzard contained a distinct set of tenascin variants. Those tissues particularly rich in extracellular matrix, such as the tendon, contained the smallest tenascin only. Intermediate-sized tenascin was present in smooth muscle, whereas the largest form was exclusively detectable underneath the epithelial lining of the villi. Thus it appears that cell type-specific forms of tenascin exist that are appropriate for the functional requirements of the respective extracellular matrices.

The antigen recognized by U1 alpha, a monoclonal antibody to the alpha chain of a chicken integrin fibronectin receptor, was identified as alpha 5. It identifies the same polypeptide as antisera raised to a sequence from the alpha 5 cytoplasmic domain. The U1 alpha antibody has the unusual functional property for alpha chain antibodies of enhancing the binding of alpha 5 beta 1 for its ligand fibronectin. U1 alpha was used to examine the function of alpha 5 beta 1 during myogenic differentiation. As myogenic cells differentiated from replicating myoblasts to bipolar myocytes there was a decrease in their adhesion to the substrate caused by inactivation of alpha 5 beta 1, which could be reversed by treatment of the cells with U1 alpha. The U1 alpha induced increased adhesion to fibronectin but did not inhibit the differentiation process as measured by formation of myotubes. However, U1 alpha did interfere with both cell migration and morphogenesis of myotubes. The resulting myotubes were smaller, more branched, and showed less regular alignment of nuclei. The results suggest that the ability of the cell to regulate alpha 5 beta 1 affinity is critical to myogenic morphogenesis.

Cartilage matrix protein (CMP) exists as a disulfidebonded homotrimer in the matrix of cartilage. Each monomer consists of two CMP-A domains that are separated by an epidermal growth factor-like domain. A heptad repeat-containing tail makes up the carboxyl-terminal domain of the protein. The secreted form of CMP contains 12 cysteine residues numbered C1 through C12. Two of these are in each of the CMP-A domains, six are in the epidermal growth factor-like domain, and two are in the heptad repeat-containing tail. Two major categories of mutant CMPs were generated to analyze the oligomerization process of CMP: a mini-CMP and a heptadless full-length CMP. The mini-CMP consists of the CMP-A2 domain and the heptad repeat-containing tail. In addition, a number of mutations affecting C9 through C12 were generated within the full-length, the mini-, and the heptad-less CMPs. The mutational analysis indicates that the heptad repeats are necessary for the initiation of CMP trimerization and that the two cysteines in the heptad repeat-containing tail are both necessary and sufficient to form intermolecular disulfide bonds in either full-length or mini-CMP. The two cysteines within a CMP-A domain form an intradomain disulfide bond.The macromolecular composition of the matrix of cartilage results from the expression of a unique repertoire of genes by chondrocytes. The matrix macromolecules synthesized by the chondrocytes have multiple domains that permit interactions with other matrix molecules or with cell surface components. These complex interactions determine the structure and the integrity of cartilage. The major components of the cartilage extracellular matrix are collagens, proteoglycans, and noncollagenous proteins. Cartilage matrix protein (CMP) 1 is one of the most abundant noncollagenous extracellular proteins in cartilage (1, 2) and has been shown to associate with the cartilage collagen fibril that consists of collagen types II, IX, and XI (3) as well as with proteoglycans (1).The deduced amino acid sequence of CMP reveals that a CMP monomer is made up of a unique combination of structural domains (4 -6). Two highly homologous domains, CMP-A1 and CMP-A2, are separated from each other by a domain with homology to epidermal growth factor (EGF). The last domain is the carboxyl-terminal tail, which contains a series of heptad repeats (7). Each domain has significant sequence or structural homology to portions of other proteins. Homology to the CMP-A domains is found in soluble proteins including von Willebrand factor, the complement components C2 and B, matrix proteins such as collagen types VI, VII, XII, and XIV, undulin, transmembrane proteins such as the ␣-chains of the integrins VLA-1, VLA-2, LFA-1, Mac-1, p150,95, and a Caenorhabditis elegans protein involved in muscle attachment as well as the dihydropyridine-sensitive calcium channel and the inter-␣-trypsin inhibitor (reviewed in Refs. 8 and 9). The A domains of several proteins have been shown to bind extracellular matrix molecules such as collagen (10 -15)...