Fibulins are a family of five extracellular matrix proteins characterized by tandem arrays of epidermal growth factor-like domains and a C-terminal fibulin-type module. They are widely distributed and often associated with vasculature and elastic tissues. In this study, we expressed the three more recently identified family members, fibulin-3, fibulin-4, and fibulin-5, as recombinant proteins in mammalian cells. The purified proteins showed short rod structures of ϳ20 nm with a globule at one end, after rotary shadowing and electron microscopy. Two forms of mouse fibulin-3 were purified, and the O-glycan profiles of the larger form were characterized. Polyclonal antibodies raised against the purified proteins did not show any crossreactivity with other family members and were used to assess the levels and localization of the fibulins in mouse tissues. Their binding interactions, cell adhesive properties, and tissue localization were analyzed in parallel with the previously characterized fibulin-1 and -2. Binding to tropoelastin was strong for fibulin-2 and -5, moderate for fibulin-4 and -1, and relatively weak for fibulin-3. Fibulin-4, but not fibulin-3 and -5, exhibited distinct interactions with collagen IV and nidogen-2 and moderate binding to the endostatin domain from collagen XV. Cell adhesive activities were not observed for all fibulins, except mouse fibulin-2, with various cell lines tested. All five fibulins were found in perichondrium and various regions of the lungs. Immunoelectron microscopy localized fibulin-4 and -5 to fibrillin microfibrils at distinct locations. Our studies suggest there are unique and redundant functions shared by these structurally related proteins.Fibulins are a family of extracellular glycoproteins with distinctive features of a fibulin-type C-terminal domain preceded by tandem calcium-binding (cb) 4 epidermal growth factor (EGF)-like modules (1-3). The five-member family can be further classified into two subgroups. Fibulin-1 and -2, the first subgroup, are substantially larger than the other three members of the family because of the presence of an extra domain with three anaphylatoxin modules and higher numbers of cbEGF modules (see Fig. 1). Fibuin-1 at 90 -100 kDa has variable C-terminal domains. Two major splice variants, fibulin-1C and -1D, are present in approximately equal amounts in most tissues of all animal species studied to date. Fibulin-2 at 200 kDa is the largest of all the fibulins, because it possesses an additional N-terminal domain of ϳ400 amino acids not found in other fibulins. Members of the second subgroup, fibulin-3, -4, and -5, are similarly small in size (50 -60 kDa) and highly homologous to one another in modular structure. They consist of a modified cbEGF domain at the N terminus followed by five tandem cbEGF modules and the fibulin-type C-terminal region (Fig. 1).Fibulin-1 and -2, the first subgroup, have been characterized extensively and shown to display distinct yet overlapping molecular interactions and expression patterns. Both proteins ar...
The N-terminal sequences of mouse laminin alpha3B and alpha5 chains have been completed and demonstrate the presence of a signal peptide followed by a complete laminin N-terminal (LN) module (domain VI). These signal peptides were released after recombinant production of larger fragments comprising domains VI/V (45-65 kDa) from this region yielding properly folded proteins, which were secreted from HEK-293-EBNA cells. Pepsin digestion of these fragments yielded products of 25-35 kDa, which consisted only of domain V. The alphaVI/V fragments were able to inhibit self-assembly of laminin-1, with those from the alpha3B and alpha5 chains being more active than those from alpha1 and alpha2 chains. Domain V fragments, however, showed a reduced activity, indicating the major contribution of the LN module in inhibition. These interactions were confirmed by surface-plasmon-resonance assays demonstrating moderate affinities (K(d)=0.02 to >6 microM) for the binding to laminin-1. This indicated that laminins containing alpha3B or alpha5 chains should also be able to form non-covalent networks by polymerization. The LN modules also showed heparin binding in affinity chromatography, which was strongest for alpha1/alpha2, moderate for alpha3B, whereas no binding was observed for alpha5. They all bound to heparan sulphate chains of perlecan and to sulphatides, with a lower variability in binding activity. Specific antibodies were raised against alpha3BVI/V and alpha5VI/V and were shown to stain basement membrane zones in various mouse tissues. These antibodies also allowed the identification of a new laminin assembly form 5B consisting of alpha3B, beta3 and gamma2 chains.
The N-terminal sequences of mouse laminin α3B and α5 chains have been completed and demonstrate the presence of a signal peptide followed by a complete laminin N-terminal (LN) module (domain VI). These signal peptides were released after recombinant production of larger fragments comprising domains VI\V (45-65 kDa) from this region yielding properly folded proteins, which were secreted from HEK-293-EBNA cells. Pepsin digestion of these fragments yielded products of 25-35 kDa, which consisted only of domain V. The αVI\V fragments were able to inhibit self-assembly of laminin-1, with those from the α3B and α5 chains being more active than those from α1 and α2 chains. Domain V fragments, however, showed a reduced activity, indicating the major contribution of the LN module in inhibition. These interactions were confirmed by surface-plasmon-resonance assays demonstrating moderate affinities (K d l 0.02 to 6 µM)
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