Fibrillin-1 and fibrillin-2 constitute the backbone of extracellular filaments, called microfibrils. Fibrillin assembly involves complex multistep mechanisms to result in a periodical head-to-tail alignment in microfibrils. Impaired assembly potentially plays a role in the molecular pathogenesis of genetic disorders caused by mutations in fibrillin-1 (Marfan syndrome) and fibrillin-2 (congenital contractural arachnodactyly). Presently, the basic molecular interactions involved in fibrillin assembly are obscure. Here, we have generated recombinant full-length human fibrillin-1, and two overlapping recombinant polypeptides spanning the entire human fibrillin-2 in a mammalian expression system. Characterization by gel electrophoresis, electron microscopy after rotary shadowing, and reactivity with antibodies demonstrated correct folding of these recombinant polypeptides. Analyses of homotypic and heterotypic interaction repertoires showed N-to C-terminal binding of fibrillin-1, and of fibrillin-1 with fibrillin-2. The interactions were of high affinity with dissociation constants in the low nanomolar range. However, the Nand C-terminal fibrillin-2 polypeptides did not interact with each other. These results demonstrate that fibrillins can directly interact in an N-to C-terminal fashion to form homotypic fibrillin-1 or heterotypic fibrillin-1/ fibrillin-2 microfibrils. This conclusion was further strengthened by double immunofluorescence labeling of microfibrils. In addition, the binding epitopes as well as the entire fibrillin molecules displayed very stable properties.

Fibrillin-1 is a major constituent of the 10 -12 nm extracellular microfibrils. Here we identify, characterize, and localize heparin/heparan sulfate-binding sites in fibrillin-1 and report on the role of such glycosaminoglycans in the assembly of fibrillin-1. When heparin or heparan sulfate was added to cultures of skin fibroblasts, the assembly of fibrillin-1 into a microfibrillar network was significantly reduced. Western blot analysis demonstrated that this effect was not due to a reduced amount of fibrillin-1 secreted into the culture medium. Inhibition of the attachment of glycosaminoglycans to core proteins of proteoglycans by ␤-Dxylosides resulted in a significant reduction of the fibrillin-1 network. These studies suggest that binding of fibrillin-1 to proteoglycan-associated heparan sulfate chains is an important step in the assembly of microfibrils.

Fibrillin-containing microfibrils in elastic and nonelastic extracellular matrices play important structural and functional roles in various tissues, including blood vessels, lung, skin, and bone. Microfibrils are supramolecular aggregates of several protein and nonprotein components. Recently, a large region in the N-terminal portion of fibrillin-1 was characterized as a multifunctional protein interaction site, including binding sites for fibulin-2 and -5 among others. Using a panel of recombinant fibrillin-1 swapped domain and deletion fragments, we demonstrate here that the conserved first hybrid domain in fibrillin-1 is essential for binding to fibulin-2, -4, and -5. Fibulin-3 and various isoforms of fibulin-1 did not interact with fibrillin-1. Although the first hybrid domain in fibrillin-1 is located in close vicinity to the self-assembly epitope, binding of fibulin-2, -4, and -5 did not interfere with self-assembly. However, these fibulins can associate with microfibrils at various levels of maturity. Formation of ternary complexes between fibrillin-1, fibulins, and tropoelastin demonstrated that fibulin-2 and -5 but much less fibulin-4, are able to act as molecular adaptors between fibrillin-1 and tropoelastin.The microfibril/elastic fiber system provides tissues, such as lung, blood vessels, and skin, with elastic properties. Microfibrils with a diameter of 10 -12 nm are typically located on the outer surface of elastic fibers and are thought to play an essential role in elastogenesis (1). Whereas elastic fibers are always associated with microfibrils, microfibrils themselves can occur in the absence of elastin in certain tissues such as ocular ciliary zonules, the kidney, or in close proximity to various basement membranes. The microfibril/elastic fiber system is a multicomponent assembly in the extracellular matrix, and for both, the microfibrils and the elastic fibers, a number of constituents have been described (for a review, see Ref 2). For most of the associated molecules, the exact relationship in terms of physical interaction with microfibrils and/or elastic fibers, and their functional relevance is not clear.The best described components of the microfibrils are a family of proteins consisting of three highly homologous members, fibrillin-1, -2, and -3 (3-9). Fibrillins, like many other extracellular glycoproteins, are characterized by a number of tandemly arranged domains. The most prominent domain is an epidermal growth factor-like domain (EGF), 2 which occurs 46 -47 times in fibrillins. These domains are stabilized by three intramolecular disulfide bonds, and the majority (42-43 domains) contain a consensus sequence for calcium binding (cbEGF) (10 -12). The tandemly arranged EGF and cbEGF domains are interspersed by two other types of domains, the transforming growth factor ␤-binding protein (TB) or 8-Cys domains and the hybrid domains. The seven TB/8-Cys domains are characterized by four intramolecular disulfide bonds, and a similar arrangement is predicted for the two hybrid domains, alth...

Mutational defects in fibrillin-rich microfibrils give rise to a number of heritable connective tissue disorders, generally termed microfibrillopathies. To understand the pathogenesis of these microfibrillopathies, it is important to elucidate the supramolecular composition of microfibrils and their interaction properties with extracellular matrix components. Here we demonstrate that the proteoglycan perlecan is an associated component of microfibrils typically close to basement membrane zones. Double immunofluorescence studies demonstrate colocalization of fibrillin-1, the major backbone component of microfibrils, with perlecan in fibroblast cultures as well as in dermal and ocular tissues. Double immunogold labeling further confirms colocalization of perlecan to microfibrils in various tissues at the ultrastructural level. Extraction studies revealed that perlecan is not covalently associated with microfibrils. High affinity interactions between fibrillin-1 and perlecan were found by kinetic binding studies with dissociation constants in the low nanomolar range. A detailed mapping study of the interaction epitopes by solid phase binding assays primarily revealed interactions of perlecan domains I and II with a central region of fibrillin-1. Analysis of perlecan null embryos showed less microfibrils at the dermal-epidermal junction as compared with wild-type littermates. The data presented indicate a functional significance for perlecan in anchoring microfibrils to basement membranes and in the biogenesis of microfibrils.

Mutations in fibrillin-1 lead to Marfan syndrome and some related genetic disorders. Many of the more than 600 mutations currently known in fibrillin-1 eliminate or introduce cysteine residues in epidermal growth factor-like modules. Here we report structural and functional consequences of three selected cysteine mutations (R627C, C750G, and C926R) in fibrillin-1. The mutations have been analyzed by means of recombinant polypeptides produced in mammalian expression systems. The mRNA levels for the mutation constructs were similar to wild-type levels. All three mutated polypeptides were secreted by embryonic kidney cells (293) into the culture medium. Purification was readily feasible for mutants R627C and C750G, but not for C926R, which restricted the availability of this mutant polypeptide to selected analyses. The overall folds of the mutant polypeptides were indistinguishable from the wild-type as judged by the ultrastructural shape, CD analysis, and reactivity with a specific antibody sensitive for intact disulfide bonds. Subtle structural changes caused by R627C and C750G, however, were monitored by proteolysis and heat denaturation experiments. These changes occurred in the vicinity of the mutations either as short range effects (R627C) or both short and long range effects (C750G). Enhanced proteolytic susceptibility was observed for R627C and C750G to a variety of proteases. These results expand and further strengthen the concept that proteolytic degradation of mutated fibrillin-1 might be an important potential mechanism in the pathogenesis of Marfan syndrome and other disorders caused by mutations in fibrillin-1.