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Fibrillin, a new 350-kD glycoprotein, is a component of extracellular microfibrils
Abstract:
A new connective tissue protein, which we call fibrillin, has been isolated from the medium of human fibroblast cell cultures. Electrophoresis of the disulfide bond-reduced protein gave a single band with an estimated molecular mass of 350,000 D. This 350-kD protein appeared to possess intrachain disulfide bonds. It could be stained with periodic acid-Schiff reagent, and after metabolic labeling, it contained [3H]glucosamine. It could not be labeled with [35S]sulfate. It was resistant to digestion by bacterial collagenase. Using mAbs specific for fibrillin, we demonstrated its widespread distribution in the connective tissue matrices of skin, lung, kidney, vasculature, cartilage, tendon, muscle, cornea, and ciliary zonule. Electron microscopic immunolocalization with colloidal gold conjugates specified its location to a class of extracellular structural elements described as microfibrils. These microfibrils possessed a characteristic appearance and averaged 10 nm in diameter. Microfibrils around the amorphous cores of the elastic fiber system as well as bundles of microfibrils without elastin cores were labeled equally well with antibody. Immunolocalization suggested that fibrillin is arrayed periodically along the individual microfibril and that individual microfibrils may be aligned within bundles. The periodicity of the epitope appeared to match the interstitial collagen band periodicity. In contrast, type VI collagen, which has been proposed as a possible microfibrillar component, was immunolocalized with a specific mAb to small diameter microfilaments that interweave among the large, banded collagen fibers; it was not associated with the system of microfibrils identified by the presence of fibrillin.
Insights
Researchers discovered a new connective tissue protein, fibrillin, crucial for microfibrils in various human tissues. This protein is essential for the structural integrity of elastic fibers and other extracellular matrix components.
Area of Science:
- Biochemistry
- Cell Biology
- Connective Tissue Research
Background:
- Connective tissues rely on complex extracellular matrix (ECM) proteins for structural integrity.
- The composition and organization of microfibrils, key ECM components, are not fully understood.
- Previous hypotheses suggested type VI collagen as a microfibrillar component.
Purpose of the Study:
- To identify and characterize novel proteins involved in connective tissue structure.
- To determine the precise localization and function of newly identified ECM proteins within various tissues.
- To differentiate the roles of fibrillin and type VI collagen in microfibril formation.
Main Methods:
- Isolation and purification of a novel protein from human fibroblast cell cultures.
- Biochemical characterization including electrophoresis, molecular mass determination, and disulfide bond analysis.
- Metabolic labeling to assess glycosylation and resistance to bacterial collagenase digestion.
- Immunolocalization using monoclonal antibodies (mAbs) and electron microscopy with colloidal gold conjugates.
Main Results:
- A 350-kD glycoprotein, named fibrillin, was isolated and characterized.
- Fibrillin is widely distributed in connective tissue matrices of skin, lung, kidney, vasculature, cartilage, tendon, muscle, cornea, and ciliary zonule.
- Immunolocalization confirmed fibrillin's presence within extracellular microfibrils (10 nm diameter), distinct from type VI collagen-associated microfilaments.
- Fibrillin showed periodic arrangement along microfibrils, potentially aligning with interstitial collagen periodicity.
- Microfibrils associated with elastic fibers and those without elastin cores were equally labeled, indicating fibrillin's broad role.
Conclusions:
- Fibrillin is a major component of connective tissue microfibrils.
- Fibrillin plays a significant role in the structural organization of the extracellular matrix, independent of elastin.
- The findings distinguish fibrillin from type VI collagen, clarifying their respective roles in microfibril and microfilament structures.