Extracellular matrix microfibrils are composed of core proteins coated with fibronectin.

Extracellular proteins of cultured calf aortic smooth muscle cells consist predominantly of microfibrils 10-20 nm in diameter typical of "elastin-associated" microfibrils described in many tissues. Chemical and immunochemical evidence is presented that microfibrils consist of at least two proteins: core protein and fibronectin. Insoluble proteins of the microfibrils were obtained in the form of a pellet and antibodies raised in rabbits against these components. The antisera reacted with the insoluble microfibrillar proteins and with soluble fibronectin in enzyme-linked immunosorbent assay, and immunostained the extracellular microfibrils in cultured cells. An immunoglobulin (Ig) fraction was prepared and absorbed with fibronectin. The absorbed IgG retained its reactivity with the microfibrillar proteins but was no longer reactive with soluble fibronectin. Immunofluorescence studies were carried out using the absorbed IgG and IgG to soluble fibronectin. Both antibodies showed immunoreactive microfibrils in the extracellular matrix of cells in log phase. However, with increasing time in culture, as the cells reached confluence, the immunofluorescence of microfibrils reacting with the absorbed IgG became less intense, whereas that of microfibrils reacting with IgG to fibronectin increased; in confluent cells, essentially no staining was detected with the absorbed IgG, and a dense network of intensely stained microfibrils was seen with IgG to fibronectin. Treatment of these cultures with urea led to partial dissociation of the fibronectin and increased visualization of the microfibrils with the absorbed IgG; double-label immunofluorescence showed that both proteins occurred on the same microfibrils. The localization of immunoreactive sites to the extracellular microfibrils was confirmed by immunoelectron microscopy. Nearly quantitative cleavage with CNBr failed to dissociate the antigenically active fragments of fibronectin from the CNBr fragments of the core proteins of the microfibrils. It was concluded that microfibrils contain core proteins and fibronectin that are codistributed in insoluble, possibly covalently cross-linked, aggregates. The core proteins are first deposited by the cell and, as a function of time in culture, fibronectin gradually coats their surface.