Cell cultures propagated from foetal bovine ligamentum nuchae synthesized and secreted two glycoproteins, designated MFP I and MFP II, that are closely related to elastic-fibre microfibrils. Glycoproteins MFP I (apparent mol.wt. 150 000) and MFP II (apparent mol.wt. 300 000) were metabolically labelled, separated from other culture-medium components by immunoprecipitation with a specific anti-(microfibrillar protein) serum, and analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and sodium dodecyl sulphate/gel-filtration chromatography. 2. Ligament cells also synthesized and secreted fibronectin, but salt-fractionation and immunoprecipitation studies with a specific anti-(cold-insoluble globulin) serum established that neither glycoprotein MFP I nor glycoprotein MFP II was related to fibronectin. 3. The secretion of glycoprotein MFP I, but not that of glycoprotein MFP II, was enhanced by the addition of ascorbate to the culture medium. 4. Ascorbate-supplemented ligament cells incorporated [3H]proline into glycoprotein MFP I, and 36% of the nondiffusible proline residues were hydroxylated, exclusively as 4-hydroxy[3H]proline. Less than 1% of the total proline residues in [3H]proline-labelled glycoprotein MFP II were hydroxylated. 5. Ascorbate-supplemented cells incorporated [14C]lysine into glycoprotein MFP I and 30% of the non-diffusible lysine residues were hydroxylated. 6. Newly secreted glycoprotein MFP I was digested by highly purified bacterial collagenase to yield polypeptide fragments of apparent mol.wts. 50 000 and 30 000. Glycoprotein MFP II was not digested by bacterial collagenase. 7. The results suggest that elastic-fibre microfibrils are composed of a novel collagenous glycoprotein MFP I in association, as yet undefined, with a non-collagenous glycoprotein MFP II.
