Proteolysis regulates exposure of the IIICS-1 adhesive sequence in plasma fibronectin.
The alternatively spliced type III connecting segment (IIICS) of fibronectin (Fn) contains an amino acid sequence, CS-1, which is recognized by the integrin receptor, alpha 4 beta 1. Plasma Fn inhibits alpha 4 beta 1-dependent binding of lymphocytes and monocytes to CS-1 containing Fn derivatives poorly, suggesting limited exposure of the CS-1 sequence in Fn. To test the availability of CS-1 in plasma Fn, an antibody was raised to the synthetic peptide CS-1. The CS-1 sequence was found to be minimally exposed in plasma Fn; and immobilization of Fn, a model of matrix deposition, caused only a modest increase in its exposure. Digestion of Fn with selected proteases, however, induced substantial expression of the CS-1 sequence. The acid protease cathepsin D generated fragments of 31-33.5 kDa from the COOH-terminal heparin-binding domain of Fn which possessed high immunoreactivity with anti-CS-1. Digestion of Fn with cathepsin B also resulted in the exposure of CS-1 sequence in a 140 kDa fragment. Although the digestion of Fn with neutral proteases (neutrophil elastase, cathepsin G, chymotrypsin, trypsin) generated fragments from the COOH-terminal heparin-binding domain of similar molecular weight as with cathepsin D, the exposure of CS-1 did not occur. Exposure of the CS-1 region by the cathepsins was supported by cell adhesion experiments; digestion of Fn with cathepsins D and B transformed inert plasma Fn to an effective inhibitor of adhesion of lymphoblastoid B and T cells (Ramos, Jurkat, Molt-4) to an immobilized CS-1 conjugate. These results suggest that exposure of the CS-1 sequence in plasma Fn by proteolysis with cathepsins D and B, enzymes implicated in several pathological processes, may serve a regulatory function in cell adhesion. The adhesive function of the CS-1 region in intact Fn appears to be suppressed by the native conformation of the molecule.
Ugarova
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TP
Joseph J. Jacobs Center for Thrombosis and Vascular Biology, Department of Molecular Cardiology, Cleveland Clinic Foundation, Ohio 44195, USA. ugarovt@cesmtp.ccf.org
Ljubimov
A V
AV
Deng
L
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Plow
E F
EF
D017398:Alternative Splicing; D000595:Amino Acid Sequence; D002448:Cell Adhesion / Q000235:genetics; D002460:Cell Line; D000939:Epitopes / Q000276:immunology; D005353:Fibronectins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D006868:Hydrolysis; D039041:Integrin alpha4beta1; D016023:Integrins / Q000276:immunology / Q000378:metabolism; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011487:Protein Conformation; D016201:Receptors, Lymphocyte Homing / Q000276:immunology / Q000378:metabolism
D000939:Epitopes; D005353:Fibronectins; D039041:Integrin alpha4beta1; D016023:Integrins; D010446:Peptide Fragments; D016201:Receptors, Lymphocyte Homing