Resolution of fibronectin and other uncharacterized proteins by two-dimensional polyacrylamide electrophoresis with thiourea

Several proteins, which are recognized components of serum, are not resolved by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) under standard conditions. One major example is fibronectin, which is detected in fairly high concentration (milligram range) by immunoassays, while undetectable in 2D-PAGE gels. Following several experiments with a combination of zwitterionic and chaotropic substances we obtained a good resolution of the protein in gels containing 0.5 M thiourea plus 8 M urea. By this technique, fibronectin was, for the first time, found to be microheterogeneous between pI values of 5.3 and 5.6. Besides fibronectin we detected three other families of uncharacterized proteins with M(r) of 130,000, 110,000 and 34,000 respectively, whose identity and function are currently under investigation.