Selection of cDNAs encoding putative type II membrane proteins on the cell surface from a human full-length cDNA bank

We have developed a simple method to test whether a hydrophobic segment near the N-terminus of a protein functions as a type II signal anchor (SA) in which the N-terminus faces the cytoplasm. A cDNA fragment containing the putative SA sequence of a target clone was fused in-frame to the 5' end of a cDNA fragment encoding the protease domain of urokinase-type plasminogen activator (u-PA). The resulting fused gene was expressed in COS7 cells. Fibrinolytic activity on the cell surface was measured by placing a fibrin sheet in contact with the transfected COS7 cells after removing the medium. When the cDNA fragment encoded a SA, the fibrin sheet was lysed by the u-PA expressed on the cell surface. The fibrinolytic activity was not detected in the culture medium, suggesting that the u-PA remains on the cell surface anchored via the SA in the membrane without being cleaved by signal peptidase. This fibrin sheet method was successfully applied to select five novel cDNA clones encoding putative type II membrane proteins from a human full-length cDNA bank.