A signal sequence detection system using secreted protease activity as an indicator

We have developed a novel expression vector, pSSD1, to detect a cDNA fragment encoding a secretory signal sequence (Ss). This vector carries a cDNA fragment encoding the protease domain of human urokinase-type plasminogen activator (u-PA) as a reporter gene under the SV40 early promoter. We inserted cDNA fragments encoding various Ss between the promoter and the reporter gene of pSSD1. These plasmids were introduced into monkey COS7 cells to produce chimeric proteins. Only when the Ss was intact, was fibrinolytic activity detected in the culture medium of the transfected cells. Thus, this system may be useful for detecting the cDNAs encoding secreted proteins or type-I membrane proteins.