Mini-plasmin found in the epithelial cells of bronchioles triggers infection by broad-spectrum influenza A viruses and Sendai virus

Extracellular cleavage of virus envelope fusion glycoproteins by host cellu lar proteases is a prerequisite for the infectivity of mammalian and nonpat hogenic avian influenza viruses, and Sendai virus. Here we report a proteas e present in the airway that, like tryptase Clara, can process influenza A virus haemagglutinin and Sendai virus envelope fusion glycoprotein. This pr otease was extracted from the membrane fraction of rat lungs, purified and then identified as a mini-plasmin. Mini-plasmin was distributed predominant ly in the epithelial cells of the upward divisions of bronchioles and poten tiated the replication of broad-spectrum influenza A viruses and Sendai vir us, even that of the plasmin-insensitive influenza A virus strain. In compa rison with plasmin, its increased hydrophobicity, leading to its higher loc al concentrations on membranes, and decreased molecular mass may enable min i-plasmin to gain ready access to the cleavage sites of various haemaggluti nins and fusion glycoproteins after expression of these viral proteins on t he cell surface. These findings suggest that mini-plasmin in the airway may play a pivotal role in the spread of viruses and their pathogenicity.