Purification and characterization of an extracellular acid protease from Neurospora crassa

An extracellular acid protease was purified 1420-fold from sulfur-starved protein-induced cultures of Neurospora crassa. The enzyme was homogeneous as determined by polyacrylamide electrophoresis. The purification procedure consisted of an ultrafiltration step, cation-exchange chromatography, and affinity chromatography on Sepharose-linked pepstatin. The enzyme is homologous to aspartyl proteases that are characterized by pepstatin inhibition and trypsinogen activation. It is extremely autolytic, especially under denaturing conditions. The protease is stable between pH 3 and 7, showing optimal activity near pH 4.0 for both trypsinogen activation and hydrolysis of bovine serum albumin. The molecular weight of the enzyme was 34,500 by gel electrophoresis and gel filtration, and 34,975 by amino acid analysis.