Molecular analysis of aminopeptidase pumAPE from Ustilago maydis encoded by APEum gene: enzyme purification and differential expression

Heterobasidiomycete Ustilago maydis is a dimorphic phytopathogenic fungus, causal agent of corn smut, a widespread disease. Recently, proteolytic system of this fungus was described and an aminopeptidase activity, probably involved in pathogenicity, was detected. The aminopeptidase pumAPE was purified from the haploid phase of U. maydis FB1 strain. The purification procedure consisted of ammonium sulphate fractionation and three chromatographic steps, resulting in a 23% recovery. The molecular mass of the dimeric enzyme was estimated to be 110 kDa and 58 kDa by gel filtration chromatography and SDS-PAGE respectively. Enzymatic activity was optimal at pH 7.0 and 35 ºC toward Lys-pNA and the pI was determined to be 5.1. The enzyme was inhibited by EDTA-Na2, 1,10-phenanthroline, bestantin, PMSF and several divalent cations (Cu2+, Hg2+ and Zn2+). The aminopeptidase exhibited a higher specificity for substrates with lysine and arginine in the N-position. The Km value was 54.4 μM and the Vmax value was 408 μmol min-1 mg-1 for Lys-pNA. A pair of primers was designed in order to amplify the gene APEum encoding this activity. In order to determine the number of copies in the genome, a APEum gene fragment was used as probe in a Southern blot. Only one copy of the gene by genome was detected. Also, differential expression of APEum was assessed under different physiological conditions. In brief, high expression levels were detected on media supplemented with corn infusion, proline, and ammonium.