Evidence that the glucoamylases and alpha-amylase secreted by Aspergillus niger are proteolytically processed products of a precursor enzyme

A 125-kDa starch hydrolysing enzyme of Aspergillus niger characterised by i ts ability to dextrinise and saccharify starch [Suresh et al, (1999) Appl. Microbiol. Biotechnol. 51, 673-675] was also found to possess activity towa rds raw' starch. Segregation of these activities in the 71-kDa glucoamylase and a 53-kDa alpha-amylase-like enzyme supported by antibody cross-reactiv ity studies and the isolation of mutants based on assay screens for the sec retion of particular enzyme forms revealed the 125-kDa starch hydrolysing e nzyme as their precursor. N-terminal sequence analysis further revealed tha t the 71-kDa glucoamylase was the N-terminal product of the precursor enzym e. Immunological cross reactivity of the 53-kDa amylase with antibodies rai sed against the precursor enzyme but not with the 71- and 61-kDa glucoamyla se antibodies suggested that this enzyme activity is represented by the C-t erminal fragment of the precursor, The N-terminal sequence: of the 53-kDa p rotein showed similarity to the reported Taka amylase of Aspergillus oryzae . Antibody cross-reactivity to a 10-kDa non-enzymic peptide and a 61-kDa gl ucoamylase described these proteins as products of the 71-kDa glucoamylase. Identification of only the precursor. starch hydrolysing enzyme in the pro tein extracts of fungal protoplasts suggested proteolytic processing in the cellular periplasmic space as the cause for the secretion of multiple form s of amylases by A. niger. (C) 2000 Federation of European Biochemical Soci eties.