Secretion, purification, and characterisation of barley alpha-amylase produced by heterologous gene expression in Aspergillus niger

Efficient production of recombinant barley alpha-amylase has been achieved in Aspergillus niger. The cDNA encoding alpha-amylase isozyme 1 (AMY1) and its signal peptide was placed under the control of the Aspergillus nidulans glyceraldehyde-3-phosphate dehydrogenase (gpd) promoter and the A. nidulans trpC gene terminator. Secretion yields up to 60 mg/l were obtained in media optimised for alpha-amylase activity and low protease activity. The recombinant AMY1 (reAMY1) was purified to homogeneity and found to be identical to native barley AMY1 with respect to size, pI, and immunoreactivity. N-terminal sequence analysis of the recombinant protein indicated that the endogenous plant signal peptide is correctly processed in A. niger. Electrospray ionisation/mass spectrometry gave a molecular mass for the dominant form of 44,960 Da, in accordance with the loss of the LQRS C-terminal residues; glycosylation apparently did not occur. The activities of recombinant and native barley alpha-amylases are very similar towards insoluble and soluble starch as well as 2-chloro-4-nitrophenol beta-D-maltoheptaoside and amylose (degree of polymerisation = 17). Barley alpha-amylase is the first plant protein efficiently secreted and correctly processed by A. niger using its own signal sequence.