Salt-dependent thermo-reversible α-amylase: cloning and characterization of halophilic α-amylase from moderately halophilic bacterium, Kocuria varians

A moderately halophilic bacterium, Kocuria varians, was found to produce active alpha-amylase (K. varians alpha-amylase (KVA)). We have observed at least six different forms of alpha-amylase secreted by this bacterium into the culture medium. Characterization of these KVA forms and cloning of the corresponding gene revealed that KVA comprises pre-pro-precursor form of alpha-amylase catalytic domain followed by the tandem repeats, which show high similarity to each other and to the starch binding domain (SBD) of other alpha-amylases. The observed six forms were most likely derived by various processing of the protein product. Recombinant KVA protein was successfully expressed in Escherichia coli as a fusion protein and was purified with affinity chromatography after cleavage from fusion partner. The highly acidic amino acid composition of KVA and the highly negative electrostatic potential surface map of the modeled structure strongly suggested its halophilic nature. Indeed, KVA showed distinct salt- and time-dependent thermal reversibility: when alpha-amylase was heat denatured at 85A degrees C for 3 min in the presence of 2 M NaCl, the activity was recovered upon incubation on ice (50% recovery after 15 min incubation). Conversely, KVA denatured in 0.1 M NaCl was not refolded at all, even after prolonged incubation. KVA activity was inhibited by proteinaceous alpha-amylase inhibitor from Streptomyces nitrosporeus, which had been implicated to inhibit only animal alpha-amylases. KVA with putative SBD regions was found to digest raw starch.