Purification of the precursor form of maltose-binding protein, a periplasmic protein of Escherichia coli.

Cells of Escherichia coli K12 strain MM18 accumulate precursors of various exported proteins when the synthesis of the malE-lacZ hybrid protein is induced by maltose. Starting from a soluble cell extract of this strain, the precursor form of the maltose-binding protein, a periplasmic protein, was purified to homogeneity. The procedure includes affinity chromatography on cross-linked amylose and chromatofocusing. The purified precursor has an intact signal sequence at the NH2 terminus. It is soluble in aqueous buffer probably in monomeric state and is proteolytically processed by the leader peptidase, which was previously purified by Wickner and co-workers as an enzyme that processes M13 phage procoat protein.