Construction of redesigned pMAL expression vector for easy and fast purification of active native antimicrobial peptides
Many protein expression and purification systems are commercially available to provide a sufficient amount of pure, soluble and active native protein, such as the pMAL system based on E. coli maltose binding protein tag (MBP). Adding specific amino acid tags to the N- or C-terminus of the protein increases solubility and facilitates affinity purification of proteins. However, many of expressed tagged proteins consequently lose functionality, particularly small peptides such as antimicrobial peptides (AMPs). Objective of this study was to redesign the pMAL expression vector in order to increase the efficacy of MBP tag separation from native peptides. Redesign of the pMAL expression vector included introduction of the His6 tag and the enterokinase cleavage site downstream from the original MBP tag and Xa cleavage site enabling purification of native and active peptide (P) following two-step affinity chromatography. In the first step the entire MBP-His6-P fusion protein is purified through binding to Ni-NTA agarose. In the second step, the purification was performed by adding mixture of amylose and Ni-NTA agarose resins following cleavage of the fusion protein with active His6 tagged enterokinase. This removes MBP-His6 and His6-enterokinase leaving pure native protein in solution. The redesigned pMAL vectors were optimized for cytoplasmic (pMALc5HisEk) and periplasmic (pMALp5HisEk) peptides expression. Two-step purification protocol was successfully applied in purification of active native AMPs, lactococcin A and human {beta}-defensin. Taken together, we established the optimal conditions and pipeline for overexpression and purification of large amount of native peptides, that can be implemented in any laboratory.