Structure-Function of a Proteinaceous Inhibitor of Plant Pectin Methylesterase

Abstract

A protein acting as a powerful inhibitor of plant pectin methylesterase has been isolated from kiwi fruit. The protein was purified by affinity chromatography on Sepharose-bound tomato pectin methylesterase. The purified protein has a molecular mass close to 16 kDa, as estimated by SDS-PAGE, whereas its molecular mass appears to be 25 kDa by gel filtration under native conditions. The inhibitor interacts with plant pectin methylesterase forming a 1:1 complex. The inhibitor primary structure, determined by direct protein analysis, comprises 152 amino acid residues; two disulphide bridges, connecting Cys9 and Cys 18, and Cys74 and Cys 114 have been identified. Amino acid sequence alignment indicates similarity with the inhibitors of invertase, and with the N-terminal non catalytic region of pectin methylesterase. Remarkably, in this alignment the four Cys residues involved in disulphide bridges in pectin methylesterase inhibitor are conserved.