Kiwi protein inhibitor of pectin methylesterase
Amino-acid sequence and structural importance of two disulfide bridges
Article first published online: 25 DEC 2001
European Journal of Biochemistry
Volume 267, Issue 14, pages 4561–4565, July 2000
How to Cite
Camardella, L., Carratore, V., Ciardiello, M. A., Servillo, L., Balestrieri, C. and Giovane, A. (2000), Kiwi protein inhibitor of pectin methylesterase. European Journal of Biochemistry, 267: 4561–4565. doi: 10.1046/j.1432-1327.2000.01510.x
- Issue published online: 25 DEC 2001
- Article first published online: 25 DEC 2001
- (Received 10 April 2000, revised 22 May 2000, accepted 24 May 2000)
- pectin methylesterase inhibitor;
- invertase inhibitor;
- disulfide bridges;
- primary structure;
- Actinidia chinensis.
A protein acting as a powerful inhibitor of plant pectin methylesterase was isolated from kiwi (Actinidia chinensis) fruit. The complete amino-acid sequence of the pectin methylesterase inhibitor (PMEI) was determined by direct protein analysis. The sequence comprises 152 amino-acid residues, accounting for a molecular mass of 16 277 Da. The far-UV CD spectrum indicated a predominant alpha-helix conformation in the secondary structure. The protein has five cysteine residues but neither tryptophan nor methionine. Analysis of fragments obtained after digestion of the protein alkylated without previous reduction identified two disulfide bridges connecting Cys9 with Cys18, and Cys74 with Cys114; Cys140 bears a free thiol group. A database search pointed out a similarity between PMEI and plant invertase inhibitors. In particular, the four Cys residues, which in PMEI are involved in the disulfide bridges, are conserved. This allows us to infer that also in the homologous proteins, whose primary structure was deduced only by cDNA sequencing, those cysteine residues are engaged in two disulfide bridges, and constitute a common structural motif. The comparison of the sequence of these inhibitors confirms the existence of a novel class of proteins with moderate but significant sequence conservation, comprising plant proteins acting as inhibitors of sugar metabolism enzymes, and probably involved in various steps of plant development.