PMEIs

PMEIs, or pectin methylesterase inhibitors, are a family of plant proteins that regulate the activity of pectin methylesterases (PMEs) in the plant cell wall. By inhibiting PMEs, PMEIs modulate the demethylesterification of homogalacturonan, altering cell wall porosity, stiffness, and the capacity for calcium cross-linking. This control affects processes such as cell expansion, organ growth, fruit ripening, pollen tube guidance, and defense responses against pathogens.

Most PMEIs are secreted into the apoplast, possessing an N-terminal signal peptide that directs them outside the cell. The core functional unit is the PMEI domain, typically about 150 amino acids in length, which folds into a disulfide-stabilized structure. Many PMEIs also contain additional domains or signal motifs, and PMEI-like domains can be found in proteins that do not inhibit PMEs, reflecting evolutionary diversification.

The interaction between PMEIs and PMEs is often specific to particular PME isoforms, enabling precise spatial and temporal control of pectin demethylesterification. This isoform specificity allows plants to tune wall rigidity and porosity in response to developmental cues and environmental challenges. Experimental approaches include biochemical assays showing PME activity inhibition by recombinant PMEI proteins and localization studies confirming apoplastic distribution.

Biological roles of PMEIs include regulation of cell wall mechanics during growth, modulation of fruit texture and ripening, and contribution to defense by limiting cell wall loosening during pathogen attack. They are studied across plant species, with extensive work in model species such as Arabidopsis thaliana and in crop crops where wall properties impact commercial traits.

See also: pectin methylesterase, plant cell wall, pectin.