Analysis of LuPME3, a pectin methylesterase from Linum usitatissimum, revealed a variability in PME proteolytic maturation.

Pectin methylesterase (PME) catalyses the de-methylesterification of pectin in plant cell walls during cell elongation. (1) Pectins are mainly composed of α(1, 4)-D-galacturonosyl acid units that are synthesised in a methylesterified form in the Golgi apparatus to prevent any interaction with Ca2+ ions during their intracellular transport. (2) The highly methylesterified pectins are then secreted into the apoplasm (3) and subsequently de-methylesterified in muro by PMEs. This can either induce the formation of pectin gels through the Ca2+ crosslinking of neighbouring non-methylesterified chains or create substrates for pectin-degrading enzymes such as polygalacturonases and pectate lyases for the initiation of cell wall loosening. (4) PMEs belong to a large multigene family. Sixty-six PME-related genes are predicted in the Arabidopsis genome. (1) Among them, we have recently shown that AtPME3 (At3g14310), a major basic PME isoform in A. thaliana, is ubiquitously expressed in vascular tissues and play a role in adventitious rooting. (5) In flax (Linum usitatissimum), three genes encoding PMEs have been sequenced so far, including LuPME3, the orthologue of AtPME3. Analysis of the LuPME3 isoform brings new insights into the processing of these proteins.