Review
New insights into pectin methylesterase structure and function

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In bacteria, fungi and plants, pectin methylesterases are ubiquitous enzymes that modify the degree of methylesterification of pectins, which are major components of plant cell walls. Such changes in pectin structure are associated with changes in cellular adhesion, plasticity, pH and ionic contents of the cell wall and influence plant development and stress responses. In plants, pectin methylesterases belong to large multigene families, are regulated in a highly specific manner, and are involved in vegetative and reproductive processes, including wood and pollen formation, in addition to plant–pathogen interactions. Although, overall, protein structures are highly conserved between isoforms, recent data indicate that structural variations might be associated with the targeting and functions of specific pectin methylesterases.

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Plant cell wall and pectin methylesterases

Plant cell walls are highly complex structures composed of diverse polysaccharides and structural proteins. In addition, depending on cell type, they might also contain other non-carbohydrate components, such as lignin or cutin. Cellulose, one of the major cell wall polysaccharides, is a linear polymer of (1,4)-linked-β-d-glucose arranged in microfibrils, and is often referred to as the cell wall skeleton owing to its strength and rigidity [1]. Microfibrils are embedded in a matrix composed of

Sequencing projects have revealed the occurrence and primary structure of PMEs

Data acquired in genome and EST sequencing projects have shown that PMEs belong to large multigene families in all plant species examined to date (CAZy, http://www.cazy.org/fam/CE8.html; TIGR, http://www.tigr.org/plantProjects.shtml); TAIR, http://www.arabidopsis.org). For instance, in Arabidopsis thaliana, 66 ORFs have been annotated as putative full-length PMEs, representing 6.81% of all carbohydrate-active enzymes (CAZymes) and expansins in the species (Table 1) [6]. The corresponding

Elucidating three-dimensional PME structures

Major advances facilitating attempts to elucidate the mechanism(s) of action of PMEs were achieved when 3D crystallographic structures were obtained of first bacterial [17], and then plant [18] PMEs. The 3D crystallographic structures of plant PMEs were obtained following the first large-scale purification of a ripe carrot root (Daucus carota) PME, which had previously been the limiting step for plant PME protein analysis, in part because of the difficulty of expressing plant PMEs in

The role of the PRO region in PME targeting and function

Both group 1 and group 2 PME proteins have been found to be localized in the cell wall of tobacco [24] and Arabidopsis 15, 25 pollen tubes. The PRO and mature regions of the protein, together with the signal peptide, have been shown to be necessary for the targeting of NtPPME1, a group 2 protein, to the pollen tube cell wall via the exocytotic pathway [12]. However, the absence of a PRO region in group 1 proteins does not preclude cell wall targeting [15]. As initially described for Phaseolus

The multiple roles of PMEs in plants

The diversity of the putative roles of PMEs in plants can be illustrated by the diversity of their expression patterns in both Arabidopsis and Populus, as revealed by microarray analyses (Figure 2, Figure 3), showing that PME isoforms cluster into several distinct groups, highlighting their putative redundancy of function, and can have organ or stress-specific expression patterns.

Outlook

Although a considerable amount of data have been generated over the past five years regarding the structure and function of plant PMEs, exciting but difficult challenges remain to be addressed to improve our understanding of these proteins. The expression of plant PMEs has been shown to be (i) tissue-specific, (ii) regulated in a wide range of developmental processes, (iii) modulated by stresses in a specific manner and (iv) involved in plant defence responses in both monocotyledonous and

Acknowledgements

We are grateful to Robert Hancock for initial revision of the manuscript and to Janne Karlsson, Stefan Jansson, Rishikesh Bhalerao, Vaughan Hurry and Björn Sundberg for providing their unpublished results on PME expression patterns in poplar.

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