The N-terminal pro region mediates retention of unprocessed type-I PME in the Golgi apparatus

The pectin matrix of the cell wall, a complex and dynamic network, impacts on cell growth, cell shape and signaling processes. A hallmark of pectin structure is the methylesterification status of its major component, homogalacturonan (HGA), which affects the biophysical properties and enzymatic turnover of pectin. The pectin methylesterases (PMEs), responsible for de-esterification, encompass a protein family of more than 60 isoforms in the Arabidopsis genome. The pivotal role of PME in the regulation of pectin properties also requires tight control at the post-translational level. Type-I PMEs are characterized by an N-terminal pro region, which exhibits homology with pectin methylesterase inhibitors (PMEIs). Here, we demonstrate that the proteolytic removal of the N-terminal pro region depends on conserved basic tetrad motifs, occurs in the early secretory pathway, and is required for the subsequent export of the PME core domain to the cell wall. In addition, we demonstrate the involvement of AtS1P, a subtilisin-like protease, in Arabidopsis PME processing. Our results indicate that the pro region operates as an effective retention mechanism, keeping unprocessed PME in the Golgi apparatus. Consequently, pro-protein processing could constitute a post-translational mechanism regulating PME activity.