The cleavable N-terminal domain of plant endopolygalacturonases from cladeB may be involved in a regulated secretion mechanism

Polygalacturonases represent the most abundant carbohydrate hydrolase famil y in the Arabidopsis thaliana genome, and they are thought to be involved i n nearly all of the developmental processes requiring cell wall modificatio ns during the life cycle of the plant. By phylogenetic analysis, plant poly galacturonases fall into at least three groups, one of which is distinguish ed from the others by the presence of an additional N-terminal domain. We h ave used RDPG1, the polygalacturonase involved in pod dehiscence in oilseed rape (Brassica napus), as a model to investigate the function of this doma in. We have confirmed that this domain is absent in the mature protein by d etermination of the N-terminal sequence of mature RDPG1 purified from oilse ed rape pod. We have furthermore investigated the accumulation and subcellu lar localization of the precursor containing the N-terminal domain and of t he mature protein throughout the development and maturation of the pod. Usi ng recombinant expression in Pichia pastoris, we have produced the RDPG1 pr ecursor, and we present evidence that the N-terminal domain of plant polyga lacturonases is not involved in folding or inactivation of the precursor bu t may play a role in the intracellular transport of this protein family via a novel regulated secretion pathway.