Pectin is a plant cell wall constituent that is mainly composed of polygalacturonic acid (PGA), a linear α1,4-d-galacturonic acid (GalUA) backbone. Polygalacturonase (PG) hydrolyzes the α1,4-linkages in PGA. Nearly all plant PGs identified thus far are secreted as soluble proteins. Here we describe the microsomal PG activity in pea (Pisum sativum) epicotyls and present biochemical evidence that it was localized to the Golgi apparatus, where pectins are biosynthesized. The microsomal PG was purified, and it was enzymatically characterized. The purified enzyme showed maximum activity towards pyridylaminated oligogalacturonic acids with six degrees of polymerization (PA-GalUA6), with a Km value of 11 μM for PA-GalUA6. The substrate preference of the enzyme was complementary to that of PGA synthase. The main PG activity in microsomes was detected in the Golgi fraction by sucrose density gradient ultracentrifugation. The activity of the microsomal PG was lower in rapidly growing epicotyls, in contrast to the high expression of PGA synthase. The role of this PG in the regulation of pectin biosynthesis or plant growth is discussed.
Keywords: Golgi; pectin; polygalacturonase; pyridylamination; sucrose linear density gradient centrifugation.
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