Nascent pectin formed in Golgi apparatus of pea epicotyls by addition of uronic acids has different properties from nascent pectin at the stage of galactan elongation

Microsomal membranes were prepared from etiolated pea (Pisum sativum L.) epicotyls and used to form nascent [Uronic acid-14C]pectin. The enzyme products were characterized by selective enzymic degradation, gel permeation chromatography and analysis of cellulose binding properties. The product obtained had a molecular weight of around 40 kDa, which was significantly lower than that of nascent [Gal-14C]pectin prepared from the same tissues. It is composed mainly of polygalacturonan and perhaps also rhamnogalacturonan (RG-I). Evidence was obtained for the presence of a protein attached to the nascent [Uronic acid-14C]pectin, but it was unaffected by endoglucanase and did not bind to cellulose. Hence, no xyloglucan appeared to be attached to the nascent [Uronic acid-14C]pectin. A model is proposed in which xyloglucan is attached to nascent pectin after formation of homogalacturonan, but before the pectin leaves the Golgi apparatus.