In maize, glutelin-2 and low molecular weight zeins are synthesized by membrane-bound polyribosomes and translocated into microsomal membranes

Abstract

Experiments to establish the site of biosynthesis and the possible translocation into microsomes of glutelins-2 (28 kD G2) and low molecular weight zeins (10, 14, 15 kD Z2) have been carried out. Free and membrane-bound polyribosomes as well as microsomal membranes were isolated from immature endosperms of W64A Zea mays L. In vitro translation studies were carried out in the presence and in the absence of membranes using [(35)S]-methionine or [(35)S]-cysteine as precursors. Cell-free translation products were characterized by electrophoretic mobility, solubility and antigenic properties. The results obtained indicate that 28 kD G2 and low molecular weight zeins are primarily synthesized on membrane-bound polysomes. From experiments using proteinase K as a probe, we also conclude that these proteins are translocated into microsomes where they accumulate. The translocated and pre-28 kD G2 proteins do not present changes in the apparent molecular weight. However we show that there are differences in their isoelectric points, a fact that indicates the existence of 28 kD G2 processing.