Species specific signal peptide cleavage of plant storage protein precursors in the endoplasmic reticulum

Abstract

Smooth microsomal membranes were isolated from immature cotyledons of Vicia faba and Phaseolus vulgaris as well as from immature endosperms of Bomi barley and its mutantlys 3a. Isolated Bomi barley endosperm polysomes direct the synthesis of hordein precursor polypeptides. Functional reconstitution of Bomi barley rough microsomes employing such polysomes and smooth microsomal membranes resulted in the ability of the membranes to perform signal peptide cleavage and co-translational transport of hordein precursor polypeptides into the lumen of the microsomes. Only a small portion of the hordein precursor polypeptides was processed and transported when the reconstitution was performed with smooth microsomal membranes isolated from cotyledons of Vicia faba and Phaseolus vulgaris or from the endosperms oflys 3a. A precursor polypeptide to the 50 kD vicilin was translated in vitro from mRNA of immature Vicia faba cotyledons. In the presence of Vicia faba smooth microsomal membranes the precursor polypeptide was co-translationally processed to the size of the polypeptide synthesized by rough microsomal membranes. Smooth microsomal membranes from cotyledons of Phaseolus vulgaris and from the endosperms of Bomi orlys 3a barley failed to process detectable amounts of the vicilin precursor polypeptide. Dog pancreatic microsomal membranes cleaved significant amounts of the hordein precursor polypeptides whereas only a small portion of the vicilin precursor polypeptides was processed.