Transfer of in vitro synthesized Vicia faba globulins and barley prolamins across the endoplasmic reticulum membrane of Vicia faba

Deposition of storage proteins

Plants store amino acids for longer periods in the form of specific storage proteins. These are deposited in seeds, in root and shoot tubers, in the wood and bark parenchyma of trees and in other vegetative organs. Storage proteins are protected against uncontrolled premature degradation by several mechanisms. The major one is to deposit the storage proteins into specialized membrane-bounded storage organelles, called protein bodies (PB). In the endosperm cells of maize and rice prolamins are sequestered into PBs which are derived from the endoplasmic reticulum (ER). Globulins, the typical storage proteins of dicotyledonous plants, and prolamins of some cereals are transported from the ER through the Golgi apparatus and then into protein storage vacuoles (PSV) which later become transformed into PBs. Sorting and targeting of storage proteins begins during their biosynthesis on membrane-bound polysomes where an N-terminal signal peptide mediates their segregation into the lumen of the ER. After cleavage of the signal peptide, the polypeptides are glycosylated and folded with the aid of chaperones. While still in the ER, disulfide bridges are formed which stabilize the structure and several polypeptides are joined to form an oligomer which has the proper conformation to be either deposited in ER-derived PB or to be further transferred to the PSV. At the trans-Golgi cisternae transport vesicles are sequestered which carry the storage proteins to the PSV. Several storage proteins are also processed after arriving in the PSVs in order to generate a conformation that is capable of final deposition. Some storage protein precursors have short N- or C-terminal targeting sequences which are detached after arrival in the PSV. Others have been shown to have internal sequence regions which could act as targeting information. In some cases positive targeting information is known to mediate sorting into the PSV whereas in other cases aggregation and membrane association seem to be major sorting mechanisms.

Secretion of plant storage globulin polypeptides by Xenopus laevis oocytes

Xenopus oocytes injected with poly(A)-containing RNA from developing cotyledons of field beans (Vicia faba L. var. minor) synthesize precursor polypeptides to the major storage globulins legumin and vicilin. These polypeptides are secreted into the medium without proteolytic cleavage of the legumin propolypeptides into the mature disulfide-linked alpha and beta chains. Similarly, storage globulin polypeptides from pea (Pisum sativum L.) and french bean (Phaseolus vulgaris L.) were secreted from oocytes. Inhibition of glycosylation by tunicamycin does not prevent secretion. This first report on the secretion of plant polypeptides by Xenopus oocytes shows that (a) intracellular deposition of storage proteins in membrane-bounded organelles (protein bodies) of plants and extracellular secretion have step(s) in common, and (b) the cell, in addition to the mRNA, determines the final destination of these polypeptides.

Low-molecular-weight polypeptides of vicilin from Vicia faba L. are products of proteolytic breakdown

Vicilin, the main 7-S globulin of Vicia faba L., undergoes cleavage during prolonged treatment at room temperature, which can be inhibited by protease inhibitors such as 1 microM leupeptin. The cleavage products show identical electrophoretic mobilities with the polypeptides normally visible after sodium dodecylsulphate gel electrophoresis of vicilin prepared from mature seeds. N-terminal amino acid analysis of electrophoretically prepared polypeptides reveals serine as common N terminus of the two largest polypeptides of Mr approximately equal to 50000 and 35000. According to serological experiments and peptide mapping the low-molecular-weight polypeptides (Mr approximately equal to 35000; 31000; 19000 and below) have antigenic determinants and amino acid sequences, respectively, that are similar to each other and are all contained within the structure of the large polypeptide of Mr approximately equal to 50000. This leads of the conclusion that polypeptides of Mr approximately equal to 35000 and below are derived by proteolysis from one (or a few closely related) polypeptide(s) of Mr approximately equal to 50000 and that proteolysis starts soon after biosynthesis as a post-translational process within the developing seed. Some experiments indicate the existence of 'nicking' points within the vicilin polypeptides, which are the major cleavage sites during preparation. These observations strongly support the view that 'native' vicilin is a trimeric or tetrameric globulin with polypeptides of Mr greater than or equal to 50000.

Origin and development of protein bodies in cotyledons of Vicia faba : Proposal for an uniform mechanism

Storage proteins of the field bean (Vicia faba L., var. minor, cv. "Fribo") are synthesized and accumulated in the cotyledons during stage 2 of seed development. Deposition of protein reserves takes place in the protein bodies. The generation of protein bodies was investigated electronmicroscopically using ultra-thin sections as well as the freeze-fracturing technique. During the initial period of storage protein formation, globulins are deposited in large vacuoles which later are transformed to give increasing numbers of small vacuoles with decreasing size. The vacuoles disappear early during the stage of storage protein formation and generate the first protein bodies. During the subsequent period of maximum storage protein formation, which takes place at the rough endoplasmic reticulum (rER), swollen ER strands appear which seem to be entirely filled with protein, and these generate ER-produced protein vacuoles (ERPVAC). The vesicles are transformed in a manner comparable to the vacuoles in the initial period of developmental stage 2 and thus generate the major quantity of protein bodies. Both processes seem to represent only two variants of an uniform mechanism of protein body generation.

In vivo synthesis and processing of cereal lectins

The synthesis and processing of cereal lectins was followed in vivo. The initial translation products of lectin genes are higher molecular weight (28 K) precursors, which are post-translationally processed in a single step into authentic lectin polypeptides (23 K). The conversion of precursor into mature product is a rather slow process (the precursor has a half life of 36 min) and is apparently not a prerequisite for biological activity since the precursor exhibits sugar binding activity. Because of the striking resemblances between the processing of cereal lectins and vectorial processing of cytoplasmatically made chloroplast, mitochondrial and glyoxysomal proteins, vectorial processing of cereal lectins might be a means of transporting these proteins through a membrane into an extra-cytoplasmic compartment.