DEPOSITION OF STORAGE PROTEINS

Plants store amino acids for longer periods in the form of specific st orage proteins. These are deposited in seeds, in root and shoot tubers , in the wood and bark parenchyma of trees and in other vegetative org ans. Storage proteins are protected against uncontrolled premature deg radation by several mechanisms. The major one is to deposit the storag e proteins into specialized membrane-bounded storage organelles, calle d protein bodies (PB). In the endosperm cells of maize and rice prolam ins are sequestered into PBs which are derived from the endoplasmic re ticulum (ER). Globulins, the typical storage proteins of dicotyledonou s plants, and prolamins of some cereals are transported from the ER th rough the Golgi apparatus and then into protein storage vacuoles (PSV) which later become transformed into PBs. Sorting and targeting of-sto rage proteins begins during their biosynthesis on membrane-bound polys omes where an N-terminal signal peptide mediates their segregation int o the lumen of the ER. After cleavage of the signal peptide, the polyp eptides are glycosylated and folded with the aid of chaperones. While still in the ER, disulfide bridges are formed which stabilize the stru cture and several polypeptides are joined to form an oligomer which ha s the proper conformation to be either deposited in ER-derived PB or t o be further transferred to the PSV. At the trans-Golgi cisternae tran sport vesicles are sequestered which carry the storage proteins to the PSV. Several storage proteins are also processed after arriving in th e PSVs in order to generate a conformation that is capable of final de position. Some storage protein precursors have short N- or C-terminal targeting sequences which are detached after arrival in the PSV. Other s have been shown to have internal sequence regions which could act as targeting information. In some cases positive targeting information i s known to mediate sorting into the PSV whereas in other cases aggrega tion and membrane association seem to be major sorting mechanisms.