Protein transport in plant cells
This thesis is a study of the routes by which proteins are incorporated into organelles in plant cells. The mechanism of protein translocation into protein bodies, leucoplasts and glyoxysomes - three major types of organelle in castor bean endosperm cells - are investigated. While the synthesis of protein body and glyoxysome matrix proteins has previously been studied, nothing is currently known about the synthesis and incorporation of proteins destined for their membranes. This work describes the purification and characterization of the major integral membrane proteins of these organelles. Rabbit antisera raised against these proteins was used to follow their synthesis both in vivo and in vitro by a variety of techniques. The results of this investigation suggest that protein body membrane proteins are initially synthesized on and inserted into the rough endoplasmic reticulum, and are subsequently transported to protein bodies by a process of membrane flow and vesiculation. Glyoxysome membrane proteins, however, appear to be posttranslationally imported directly into the organelle from the cytoplasm. Neither class of protein appear to be synthesized as precursors nor are any co- or post-translationa1 modifications evident. Leucoplasts also import proteins from the cytoplasm, apparently by mechanisms similar to those operating in chloroplasts and mitochondria. Chloroplast precursor proteins are competent for translocation into leucoplasts and are apparently correctly processed on import by a leucoplast stromal peptidase. Other features of leucoplast and chloroplast protein import are also similar. The results presented suggest that plant cells utilize a variety of routes and mechanisms for transporting proteins. These mechanisms are compared with those employed, and more extensively studied, in animal and yeast cells.