All cells transport proteins across membranes, but the complexity of protein traffic in plant cells is especially striking because of the variety of organelle types involved. Many proteins are inserted, during translation, into the lumen of the endoplasmic reticulum, after which they are transported via the endomembrane system to the Golgi apparatus, vacuole, protein bodies or plasma membrane. Other proteins are transported posttranslationally

into glyoxysomes, mitochondria and plastids. In each case, the protein is synthesised with an appropriate signal which ensures targeting to the correct organelle, and a number of studies have attempted to define the characteristics of these targeting signals (reviewed by Bennett & Osteryoung, 1991; Robinson, 1991).