Introduction/Overview

This chapter introduces the gist of the complete book as well as the particular topic discussed in each chapter. All living cells contain proteins that carry out specialized functions within various subcellular membrane or aqueous spaces. Bacterial cells have at least one membrane that separates the inside of the cell from its environment. Most proteins are synthesized in the cytoplasm of the cell, except for a small number that are encoded in the mitochondrial and chloroplast genomes. This raises the question of how proteins are transported from the cytoplasm to other destinations within or outside of the cell. Proteins are imported directly from the cytoplasm into the endoplasmic reticulum (ER), mitochondria, peroxisomes, and chloroplasts by mechanisms that use a targeting sequence and a translocation machinery. In addition to importing proteins from the cytoplasm into the organelle, mitochondria and chloroplasts also export proteins from the mitochondrial matrix or the chloroplast stroma where proteins are encoded by their respective organellar genomes. Overall this chapter reveals that this book brings together a number of important topics in the protein localization field.