Protein synthesis, processing, and trafficking

The final step in the transfer of the genetic information stored in DNA into proteins is the translation of the intermediary messenger molecules, mRNAs (see Chapter 3). Protein synthesis occurs in the cytoplasm and generates a great variety of products endowed with a wide spectrum of functions. The complete set of proteins produced by a cell is called the “proteome” and is responsible for the remarkable diversity in cell specialization that is typical of metazoan organisms. In order to be functional, proteins need to be properly folded, assembled, often modified and transported to the final destination. The cell has in its interior several membrane-bound compartments, termed organelles, such as the mitochondria, the peroxisomes, the nucleus and the endoplasmic reticulum to which the proteins may be targeted. Since each compartment serves a particular purpose, protein transport is crucial to maintain the identity and functions of each organelle. Intracellular physiology depends on the proper and coordinated functioning of the organelles. In many cases protein folding and processing are coupled with protein trafficking so that the targeting process is unidirectional and irreversible. This chapter briefly describes how proteins are synthesized and then focuses on their processing and delivery to their appropriate destinations within the cell. An understanding of the machines that catalyze protein folding, assembly, processing and targeting is relevant to the study of hematology providing a basis for an explanation of how malfunctions in these processes can cause hematological disorders.