Molecular biology of intracellular protein trafficking

The evidence accumulated to date indicates that protein compartmentalization is mediated through specific regions of proteins destined for translocation into subcellular organelles. Proteins targeted to mitochondria, chloroplasts or the endoplasmic reticulum have 'transit' sequences contained in amino-terminal peptide extensions. However, most peroxisomal proteins do not have amino-terminal extensions. Protein importation into mitochondria has been extensively studied and characterized. This post-translational process appears to involve receptors on the mitochondrial outer membrane, and is dependent upon the electrochemical gradient across the inner membrane. Translocation to one of the submitochondrial compartments is determined by the type of transit sequence contained in a mitochondrial protein. The majority of imported mitochondrial proteins are proteolytically altered prior to assembly into oligomeric enzyme complexes. Protein importation into peroxisomes is distinctly different from importation into mitochondria. Although both processes are post-translational, their only other similarity is a requirement for ATP. In this review, we present and compare recent evidence for both mitochondrial and peroxisomal protein importation.