Compartmentation of Newly Synthesized Protein (1982)

In the ten years since the proposal8 and evidence10 that translocation of secretory proteins across the endoplasmic reticular membrane depended upon an amino acid sequence “signal”, our understanding of the events which allow compartmentation of eukaryotic proteins into different organelles has progressed rapidly. During this time, segregation of bacterial proteins has also been found to share many similar features.2 While the proposal that all proteins destined to cross organellar15 membranes might utilize an identical mechanism for translocation has been disproved, the impetus provided by the “signal hypothesis” greatly accelerated investigation in this area.Present evidence indicates that intracellular translocation across membranes, i.e., compartmentation, can occur by four mechanisms1. Removal of NH2-terminal signal (pre) peptide of 15 to 30 amino acids from secretory, some viral envelope, integral plasma membrane and lysosomal proteins occur when they cross the ER membrane during translation. Further co-translational and post-translational modifications including glycosylation, processing of oligosaccharide side chains, phosphorylation, further proteolytic processing, acetylation, amidation and fatty acylation also play a role in determining the ultimate localization of these newly synthesized proteins2. Alternatively, for ovalbumin and some ER integral membrane proteins, compartmentation occurs during translation but without the removal of any peptide3. For most chloroplast and mitochondrial proteins, translocation from the cytoplasm into the organelle occurs after translation is completed and involves the removal of an NH2-terminal peptide. In contrast to secretory proteins, however, this transit peptide is larger and probably hydrophilic4. Finally, a few proteins destined for either the ER membrane, mitochondria, or peroxisomes are incorporated post-translationally but without the removal of any peptideWhile compartmentation has been shown to occur by these mechanisms, the component parts of the translocation apparatus are not well characterized. One of the major directions of current investigations is to separate, purify and characterize these components and, in most cases, proteases. In vitro reconstitution of the translocation event is essential for elucidating the mechanism of this reaction. A second approach, which has already provided useful information, is to construct genes with modifications in the coding regions of translocated proteins and to insert such genes into the bacterial genome. An alternative method is to introduce amino acid analogs into the proteins and assess alterations in translocation. In conjunction with these studies, it is clear that further structural analysis of larger precursors, particularly of mitochondrial protein subunits, will be essential.When the above information is available, questions concerning the unique structural features of proteins which direct them to a specific organelle, and the components of the translocation apparatus which are common among different organelles can then be answered. Finally, the importance of compartmentation as a regulatory event in modifying localization of active proteins can then be addressed.