Regulation of Organelle Biogenesis Review

an organism are compared. In mammals, for example, the ER is vastly abundant in exocrine pancreatic secre-where there is no appreciable demand for the secretion of any protein (Bolender, 1974). Using representative examples primarily from the yeast Saccharomyces cere-One hallmark of eukaryotic cells is that they contain visiae, we highlight in this review some recent insights many functionally distinct membrane-bounded com-into the biogenesis of organellar proteins and lipids and partments, or organelles. Each organelle carries out discuss some of the known mechanisms that are in-highly specialized functions because it possesses its volved in regulation of organelle biogenesis. peroxisomes each have a unique set of receptors that by Redundant Pathways allow them to select faithfully their characteristic set of Proteins are targeted to organelles by signal sequences proteins. Although there are only a few initial entry points (Blobel and Sabatini, 1971). These sequences are recog-from the cytosol into subcellular organelles, many pro-nized by specific targeting factors that mediate their teins are sorted and distributed further before reaching delivery to the appropriate organelle. In most cases, their final destinations. The ER, for example, serves as soluble cytosolic targeting factors work together with entry point into all compartments on the secretory and receptors bound to the surface of the organelles. These endocytic pathways (Palade, 1975), and proteins enter-components pass the proteins to a translocation appa-ing the mitochondria are subsequently distributed into ratus in the organellar membrane, which in turn cata-mitochondrial subcompartments. There is, however, no lyzes the movement of the protein across the membrane protein traffic among the ER (or compartments of the into the organelle or, as it is the case for integral mem-secretory pathway), the mitochondria, and the peroxi-brane proteins, catalyzes their proper membrane inte-somes, possibly because each of the three organelles gration. is of distinct evolutionary origin (Blobel, 1980). Neverthe-Recent developments have shown that targeting of less, all three of these organelles coexist in the cell, and proteins to organelles is more complicated than was their functions must be coordinated and adapted to originally appreciated. It is now clear that, in the case cellular needs. Interorganellar communication is estab-of the ER, mitochondria, and peroxisomes, more than lished via metabolites that can move between organ-one targeting pathway exists for each organelle. There elles. Such metabolic communication renders organ-are at least two pathways that deliver proteins to the elles dependent upon one another for their function and ER: one pathway utilizes the …