Protein Targeting in the Neuron

The morphology of the neuron is essential to its function. Each mor­ phological specialization of the neuron, the dendrite, the dendritic spines, the axon, the axon hillock, the nodes of Ranvier, the active zone, and the synaptic vesicle appears to have a unique protein composition. To explain the architecture of the neuron in molecular terms, therefore, we need to know how unique proteins achieve their distributions within the cell. Because neurons have such morphological complexity, we may wonder whether they have evolved unique mechanisms of protein sorting, or use the same mechanisms found in non neuronal cells. Although scanty, the existing data on sorting of proteins in neuronal cells can still be adequately explained by mechanisms that exist in other cell types. What may be surprising is the diversity of sorting mechanisms that are used. In this review, we summarize evidence for selective targeting, selective retention, selective stabilization, selective exclusion, and RNA targeting. Protein sorting is conveniently divided into two issues: how proteins are targeted to organelles and how organelles reach their cellular destination. A second major subdivision is between sorting mechanisms that are inherent to cells and occur in dissociated cells and those that are induced by a cell's interaction with other cells or with extracellular matrix. In this review, we attempt to organize the available data into these conventional categories. It is also conventional to review membrane protein targeting or cytoskeletal assembly as two separate topics. Because the morphology