Protein Targeting in Neurons and Endocrine Cells

Abstract

The functional polarity of axons is reflected in the polarized distribution of a number of proteins, both membrane and cytoskeletal. Targeting to each domain of the neuron may require a specific signal; alternatively, targeting to only one domain may be specific, while localization to the other may occur by default. In addition to sorting to the proper domain, proteins in neurons must also be sorted to the correct subcellular organelle. For example, synaptic vesicle proteins synthesized in the endoplasmic reticulum (ER) and posttranslationally modified in the Golgi must first be targeted to the axon and then to the nerve terminal. A number of studies suggest that synaptic vesicle proteins are finally assembled into mature synaptic vesicles at the synapse; consequently, synaptic vesicle proteins may contain multiple targeting signals to reach their final destination. Research has been going on investigating the molecular basis for targeting of membrane proteins in neurons and to synaptic vesicles in two different systems. Using dissociated cultures of rat embryonic hippocampal neurons, the mechanism for sorting of proteins to dendrites or axons is examined. In the pheochromocytoma cell line PC12, the sequences that are necessary to target an exogenous protein to synaptic vesicles is being determined. It is found that the final stage of sorting to synaptic vesicles in neurons is mediated by similar processes as in PC12 cells. To determine this, chimeras are made between cDNAs encoding the synaptic vesicle protein SV2 and the glucose transporter GLUT1, in which the cytoplasmic loop between TM6 and TM7 is replaced by the cytoplasmic loop of SV2. The distribution of the chimeras have also been analyzied on sucrose gradients designed to separate large dense-core vesicles from endosomes, synaptic vesicles, and other membranes to determine if the same targeting information is used for both types of regulated secretory vesicles.