Activity dependent changes in synaptic efficacy, such as those which occur during long-term potentiation (LTP) and long-term depression (LTD), are believed to underlie learning and memory. A key event in both LTP and LTD is the induction of a set of immediate-early gene products, including the activity-regulated cytoskeletal- associated protein (arc, also known as arg 3.1). The best characterized function of Arc is enhancement of the endocytic internalization of AMPA receptors in dendritic spines, a process associated with LTD. Arc has also been implicated in the proteolytic processing of amyloid precursor protein (APP) on the surface of endosomes. To mediate these activities, Arc must associate with cellular membranes, but the mechanism of its binding to membranes is not understood. In addressing this question, we found that Arc undergoes palmitoylation in neurons, allowing it to insert directly into the lipid bilayer. Unlike other forms of protein lipidation, palmitoylation is reversible and, hence, may be subject to conditional regulation. In Aim 1 of this project we propose to define the mechanism and biochemical consequences of Arc palmitoylation. We will determine if the level of Arc palmitoylation is responsive to LTP- and LTD-inducing paradigms, and we will identify which of the 23 known human forms of palmitoyl acyltransferase is (are) responsible for modifying Arc. Using a combination of biochemical and imaging approaches, we will determine how palmitoylation influences the subcellular distribution of Arc, its ability to self-assemble on the membranes of living cells, and its susceptibility to undergo tyrosine phosphorylation, another Arc modification identified in our studies. In Aim 2 we will carry out electrophysiological investigations aimed at understanding how Arc palmitoylation affects synaptic plasticity. Our previous work implicated Arc in the weakening of synaptic strength and the elimination of excitatory synapses. Preliminary data indicate that these effects may be mediated by Arc palmitoylation. Changes in Arc expression have been linked to numerous cognitive disorders, including mental retardation, Alzheimer's disease, and substance abuse. Therefore, elucidation of novel mechanisms of Arc regulation has potential clinical significance.