Understanding how local protein synthesis leads to synaptic plasticity is a fundamental problem, but it is also highly relevant to mental illness. It as been hypothesized that a significant fraction of the genetic defects associated with autism spectrum disorders (ASD) may cause disease through a common mechanism - the dysregulation of protein synthesis at synapses. The interaction of eukaryotic initiation factor 4E (eIF4E) with eIF4G is the rate limiting step for cap-dependent protein synthesis. Mouse models in which eIF4E is overexpressed, or in which the competitive inhibitor 4EBP2 is knocked out, display autistic-like behaviors. A small molecule inhibitor of the 4E-4G interaction, 4EGI-1, shows exciting potential for reversing autistic symptoms in these mouse models. However, the neural circuits that are altered in ASD exhibit very high degrees of both spatial and temporal complexity so that therapeutic interventions in ASD will likely need to be directed at relevant neural circuits during specific time windows, rather than broadly at all areas of the brain. This i hard to achieve with small molecules like 4EGI-1 or indeed with any currently available molecular tool. We propose to develop genetically-encoded light-controlled ('optogenetic') tools that permit control of the 4E-4G interaction. Specifically we will develop: (i) opto-4EBP2, a tool that will permit blue light controlled blocking of the 4E-4G interaction. Conceptually this is a genetically-encoded, protein-based and reversible version of the small molecule inhibitor 4EGI that was found to reverse autism-like behaviors in mouse models. (ii) opto-4E. This tool will permit blue light triggered up-regulation of local translation. It can be used to test whether time up-regulation of protein synthesis in discrete brain regions leads to ASD-like behaviors. Our approach is two-stage. First, we will carry out structure-based design of first-generation opto-4EBP2 and opto-4E tools. The second stage is optimization, which is critical for effective in vivo function. We will develop a cell-based 4E-4G interaction assay based on fluorescence screening using dimerization dependent fluorescent proteins. This methodology will allows us to rapidly screen thousands of opto-4EBP2 and opto-4E designs. The optogenetic tools we create will permit fundamental studies on the mechanisms of synaptic plasticity. In addition, these tools it will make it possible to determine whether targeting the 4E-4G interaction with appropriate spatiotemporal control is a valid approach for therapeutic intervention in ASD.