Pinpointing the underlying cause of many neurodegenerative diseases has been difficult due to the complexity of neuronal cellular biology and the lack of basic information about impaired mechanisms that contribute to disease. Mutations in the human DCTN1 gene, which encodes a component of the dynactin complex, have been strongly linked to both familial and sporadic cases of amyotrophic lateral sclerosis (ALS). Many of the neurodegenerative phenotypes associated with human disease have been recapitulated in flies and mice harboring dynactin complex mutations demonstrating a conserved pathogenesis in dynactin complex mutants. To clarify the mechanisms that contribute to the pathogenesis of neurodegeneration observed in dynactin complex mutants, especially the degeneration of synaptic contacts, we will use a combination of forward genetic screens and quantitative cellular assays to identify and characterize important modifiers of dynactin complex function at the synapse. We believe that this approach has the potential to identify and describe new genes and pathways required within the nervous system for normal synaptic growth, synapse stabilization, and function. A genetic screen designed to specifically identify genetic modifiers of the Drosophila DCTN1 homolog, glued, within the nervous system has identified the Drosophila homologue of the Arfaptin2 gene, Darfaptin2 (Darf2). We find that Darf2 is expressed in motorneurons and required for normal synaptic growth. The goal of this proposal is to investigate the hypothesis that Darfaptin2 (Darf2) represents a novel component of the dynactin complex required for normal synaptic growth, stabilization, and neurotransmission. Models of Arfaptin2 function include the mediation of cross-talk between Rho-like GTPases and Arf family GTPases during vesicle formation, and the regulation of proteasome activity within neurons. Using standard genetic techniques and synaptic analyses, we will first define the role of Darf2 in the regulation of synaptic growth and synapse stabilization. This will include the biochemical analysis of its association with the dynactin complex in the nervous system (Aim1). We will further investigate the role of Darf2 in the nervous system using a structure-function approach to define protein domains required for normal Darf2 activity. This will include determining the signaling context for Darf2 during synapse growth and stabilization (Aim2). Finally, we are developing reagents and assays to directly investigate the regulation of proteasome function in the nerve terminal by the dynactin complex, Darf2, and during synapse retraction (Aim3). It is expected that these studies will reveal a novel regulatory mechanism during synaptic growth, stabilization, and neurotransmission that will have important implications for the pathogenesis of late onset neurological diseases.