Syntaxin 1 is an essential component of the neurotransmitter release machinery, and regulation of syntaxin 1 expression levels is thought to contribute to the mechanism underlying learning and memory. However, the molecular events that control the degradation of syntaxin 1 remain undefined. Here we report the identification and characterization of a novel RING finger protein, Staring, that interacts with syntaxin 1. Staring is expressed throughout the brain, where it exists in both cytosolic and membrane-associated pools. Staring binds and recruits the brain-enriched E2 ubiquitin-conjugating enzyme UbcH8 to syntaxin 1 and facilitates the ubiquitination and proteasome-dependent degradation of syntaxin 1. These findings suggest that Staring is a novel E3 ubiquitin-protein ligase that targets syntaxin 1 for degradation by the ubiquitin-proteasome pathway.Modulation of protein degradation is a major mechanism by which cells regulate the expression levels of specific proteins and consequently the cellular processes that these proteins participate in (1, 2). The ubiquitin-proteasome pathway plays a crucial role in the degradation of proteins involved in a variety of cellular processes, including differentiation, proliferation, and apoptosis. However, the role of the ubiquitin-proteasome pathway in the degradation of presynaptic proteins remains poorly characterized, despite the presence of ubiquitin at nerve terminals (3-5). In the ubiquitin-proteasome pathway, substrates are marked for degradation by covalent linkage to ubiquitin. The ubiquitinated substrate proteins are then recognized and degraded by the 26 S proteasome (1, 2, 6). Ubiquitination involves a highly specific enzyme cascade in which ubiquitin is first activated by an E1 1 ubiquitin-activating enzyme and then transferred to an E2 ubiquitin-conjugating enzyme and finally ligated to the substrate by an E3 ubiquitin-protein ligase (1,7,8). The E3 ubiquitin-protein ligase plays an essential role in determining the specificity of ubiquitination and subsequent protein degradation. Consistent with this role, it is estimated that an organism such as a human contains over 100 E3 ubiquitin ligases, in contrast to a single E1 ubiquitin-activating enzyme and about a dozen E2 ubiquitin-conjugating enzymes (9). Despite the importance of E3 ubiquitin-protein ligases in specific protein degradation and the estimated presence of more than 100 E3 ligases in the human genome, only a few E3 ligases have been characterized at the molecular level.Syntaxin 1 is a neuronal membrane protein that was originally identified as a binding partner for synaptotagmin and the N-type calcium channel (10 -12). It is well established that syntaxin 1 functions as a synaptic t-SNARE to mediate synaptic vesicle exocytosis at nerve terminals (13)(14)(15). Syntaxin 1 appears early during embryonic development (16, 17), and its expression level is dramatically up-regulated during synapse formation and brain maturation (16 -19). Regulation of syntaxin 1 levels may contribute to the mechanism unde...