||Eph, the largest family of receptor tyrosine kinases (RTKs), is involved in axon guidance and synapse formation during development. Distinct from other RTKs, Ephs are activated by membrane-bound ligands, ephrins. Co-localization of EphA4 and its ligand, ephrin-A2, at the neuromuscular junction (NMJ), together with the observation that activation of EphA4 regulates acetylcholinesterase expression in muscle, suggests that EphA4 receptor is involved in NMJ development and/or maintenance. To further elucidate the downstream signaling of EphA4 at the NMJ, a yeast two-hybrid screen was performed using the cytoplasmic region of EphA4 as the bait to identify EphA4 interacting partners in muscle. In this project, seven positive clones were identified as EphA4-interacting proteins, including a novel isoform of anaphase promoting complex subunit 2 (APC2), which we designated as APC2i. While the functions of APC2i remain to be elucidated, APC2 is known to be involved in cell cycle progression via the regulation of protein degradation, and has recently been demonstrated to play an important role in synapse formation or maintenance. As there is no scientific publication on mammalian APC2i, the presence of APC2i was first examined using RT-PCR. We found that both APC2 and APC2i transcripts were expressed in mouse adult brain. Furthermore, EphA4 was found to be associated with APC2i or APC2 when overexpressed in HEK293T cells. The overexpression of APC2i, but not APC2, resulted in reduced EphA4 protein level, which was reversed by treatment with proteasome inhibitor. Taken together with the observation that ephrin-A1 stimulates EphA4 down-regulation in neurons by enhancing proteasome assembly, our findings support the notion that APC2i may regulate EphA4 expression in neurons by affecting proteasome-dependent degradation of EphA4.