Growth-associated protein-43 (GAP-43) is a phosphoprotein whose expression in neurons is related to the initial establishment and remodeling of neural connections. GAP-43 gene expression is known to be regulated at both the transcriptional and the postranscriptional levels. However, very little is known about the cellular mechanism involved in the degradation of this protein. Ubiquitin (Ub) is well known for its role in targeting cytoplasmic proteins for degradation by the 26S proteasome. The ubiquitin-proteasome system (UPS) consists of a conserved cascade of three enzymatic components that attach Ub covalently to various substrates and control the degradation of protein involved in several important cellular processes. In this study, we investigated the degradation of GAP-43 in transfected NIH 3T3 cells and neuronal cultures. We found that the proteasome inhibitors, lactacystin and MG132 increased the cellular GAP-43 level, leading to the accumulation of polyubiquitinated forms of this protein in transfected cells and that the Ub-proteasome pathway is also involved in the turnover of this protein in neurons. We conclude based on our findings that GAP-43 is a substrate of the UPS. © 2005 Wiley-Liss, Inc.