Atrogin-1/MAFbx/FBXO32 is a muscle-specific ubiquitin-ligase (E3) that is dramatically increased in atrophying muscle. Here we have investigated the functional relationship between atrogin-1 and FBXO25 which shares 65% amino acid identity. Using a RT-PCR we demonstrated that FBXO25 is highly expressed in brain, kidney, and intestine, whereas atrogin-1 expression is largely restricted to striate muscle. FBXO25 was shown here to contain a functional F-box domain that binds to Skp1 and thereby to Roc1 and Cul1, the major components of SCF-type E3s. In addition, the productive SCF complex containing FBXO25 showed ubiquitin ligase activity. Immunofluorescence studies in transfected B16-F10 cells showed that FBXO25 colocalize in the nucleus with Skp1, thus, indicating that putative substrates of FBXO25 may be nuclear proteins. We investigated the differential expression of atrogin-1 and FBXO25 in fasted and dexamethasone-treated mice and also in rats with streptozotocin-induced diabetes. Although the atrogin-1 was strongly induced in muscle in all three models, no changes were observed in the expression of FBXO25. Therefore, here we have shown that FBXO25 is a novel F-box protein analogous to atrogin-1, which is not involved in muscle atrophy. These data contribute to elucidate the role of FBXO25 in the ubiquitin-proteasome pathway.
Supported by FAPESP and FAEPA.