The steroid receptor coactivator oncogene, AIB1 (Amplified In Breast cancer 1) (also known as ACTR/SRC-3/RAC-3/TRAM1) is amplified and overexpressed in a variety of epithelial tumors. In this report we describe that the cellular levels of AIB1 are controlled through regulated proteasomal degradation. We found that serum withdrawal or contact inhibition due to high cell density caused rapid degradation of AIB1 protein in immortalized cell lines. Proteasome inhibitors prevented this process and high molecular weight ubiquitylated species of AIB1 were detected. Immunofluorescence analysis of AIB1 indicated that the proteasomal degradation occurred predominantly in the nuclear compartment. However, treatment with the CRM1 dependent nuclear export inhibitor leptomycin B prevented induced degradation of AIB1 indicating that nuclear export was required for AIB1 degradation. Interestingly, pretreatment of the cells with the IKK inhibitor wedelolactone or the MEK 1/2 inhibitor UO126 also prevented induced degradation of AIB1. This data indicates that phosphorylation of AIB1 through either the IKK or MAPK signaling pathways might be required for AIB1 degradation. We have determined that the ubiquitin ligase E6-AP is also involved in induced degradation of AIB1. AIB1/E6-AP complexes were detected in cellular extracts and reduction of cellular E6-AP levels with E6-AP siRNA prevented proteasomal degradation of AIB1. Conversely, overexpression of E6-AP promoted AIB1 degradation. The C-terminus of AIB1 interacted with E6-AP in vitro and deletion of this region in AIB1 rendered it resistant to degradation in cells. Furthermore, the naturally occurring Δ3AIB1 isoform also showed resistance to degradation suggesting that the N-terminal HLH-PAS domain of AIB1 may be involved in its degradation. We propose a model whereby cellular signals, promoted by changes in the cellular milieu, initiate E6-AP-mediated degradation of AIB1 in human cancer cells.