The ubiquitin proteasome system classically selects its substrates for degradation by tagging them with ubiquitin. Here, we describe another means of controlling proteasome function in a global manner. The 26S proteasome can be inhibited by modification with the enzyme, O-GlcNAc transferase (OGT). This reversible modification of the proteasome inhibits the proteolysis of the transcription factor Sp1 and a hydrophobic peptide through inhibition of the ATPase activity of 26S proteasomes. The Rpt2 ATPase in the mammalian proteasome 19S cap is modified by O-GlcNAc in vitro and in vivo and as its modification increases, proteasome function decreases. This mechanism may couple proteasomes to the general metabolic state of the cell. The O-GlcNAc modification of proteasomes may allow the organism to respond to its metabolic needs by controlling the availability of amino acids and regulatory proteins.

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Background Stromal fibroblasts associated with in situ and invasive breast carcinoma differ phenotypically from fibroblasts associated with normal breast epithelium, and these alterations in carcinoma-associated fibroblasts (CAF) may promote breast carcinogenesis and cancer progression. A better understanding of the changes that occur in fibroblasts during carcinogenesis and their influence on epithelial cell growth and behavior could lead to novel strategies for the prevention and treatment of breast cancer. To this end, the effect of CAF and normal breastassociated fibroblasts (NAF) on the growth of epithelial cells representative of pre-neoplastic breast disease was assessed.

Although the knowledge that nuclear and cytoplasmic proteins are modified with N-acetylglucosamine has existed for decades, little has been shown as to its function until recently. There are now substantial data highlighting the significance of proper regulation of this modification in multiple cellular processes. Currently, only two enzymes are known that regulate this modification. O-GlcNAc transferase (OGT) modifies protein substrates posttranslationally by adding the N-acetylglucosamine. Bifunctional nuclear/cytoplasmic O-GlcNAcase and acetyl transferase (NCOAT) is responsible for cleaving the modification from target proteins. Here, we demonstrate for the first time an unusual association of these two opposing enzymes into a single O-GlcNAczyme complex. NCOAT and OGT associate strongly through specific domains such that NCOAT accompanies OGT, with histone deacetylases (HDACs), into transcription corepression complexes. Exclusion of NCOAT activities from OGT association blocks proper estrogen-dependent cell signaling as well as mammary development in transgenic mice. This demonstrates that NCOAT is in a strategic position to rapidly counteract OGT and HDAC without requiring its recruitment.