Note to users. If you're seeing this message, it means that your browser cannot find this page's style/presentation instructions -- or possibly that you are using a browser that does not support current Web standards. Find out more about why this message is appearing, and what you can do to make your experience of our site the best it can be.

Subscribe

Sci. Signal., 4 March 2008
Vol. 1, Issue 9, p. ec82
[DOI: 10.1126/stke.19ec82]

EDITORS' CHOICE

Protein Stability Preventing Interaction with the Proteasome

Nancy R. Gough

Science Signaling, AAAS, Washington, DC 20005, USA

Steroid receptor coactivator 3 (SRC-3) interacts with estrogen receptors to promote estrogen-mediated transcription and genetic amplification, and overexpression of SRC-3 has been associated with various cancers, including breast cancer. Phosphorylation by p38 mitogen-activated protein kinase or glycogen synthase kinase 3β has been associated with enhanced degradation of SRC-3. Yi et al. sought to determine whether other kinases regulated SRC-3 activity or abundance and, through cotransfection experiments, identified atypical protein kinase C, both aPKC{zeta} and PKC{iota}, as kinases that when coexpressed with tagged SRC-3 and the estrogen receptor ER{alpha} increased the abundance of SRC-3. The stabilized SRC-3 was functional and stimulated estrogen-dependent gene expression both in transfected cells and in an in vitro transcription assay. Knockdown experiments with siRNAs confirmed that endogenous SRC-3 abundance was decreased in cells in which abundance of both aPKCs was decreased. The ability of aPKC{zeta} to increase the abundance of SRC-3 depended on ER{alpha} and estrogen and did not occur when the cells were cotransfected with the ERβ estrogen receptor. Coimmunoprecipitation experiments showed that both ER{alpha} and SRC-3 interacted with aPKC{zeta}. Although the ability of aPKC{zeta} to stabilize SRC-3 required ER{alpha} and estrogen and SRC-3 mutants that did not bind ER{alpha} were not stabilized by aPKC{zeta}, the interaction between SRC-3 and aPKC{zeta} did not require the presence of ER{alpha}. Instead, in vitro kinase assays suggested that ER{alpha} promoted the phosphorylation of SRC-3 by aPKC{zeta}. Mutation analysis indicated that the aPKC-mediated stabilization of SRC-3 involved an acidic region in the C-terminal region of SRC-3. SRC-3 interacted with the C8 subunit of the 20S proteasome in glutathione S-transferase pull-down experiments, and SRC-3 isolated from cells cotransfected with ER{alpha} and aPKC{zeta} exhibited reduced interaction with this proteasomal subunit. Proteasomal degradation can be mediated by ubiquitin-dependent and -independent mechanisms, the latter involving a REG{gamma} complex. In vitro assays for ubiquitin-dependent or REG{gamma}-dependent proteasomal degradation showed that SRC-3 from cells cotransfected with ER{alpha} and aPKC{zeta} was resistant to degradation. The authors suggest that phosphorylation of SRC-3 by aPKCs leads to a conformational change that exposes the acidic region and prevents the interaction between SRC-3 and the C8 proteasomal subunit.

P. Yi, Q. Feng, L. Amazit, D. M. Lonard, S. Y. Tsai, M.-J. Tsai, B. W. O'Malley, Atypical protein kinase C regulates dual pathways for degradation of the oncogenic coactivator SRC-3/AIB1. Mol. Cell 29, 465-476 (2008). [PubMed]

Citation: N. R. Gough, Preventing Interaction with the Proteasome. Sci. Signal. 1, ec82 (2008).


To Advertise     Find Products


Science Signaling. ISSN 1937-9145 (online), 1945-0877 (print). Pre-2008: Science's STKE. ISSN 1525-8882