Ubiquitin-dependent and Ubiquitin-independent Control of Subunit Stoichiometry in the SWI/SNF Complex

Brian R Keppler,Trevor K Archer

doi:10.1074/jbc.m110.173997

Abstract

The mammalian SWI/SNF chromatin remodeling complex is a key player in multiple chromatin transactions. Core subunits of this complex, including the ATPase, Brg-1, and various Brg-1-associated factors (BAFs), work in concert to maintain a functional remodeling complex. This intra-complex regulation is supervised by protein-protein interactions, as stoichiometric levels of BAF proteins are maintained by proteasomal degradation. We show that the mechanism of BAF155-mediated stabilization of BAF57 involves blocking its ubiquitination by preventing interaction with TRIP12, an E3 ubiquitin ligase. Consequently, as opposed to complexed BAF57, whose principal lysines are unavailable for ubiquitination, uncomplexed BAF57 can be freely ubiquitinated and degraded by the proteasome. Additionally, a BAF57 mutant, which contains no lysine residues, was found to retain its ability to be stabilized by interaction with BAF155, suggesting that in addition to the ubiquitin-dependent mechanism of BAF57 degradation, there exists a ubiquitin-independent mechanism that may involve the direct interaction of BAF57 with the proteasome. We propose that this regulatory mechanism exists to ensure functional fidelity of the complex and prevent the accumulation of uncomplexed proteins, which may disrupt the normal activity of the complex.

Highlights

Mammalian DNA is packaged into chromatin with the help of nucleosomes as a means to regulate transcription, replication, DNA repair, and recombination, all highly dynamic processes [1]
BAF155 Blocks the Ubiquitination of BAF57—The SWI/SNF chromatin remodeling complex is composed of a number of core subunits including Brg-1, which possesses ATPase activity, and accompanying Brg1-associated factors (BAFs)
Using the BAF155-BAF57 relationship as a model, we hypothesized that because BAF155 stabilizes BAF57 by rescuing it from proteasomal degradation, perhaps BAF155 blocks the ubiquitination of BAF57

Summary

Introduction

Mammalian DNA is packaged into chromatin with the help of nucleosomes as a means to regulate transcription, replication, DNA repair, and recombination, all highly dynamic processes [1]. BAF155 Blocks the Ubiquitination of BAF57—The SWI/SNF chromatin remodeling complex is composed of a number of core subunits including Brg-1, which possesses ATPase activity, and accompanying BAFs. Previous reports have shown that the stoichiometry of these BAFs is highly important to the complex and that BAF155 plays a pivotal scaffolding role in regulating the protein levels of other complex members [14].

Results

Conclusion