Chaperone Hsp27, a Novel Subunit of AUF1 Protein Complexes, Functions in AU-Rich Element-Mediated mRNA Decay

Abstract

Controlled, transient cytokine production by monocytes depends heavily upon rapid mRNA degradation, conferred by 3′ untranslated region-localized AU-rich elements (AREs) that associate with RNA-binding proteins. The ARE-binding protein AUF1 forms a complex with cap-dependent translation initiation factors and heat shock proteins to attract the mRNA degradation machinery. We refer to this protein assembly as the AUF1- and signal transduction-regulated complex, ASTRC. Rapid degradation of ARE-bearing mRNAs (ARE-mRNAs) requires ubiquitination of AUF1 and its destruction by proteasomes. Activation of monocytes by adhesion to capillary endothelium at sites of tissue damage and subsequent proinflammatory cytokine induction are prominent features of inflammation, and ARE-mRNA stabilization plays a critical role in the induction process. Here, we demonstrate activation-induced subunit rearrangements within ASTRC and identify chaperone Hsp27 as a novel subunit that is itself an ARE-binding protein essential for rapid ARE-mRNA degradation. As Hsp27 has well-characterized roles in protein ubiquitination as well as in adhesion-induced cytoskeletal remodeling and cell motility, its association with ASTRC may provide a sensing mechanism to couple proinflammatory cytokine induction with monocyte adhesion and motility.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/ .

ACKNOWLEDGMENTS

We thank Andy Clark and Daiya Takai for plasmids, Nahum Sonenberg for eIF4G antibodies, Daniel Sinsimer for assistance with molecular beacon design and methods, and Gerald Wilson for reporter quantitative RT-PCR methods and assistance with statistical analyses. We thank Lori Covey for comments on the manuscript.

This work was supported by grants P01 AI057596 from the NIH to S.P. and G.B. and R01 AI059465 from the NIH to S.P. K.S. was supported by training grant T32 AI00743 from the NIH to S.P. F.M.G. and A.M.K. were supported by Integrative Graduate Education and Research Traineeship DGE0333196 from the NSF to Prabhas Moghe (Department of Biomedical Engineering, Rutgers University). F.M.G. was also supported by Initiative for Minority Students R25 GM058389 from the NIH to Michael Leibowitz (UMDNJ).