Chaperone Hsp27 inhibits translation during heat shock by binding eIF4G and facilitating dissociation of cap-initiation complexes

Inhibition of protein synthesis during heat shock limits accumulation of un folded proteins that might damage eukaryotic cells. We demonstrate that cha perone Hsp27 is a heat shock-induced inhibitor of cellular protein synthesi s. Translation of most mRNAs requires formation of a cap-binding initiation complex known as eIF4F, consisting of factors eIF4E, eIF4A, eIF4E kinase M nk1, poly(A)-binding protein, and adaptor protein eIF4G. Hsp27 specifically bound eIF4G during heat shock, preventing assembly of the cap-initiation / eIF4F complex and trapping eIF4G in insoluble heat shock granules. eIF4G is a specific target of Hsp27, as eIF4E, eIF4A, Mnk1, poly(a)-binding protein , eIF4B, and eIF3 were not bound by Hsp27 and were not recruited into insol uble complexes. Dissociation of eIF4F was enhanced during heat shock by ect opic overexpression of Hsp25, the murine homolog of human Hsp27. Overexpres sion of Hsc70, a constitutive homolog of Hsp70, prevented loss of cap-initi ation complexes and maintained eIF4G solubility. Purified Hsp27 specificall y bound purified eIF4G in vitro, prevented in vitro translation, eliminated eIF4G interaction with protein binding factors, and promoted eIF4G insolub ilization. These results therefore demonstrate that Hsp27 is a heat-induced inhibitor of eIF4F-dependent mRNA translation.