In this study, in vitro RNA binding by members of the mammalian 70-kDa heat 
shock protein (Hsp) family was examined. We show that Hsp/Hsc70 and Hsp110 
proteins preferentially bound AU-rich RNA in vitro. Inhibition of RNA binding by 
ATP suggested the involvement of the N-terminal ATP-binding domain. By using 
deletion mutants of Hsp110 protein, a diverged Hsp70 family member, RNA binding 
was localized to the N-terminal ATP-binding domain of the molecule. The 
C-terminal peptide-binding domain did not bind RNA, but its engagement by a 
peptide substrate abrogated RNA binding by the N terminus of the protein. 
Interestingly, removal of the C-terminal alpha-helical structure or the 
alpha-loop domain unique to Hsp110 immediately downstream of the peptide-binding 
domain, but not both, resulted in considerably increased RNA binding as compared 
with the wild type protein. Finally, a 70-kDa activity was immunoprecipitated 
from RNA-protein complexes formed in vitro between cytoplasmic proteins of human 
lymphocytes and AU-rich RNA. These findings support the idea that certain heat 
shock proteins may act as RNA-binding entities in vivo to guide the appropriate 
folding of RNA substrates for subsequent regulatory processes such as mRNA 
degradation and/or translation.