GrpE, Hsp110/Grp170, HspBP1/Sil1 and BAG Domain Proteins: Nucleotide Exchange Factors for Hsp70 Molecular Chaperones - The Networking of Chaperones by Co-chaperones - page 11

stabilize open states in which a subset of the interactions between NBD and ADP is
disabled, thereby lowering ADP affinity. Substantial parts of the NBD contact area
with GrpE become buried near the lobe interface in the ADP-bound conformation
of DnaK, suggesting that GrpE captures open conformations, but cannot 'force' the
NBD to open. ATP binding induces a conformational change in the NBD of DnaK,
displacing the binding sites on lobes I and II by inter-lobe shearing, resulting in
strongly decreased affinity to GrpE. So both ADP and ATP compete with GrpE for
binding to DnaK.
The Hsp110 Family of Nucleotide Exchange Factors
The Hsp110/Grp170 proteins belong to the Hsp70 protein family (Easton et al.
2000 ). Crystal structures of the yeast Hsp110 protein Sse1p revealed a shared do-
main composition comprising a N-terminal actin-type nucleotide binding domain,
followed by a ʲ-domain and a α-helix bundle (Liu and Hendrickson 2007 ; Polier
et al. 2008 ; Schuermann et al. 2008 ). Hsp110 family protein sequences are however
much less conserved than canonical Hsp70, with the greatest divergence found in
the C-terminal domains. Backbone extensions compared to canonical Hsp70 pro-
teins are found at the C-terminus and within the ʲ-domain (Fig. 1.3 ). The Grp170
homologs have even larger extensions than cytosolic homologs and always bear N-
terminal import and C-terminal ER-retention signal sequences (Table 1.1 ).
In the crystal structures of Sse1p, the α-helix bundle is associated with the flank
of the NBD, resulting in a compact conformation (Fig. 1.2 ). The ʲ-domain under-
goes extensive interactions with the bottom of the NBD, but not with the α-helix
bundle domain, which extends in the opposite direction. Sse1p exhibits a pro-
nounced twist of the NBD lobes, revealing a bound ATP molecule in the center.
Structures of an ATPase-inactive DnaK mutant later demonstrated that the binding
of ATP induces a very similar conformation in canonical Hsp70 proteins (Kityk
et al. 2012 ; Qi et al. 2013 ).
In the crystal structures of the complex, the NBDs of Sse1p and mammalian
Hsp70 face each other in a pseudo-symmetrical fashion (Polier et al. 2008 ; Schuer-
mann et al. 2008 ). The NBD of Hsp70 is captured in an open conformation by ad-
ditional interactions of subdomain IIB with the α-helix bundle domain of Sse1p. In
this conformation, ADP cannot simultaneously engage in direct interactions with
all four subdomains and is thus more likely to dissociate, explaining the nucleotide
exchange activity of Sse1p. The residues mediating key contacts to Hsp70 are con-
served in all Hsp110/Grp170 proteins (Andreasson et al. 2010 ; Hale et al. 2010 ).
Only the compact, ATP-bound conformation of Hsp110/Grp170 proteins provides
the necessary geometry required for simultaneous interactions between NBDﾷNBD
and α-helix bundle·subdomain IIB of Hsp110/Grp170 and Hsp70, respectively
(Raviol et al. 2006b ; Shaner et al. 2004 ; Andreasson et al. 2008 ).
Besides serving as essential NEFs for Hsp70, Hsp110/Grp170 proteins po-
tently stabilize denatured proteins against aggregation (Goeckeler et al. 2002 ;