GrpE, Hsp110/Grp170, HspBP1/Sil1 and BAG Domain Proteins: Nucleotide Exchange Factors for Hsp70 Molecular Chaperones - The Networking of Chaperones by Co-chaperones - page 19

of nucleotide exchange and premature conversion of Ssc1p to the low affinity state,
thus causing rapid release of the protein in transit (Schneider et al. 1996 ). In vitro
studies have shown that ATP but not ADP effectively releases Mge1p interaction
with mtHsp70 (Miao et al. 1997 ). In the mitochondrial matrix, Mge1p furthermore
helps to fold newly imported proteins. A strain harboring the temperature sensitive
allele mge1-100 showed reduced rates of maturation of the Yfh1p protein, similar
to the defect observed in a deletion strain for the mitochondrial Hsp70 isoform
Ssq1p, suggesting a close relationship between the two proteins in substrate folding
(Schmidt et al. 2001 ). Ssc1p and the much less abundant Ssq1p compete for bind-
ing to Mge1p (Schmidt et al. 2001 ). Overexpression of Mge1p increases the activity
of Ssq1p, indicating that Mge1p availability is limiting for Ssq1p function. A role
of Mge1p as a possible sensor of stress is attributed to the reversible cessation of
the interaction between Mge1p and mitochondrial Hsp70s at heat shock tempera-
tures and under conditions of oxidative stress (Marada et al. 2013 ; Moro and Muga
2006 ). A similar role has been discussed for bacterial GrpE (Nakamura et al. 2010 ).
The loss of interaction with Hsp70 has been attributed to a transition from active
dimer to inactive monomer.
The ER is a major folding compartment of the cell, handling the folding, matura-
tion and post-translational modification of secretory and membrane proteins, and
is therefore rich in molecular chaperones. Through the 'unfolded protein response'
(UPR) pathway, the folding capacity is adapted to the protein load. Aberrant pro-
teins are efficiently cleared by ER-associated decay (ERAD), a process which retro-
translocates substrates into the cytosol for proteasomal degradation. Therefore it
is not surprising that the combined function of the ER-NEFs Lhs1p and Sil1p is
essential and the double-deletion lethal (Tyson and Stirling 2000 ). Both proteins
contribute to co-translational import of proteins into the ER lumen together with the
luminal Kar2p and the translocon-associated J-domain protein Sec63p, and to sub-
sequent folding. Both LHS1 and SIL1 are up-regulated by the ER stress response,
the 'unfolded protein response' (UPR), thus increasing the folding capacity of the
ER. Deletion of either factor triggers the UPR. This may explain why the single
deletions have comparatively mild phenotypes such as altered protein maturation
in the ER lumen and increased ERAD. Lhs1p and Sil1p are however only partially
redundant (de Keyzer et al. 2009 ; Tyson and Stirling 2000 ). Although Sil1p appears
to be about one order of magnitude more abundant than Lhs1p (but constitutes less
than 0.1 % of the Kar2p content) under normal growth conditions (Ghaemmaghami
et al. 2003 ), lhs1 Δ cells exhibit a slight import defect, as indicated by the accumula-
tion of pre-proteins.
The cytosolic Hsp110 family proteins, Sse1p and Sse2p, are closely related, hav-
ing 76 % sequence identity (Mukai et al. 1993 ). Sse1p and Sse2p probably origi-
nated from a recent genome duplication event in S. cerevisiae . Sse2p is 10 times less
abundant that Sse1p, although both proteins are expressed under normal conditions.
Sse1p is the most abundant NEF in the yeast cytosol, but the concentration of cyto-
solic Hsp70 proteins is one order of magnitude higher. Under stress, SSE2 gene ex-
pression is strongly induced by the heat shock response (HSR) pathway, while that
of SSE1 is only modestly increased (Mukai et al. 1993 ). The loss of Sse1p, but not