p23 and Aha1 - The Networking of Chaperones by Co-chaperones - page 123

Hsp90 client GR, deletions of Aha1 and Hch1 in yeast as well as overexpression
and silencing of Aha1 in human cells demonstrated that Aha1 contributes to the
activation of this steroid hormone receptor (Harst et al. 2005 ). These findings led
to the conclusion that Aha1 generally enhances Hsp90 function due to its ability
to activate the ATPase activity. Nevertheless and in contrast to p23, no client pro-
tein complexes with Hsp90 and Aha1 were found for v-src or SHRs, even under
complex stabilizing conditions like the presence of molybdate (Sun et al. 2012 ). A
current study even shows that simultaneous binding of GR and Aha1 is not possible
due to an overlapping binding site on Hsp90 (Lorenz et al. 2014 ). The absence of
evidence for a Aha1/Hsp90/client-complex in combination with the observed in
vivo effects raises the possibility that the interaction is either indirect, happens after
client maturation or is simply too transient to detect (Sun et al. 2012 ).
Even though no direct interaction between Aha1, Hsp90 and the classic clients (v-
src and SHRs) are detectable, Aha1 was found in Hsp90-complexes with the cystic
fibrosis regulator CFTR (Wang et al. 2006 ; Koulov et al. 2010 ), the adenine nucleo-
tide transporter ANT1 (Bhangoo et al. 2007 ) and Akt kinase (Sun et al. 2012 ). In the
case of CFTR and especially its disease-related mutant ΔF508, the knockdown of
Aha1 promoted the translocation to the plasma membrane (Wang et al. 2006 ). This
result is consistent with the observation that Aha1 overexpression led to a decrease
in Akt activation, implying that stimulation of the ATPase is not generally correlated
to Hsp90's ability to facilitate protein folding or activation (Sun et al. 2012 ).
Pull-down of Aha1 and subsequent MS analysis revealed several more Aha1
interacting proteins that play a role in several cellular processes, among them DNA-
PKcs and Topo IIa. Most of the identified proteins are part of oligomeric complexes
that contain components known to interact with Hsp90 (Sun et al. 2012 ). Therefore,
Aha1 is thought to be involved in several further Hsp90-related processes. Until
today no Hsp90-independent function of Aha1 has been described.
The Interplay Between p23 and Aha1 in the Hsp90 Cycle
p23 and Aha1 are Hsp90 cochaperones with opposing effects on Hsp90. While
p23 inhibits the ATPase to about 50 % (Richter et al. 2004 ; Siligardi et al. 2004 ),
Aha1 stimulates the ATP turnover up to 30-fold (Panaretou et al. 2002 ; Retzlaff
et al. 2010 ). It therefore seems obvious that both cochaperones should not influence
Hsp90 simultaneously. As Aha1 is able to bind to Hsp90 in the nucleotide-free as
well as in the ATP-bound state, the Aha1-Hsp90 complex seems to be present dur-
ing different steps of the cycle while the p23-Hsp90 complex can only be formed in
the nucleotide-bound state. The ability of Aha1 to bind to different conformations of
Hsp90 entails at the same time the challenge of releasing this cochaperone to enable
further cofactor or client protein binding. In this regard, p23 was shown to replace
Aha1 from Hsp90 in a nucleotide-dependent manner. This is possible because both
cochaperones share a binding site in the middle domain of Hsp90 (Meyer et al.
2004 ; Ali et al. 2006 ; R￶hl et al. 2013 ) and therefore compete for Hsp90 binding.