p23 and Aha1 - The Networking of Chaperones by Co-chaperones - page 113

Fig. 6.1 The ATPase cycle of
Hsp90. During one ATPase
cycle, Hsp90 changes from
an open, nucleotide-free
state to a closed, ATP-bound
conformation. After ATP
binding, the N-terminal
domains (ND) dimerize,
leading to the closed 1 state.
Rearrangement of the middle
domains (MD) leads to the
hydrolysis-competent closed
2 conformation. ADP and Pi
are released after hydrolysis,
leaving Hsp90 again in an
open conformation, ready for
a further cycle

ND

CR

A i P

A

MD

CD

open

closed 2

closed 1

1997 ), is connected via a charged region (CR) to the middle domain (Hainzl et al.
2009 ; Wayne and Bolon 2010 ; Tsutsumi et al. 2012 ). The middle domain is thought
to be important for client protein binding (Meyer et al. 2003 ; Hawle et al. 2006 ;
Lorenz et al. 2014 ). The C-terminal domains are responsible for the dimerization of
the protein (Nemoto et al. 1995 ). They are essential for viability and client activa-
tion (Wayne and Bolon 2007 ).
During one cycle of ATP hydrolysis, as shown in Fig. 6.1 , Hsp90 changes from
an open, nucleotide-free conformation to a closed, ATP-bound state (Sullivan et al.
1997 ; Ali et al. 2006 ; Hessling et al. 2009 ; Mickler et al. 2009 ). It has been recently
shown that binding of ATP to both subunits, but not the hydrolysis of both nucleo-
tides is essential (Mishra and Bolon 2014 ). Binding of the nucleotides induces the
closing of the ATP lid and the subsequent dimerization of the N-terminal domains,
leading to a conformation called “closed 1” state (Li et al. 2013 ). This is only par-
tially closed and ATP exchange is still possible. Next, a reorientation of the middle
domains occurs, leading to contacts between the N- and M-domains. This “closed
2” position represents the state in which ATP is hydrolyzed (Li et al. 2013 ). After
hydrolysis, Hsp90 returns to the open state thereby releasing the hydrolysis prod-
ucts ADP and Pi.
Even though Hsp90's chaperone function relies on the ATPase activity (Ober-
mann et al. 1998 ; Panaretou et al. 1998 ), the ATP turnover is very slow. For yeast
Hsp90 1 ATP per minute is hydrolyzed (Panaretou et al. 1998 ). For human Hsp90
one turnover takes 10 min (McLaughlin et al. 2002 ). This slow ATPase activity
is not a consequence of a slow binding or hydrolysis of ATP, but rather due to the
large conformational changes within the protein (Weikl et al. 2000 ; Hessling et al.
2009 ). These conformational changes and hence the ATPase activity of Hsp90 can
be modulated by the cohort of Hsp90 cochaperones.
Hsp90 cochaperones are divided into two subgroups according to the presence
of a TPR-domain (tetratricorepeat): TPR-cochaperones and non-TPR-cochaperones