The Role of HSP70 and Its Co-chaperones in Protein Misfolding, Aggregation and Disease - The Networking of Chaperones by Co-chaperones - page 261

At the C-terminus of SACS is a higher eukaryotes and prokaryotes nucleotide-
binding (HEPN) domain (Grynberg et al. 2003 ), which binds to various nucleotides,
such as ATP, ADP, and GTP, but does not exhibit any GTPase or ATPase activity
(Kozlov et al. 2011 ). Crystal structure of the human SACS-HEPN domain revealed
that it exists as a dimer. An ARSACS patient mutation, N4549D, at the dimer in-
terface hinders protein folding and dimerization of the domain, which is likely re-
sponsible for its loss of function (Kozlov et al. 2011 ). HEPN domains are widely
distributed in eubacteria and archaea but are restricted to animals in eukaryotes
(Grynberg et al. 2003 ). In humans, the HEPN domain occurs only in the protein
SACS. It has been suggested that the close proximity of the SACS J-domain and
HEPN domain could be important for the hypothesised function of SACS as a co-
chaperone. The HEPN domain may increase the local concentration of GTP or ATP
to promote nucleotide exchange onto HSP70 (Kozlov et al. 2011 ).
Additionally, the N-terminus of SACS contains an ubiquitin-like (UbL) do-
main, which shares 43 % homology to the Rad23A UbL domain over 65 resi-
dues, and has been shown by co-immunoprecipitation to interact with the 20S
proteosomal alpha subunit C8 (Parfitt et al. 2009 ). Towards the C-terminus is
the XPCB domain, which shares 35 % homology with the hHR23 XPCB domain
(Kamionka and Feigon 2004 ). Interestingly, the hHR23 protein also contains an
UbL domain, which similar to SACS, interacts with the 19S regulatory subunit
of the 26S proteasome (Mueller and Feigon 2002 ). The SACS-XPCB domain
was recently identified as a potential binding domain for the E3 ubiquitin ligase
Ube3A, which is non-functional in Angelman's syndrome (Greer et al. 2010 ). In
Ube3A KO mice levels of ubiquitinated SACS were severely reduced, suggest-
ing Ube3A is responsible for ubiquitinating SACS (Jana 2012 ). Bioinformatic
analyses also suggests two UIMs in SACS, located either side of the XPCB do-
main. The presence of the UbL domain, XPCB domain and UIM motifs all point
towards a function for SACS in the ubiquitin-proteosome pathway of protein
degradation. This along with the presence of the J-domain and HSP90 like chap-
erone domains suggest that SACS plays a role in proteostasis. Interestingly, other
proteins that function in the ubiquitin-proteosome pathway are linked to ataxias,
for example, ataxin-3 contains a UIM and functions as a deubiquitination enzyme
(Burnett et al. 2003 ).
SACS's localisation indicates it may function at mitochondria. This is sup-
ported by recent findings which show that siRNA-mediated SACS knockdown
in SH-SY5Y cells leads to a more interconnected mitochondrial network (Girard
et al. 2012 ). Similarly, fibroblasts from ARSACS patients display a hyperfused
mitochondrial phenotype indicated by the presence of balloon-like or bulbed mi-
tochondria (Girard et al. 2012 ). This mitochondrial phenotype parallels that seen
when the mitochondrial fission protein Drp1 is silenced or when mutant forms of
Drp1 are overexpressed (Frank et al. 2001 ; Smirnova et al. 2001 ; Lee et al. 2004 ;
Estaquier and Arnoult 2007 ). This suggests that loss of SACS function leads to a
disruption in normal mitochondrial dynamics. Furthermore, a decrease in mito-
chondrial fission seems more likely than enhanced fusion due to the identification