GrpE, Hsp110/Grp170, HspBP1/Sil1 and BAG Domain Proteins: Nucleotide Exchange Factors for Hsp70 Molecular Chaperones - The Networking of Chaperones by Co-chaperones - page 14

BAG domain-containing NEFs
BAG (Bcl-2 associated athanogene) family proteins have a modular domain ar-
chitecture comprising a conserved region of ~ 100 amino acids at the C-terminus,
called the BAG domain (Takayama et al. 1999 ). In the N-terminal part diverse do-
mains and sequence motifs were found for BAG domain proteins (Fig. 1.3 ). The
human genome comprises six BAG family protein sequences, which were num-
bered Bag1-6 (Takayama and Reed 2001 ) (Table 1.1 ). As pointed out above, these
proteins are structurally and functionally quite heterogeneous, and will be discussed
here one after the other. Only Bag1 and Bag3 appear to be conserved in most meta-
zoans. Homologs have been described in the fruit fly Drosophila melanogaster
(Arndt et al. 2010 ), the nematode worm Caenorhabditis elegans (Nikolaidis and
Nei 2004 ) and the tunicate Ciona intenstinalis (Wada et al. 2006 ).
The first structures to be solved were the BAG domain of Bag1 in isolation and
in complex with the NBD of Hsc70, revealing a bundle structure with three long
α-helices for the BAG domain (Sondermann et al. 2001 ; Briknarova et al. 2001 )
(Fig. 1.2 ). Interactions with α-helices 2 and 3 of Bag1 stabilize a conformational
change in the Hsc70 NBD similar to the GrpEﾷDnaK complex (Harrison et al. 1997 ;
Sondermann et al. 2001 ). Three different isoforms of Bag1 exist in cells, which are
generated by alternative translation initiation from a single mRNA (Fig. 1.3 ). All
Bag1 isoforms contain an ubiquitin-like (Ubl) domain that serves as a sorting signal
to facilitate interaction with the 26S proteasome (Alberti et al. 2003 ). The Bag1 L
isoform contains an additional nuclear localization signal (NLS) at the extreme N-
terminus, whereas the other two isoforms are present in the cytosol (Takayama et al.
1998 ). Interestingly, the BAG domain shares binding sites with Hsc70 and Raf1, a
stress-signaling anti-apoptotic kinase, and the two proteins bind Bag1 in a mutually
exclusive manner (Song et al. 2001 ). The structure of the Ubl domain from mouse
Bag1 has been solved by NMR, revealing a characteristic ubiquitin-like fold (Huang
and Yu 2013 ). In mice, this domain of Bag1 mediates interaction with the cytoplas-
mic tail of the heparin-binding EGF-like growth factor (HB-EGF) precursor, thereby
altering cell adhesion and secretion of the mitogen HB-EGF (Lin et al. 2001 ).
Bag3 is expressed prominently in striated muscle tissue, but is also necessary
for development and blood cell formation. Bag3 deletion in mice resulted in severe
myopathy (Homma et al. 2006 ) and loss of hematopoietic stem cells (Kwon et al.
2010 ). Interestingly, Bag3 is the only heat stress-inducible BAG-domain protein
(Franceschelli et al. 2008 ; Jacobs and Marnett 2009 ). Bag3 contains various se-
quence motifs and domains, such as WW domains and proline-rich repeats (PXXP),
which mediate interactions with numerous partner proteins other than Hsp70. For
example, the first WW domain was shown to interact with PXXP motifs at the
C-terminus of PDZGEF2, a regulatory protein involved in cell adhesion (Iwasaki
et al. 2010 ); binding to the small heat shock proteins HspB8 and HspB6 is mediated
by two IPV motifs (Fuchs et al. 2010 ). The PXXP repeats of Bag3 likely inter-
act with SH3 domains found in regulatory proteins of cell adhesion and migration
(Doong et al. 2000 ). These interactions link Bag3 to processes such as development,
autophagy and cytoskeletal organization (reviewed in (Rosati et al. 2011 )). The