The Evolution and Function of Co-Chaperones in Mitochondria - The Networking of Chaperones by Co-chaperones - page 206

Pam17 is an integral membrane protein with no homology to known Hsp70 co-
chaperones (van der Laan et al. 2005 ). Disruption of the Pam17 function by genetic
depletion or mutagenesis results in the decrease in the formation of the Pam18/
Pam16 heterodimer and its association with the TIM23 machinery (Hutu et al.
2008 ; Popov-Celeketic et al. 2008 ; Schilke et al. 2012 ). Pam17 likely plays a role
in regulating the dynamics of this machinery during protein import, and does not
interact with the mitochondrial chaperones (Chacinska et al. 2010 ; Schiller 2009 ;
Lytovchenko et al. 2013 ).
The PAM components Ssc1, Mge1, Tim44, Pam16 and Pam18 appear to be found
in all eukaryotic organisms, and are highly conserved (Chen et al. 2013 ; Clements
et al. 2009 ; Dolezal et al. 2005 ; Rada et al. 2011 ; Sinha et al. 2010 ). Two proteins
found in extant alphaproteobacteria, TimA and TimB, have significant structural
similarities to Tim44 and Pam18, respectively. The J-domain of the bacterial TimB
can fully replace the J domain of yeast Pam18 upon single amino acid mutation
enabling the interaction with Pam16's J- like domain (Clements et al. 2009 ). These
proteins, with LivH (amino acid translocon) and DnaK are proposed to be the pro-
karyotic progenitors of the mitochondrial protein import machinery (Clements et al.
2009 ). The J and J-like domains of Pam18 and Pam16 are interchangeable between
human and yeast proteins in their ability to form a functional heterodimer and stim-
ulate Ssc1 (Elsner et al. 2009 ). Similarly, highly conserved proteins are present in
plants, and in parasitic organisms with highly divergent mitochondria (Rada et al.
2011 ; Dolezal et al. 2005 ; Chen et al. 2013 ). These findings strongly indicate the
importance of the function of mitochondrial co-chaperones in essential cellular pro-
cesses and the conservation of interactions between these components.
The number of components involved, and the unique nature of their interactions,
makes the PAM complex the most complicated Hsp70 machine known to date. Its
further study is likely to lead to greater understanding of the molecular and bio-
chemical properties of Hsp70 systems in general.
Molecular Chaperones and FeS Cluster Assembly
Iron-sulfur (FeS) cluster proteins are essential cellular components found in virtu-
ally all organisms studied so far (Balk and Lill 2004 ). In mitochondria, they are
involved in redox chemistry as components of respiratory chain (NADH dehydro-
genase, succinate dehydrogenase, Rieske protein) and metabolic conversions (acon-
itase, a key enzyme in the citric acid cycle). Machinery devoted to the assembly
of FeS clusters is highly conserved from prokaryotes to humans (Muhlenhoff and
Lill 2000 ); in bacteria and mitochondria alike, a dedicated Hsp70/J protein system
mediates this essential process (Craig and Marszalek 2002 ).
In Escherichia coli , the Isc operon encodes proteins essential for FeS cluster as-
sembly, such as IscU (Agar et al. 2000 ) as well as the Hsp70 protein Hsc66 (Seaton
and Vickery 1994 ) and the type III J protein Hsc20 (Kawula and Lelivelt 1994 ).
These two proteins interact with IscU and are important in FeS assembly in bacteria