Thus far, the only known function of eEF2 is its role in mRNA
translation by facilitating the GTP-dependent translocation of the
ribosome. Unexpectedly, we found that, in addition to its presence in
the cytoplasm, eEF2 also exists in the mitochondria (Fig. 1)
and interacts with Drp1 to promote mitochondrial fission (Fig.
4). Although these novel findings appear to stray from the current
knowledge about eEF2, they clearly show that this protein has an
important regulatory role in mitochondrial dynamics. Noteworthily, novel
functions of other elongation factors have reported recently. For
instance, it was found that elongation factor-Tu could specifically bind
to T6S integral membrane toxin, and this interaction is required for
delivery of this toxin to target cells.(13) This finding provides
another evidence that elongation factors may have the aptitude for
interacting with other proteins, affecting certain cellular functions.
It was shown in a previous study that silencing of eEF2 expression
caused morphological alterations of mitochondria and other
organelles,(14) but no further investigation on the role of eEF2
in mitochondria regulation has been reported so far. Here, we show that
eEF2 is not only present in the cytoplasm, but also exists in the
mitochondria and has a promotive role in mitochondrial fission. The
positive regulation of mitochondrial fission by eEF2 was evidenced by
our observation that knockdown of eEF2 increased the length of
mitochondria (Fig. 2a, b and c) and forced expression of eEF2
decreased the length of this organelle (Fig. 2d). Expression of
eEF2 also affected mitochondrial metabolism (Fig. 3) and cell
proliferation (SFig. 5), the likely consequences of the altered
mitochondrial fission mediated by eEF2 expression. Mechanistically, eEF2
appears to stimulate the activity of GTPase of Drp1 through its
interaction with this major component of the fission machinery, as these
two proteins are physically associated and co-localized (Fig.
4), and deletion of the GTP binding site of eEF2 substantially weakened
its binding to Drp1 (Fig. 4) and impaired its ability to
activate GTPase of Drp1 (Fig. 6) and to support mitochondrial
fission (Fig. 5).
Drp1 is a cytosolic protein, which upon activation translocates to the
outer membrane of the mitochondria and plays a central role in inducing
mitochondrial fission. Nevertheless, how precisely Drp1 is targeted and
translocated from the cytosol to the mitochondria remains less clear. We
show here that silencing eEF2 expression not only decreased the
mitochondrial eEF2 but also Drp1 in the mitochondria (Fig. 4b);
by contrast, forced expression of eEF2 increased the mitochondrial Drp1
(Fig. 4a). Therefore, it is likely that binding of eEF2 to Drp1
facilitates the translocation of Drp1 to the outer membrane of the
mitochondria. It is known that phosphorylation of eEF2 by eEF2 kinase
reduces its binding affinity to the ribosome, thereby inhibiting peptide
elongation. Our experiments showed that the cells with depletion of eEF2
kinase had a significantly increase of mitochondrial fission
(SFig. 4b), (15) suggesting that phosphorylation of eEF2
limits its interaction with Drp1. Additionally, it was reported that
eEF2 can be phosphorylated by cyclin A-CDK2 at Se-595; thus, cyclin
A-CDK2 might affect the eEF2-mediated mitochondrial fission as well.
As mitochondrial fusion and fission are important determinants of the
functions of this organelle and defects in these processes are
implicated in various human diseases such as cardiovascular,
neurodegenerative, endocrine and neoplastic diseases,(16-18 ) our
revelation of eEF2 as a new regulator of mitochondrial dynamics might
provide a potential target for therapeutic intervention of those human
disorders resulting from mitochondria dysfunction. Notably, small
molecule inhibitors of eEF2 have just been developed
recently,(19 ) and this may provide a promising opportunity to
test their effects on the mitochondrial activity and the
mitochondria-associated biological functions and their potential as
therapeutic agents for the mitochondria-associated diseases.