Identifizierung und Charakterisierung der mitochondrialen Translokationspore Tim23.2 als Calmodulin-bindendes Protein

In order to react to changes within their environment, plants
developed a specific signaling network that enables the cells to
convert external stimuli including light, abiotic and biotic stress
as well as hormones into cellular signals. One example is the
influx of calcium, a second messenger stored in apoplasts or
internal reservoirs, into the cytosol. This causes changes in the
calcium-ion-concentration within the cell that are recognized by
specific sensors including Calmodulin and lead to the induction of
a cellular signal response. Calcium signals do not only occur in
the cytosol, but also appear within the nucleus, chloroplasts,
mitochondria as well as peroxisoms (Bachs et al. 1992, Chigri et
al. 2005, Kuhn et al. 2009, Dolze et al. 2013). The import of
nuclear encoded proteins into the mitochondria is regulated by
calcium and Calmodulin at level of the TIM23- and TIM22-complex.
This study identified atTim23.2, the pore-forming component of the
TIM23-complex, as a Calmodulin-binding protein. Pull-down-assays
using Calmodulin-agarose revealed a specific and calcium-dependent
binding. Furthermore, in silico analysis identified two potential
Calmodulin-binding domains (CaMBD). Topology studies of atTim23.2
demonstrated that the proposed N-terminal CaMBD is located within
the intermembrane space, the binding region within the first loops
is located in the matrix of the mitochondria. Moreover, a topology
of four transmembran domains of the protein could be shown. The
recently in the mitochondria identified Calmodulin-like protein
CML30 appeared to be a potential binding partner for atTim23.2.
CML30 could be indeed detected in the intermembrane space of the
mitochondria, but a direct interaction of the two proteins could
not have been detected so far. Furthermore, using the
split-ubiquitin system proved the ability of atTim23.2 to dimerize
which might be responsible for the regulation of opening and
closing of the importpore as it was already shown in S.cerevisiae.
However, a correlation between the two functions of atTim23.2 to
bind Calmodulin as well as to dimerize could not have been
confirmed, yet. Nevertheless, the regulation of the pore via the
calcium/Calmodulin signaling network could connect the import
process of matrix proteins with the stress regulation of the cell.