Project 1. Structures, Functions and Substrates of Multi-subunit Clp
Protease Complexes in Plastids of Arabidopsis thaliana
Proteolysis in chloroplasts and non-photosynthetic
plastids is poorly understood, but it is critical for plant development and
plastid function. We are focusing on the function of Clp proteases in plastids
of Arabidopsis thaliana. Tetradecameric Clp protease core complexes in
non-photosynthetic plastids and chloroplasts of leaves of different Brassicaceae
were purified based on native mass and isoelectric point and identified by mass
spectrometry (refs #23, 33).
The protease complex consisted of five different
serine type protease Clp proteins (ClpP1,3-6) and four non-proteolytic ClpR
proteins (ClpR1-4). In addition, ClpS1,2, unique to land plants, tightly
interacted with this core complex. 3-D homology modeling, based on
high-resolution crystal structures of Escherichia coli ClpP subunits, showed
that the ClpP/R proteins fit well together in a tetradecameric complex and that
the ClpS proteins fit well on the axial sites of the ClpPR cores. In contrast to
plastids, plant mitochondria contained a single ~320 kDa homo-tetradecameric
ClpP2 complex, without association of ClpR or ClpS proteins (ref #33).
We are further characterizing the function and structure and substrates of this
proteolytic complex to answer why and how the Clp complex has evolved towards
such a tremendous complexity in the chloroplasts of photosynthetic organisms.
