Biochemical characterization of an intermediate membrane subfraction in cyanobacteria involved in an assembly network for photosystem II

Oxygenic photosynthesis converts light energy into chemical energy
and is responsible for generating most of our atmosphere’s oxygen
and biomass on earth. Several multimeric protein complexes are
involved in the underlying photosynthetic electron transfer chain
with photosystem II (PSII) representing the initial complex
mediating the extraction of electrons from water molecules, thus
generating molecular oxygen as a by-product. During recent years,
the structural details and components of the PSII complex,
including its inorganic and organic cofactors, have been elucidated
in great detail. However, little is known about the assembly
pathway of this at least 20 protein subunits containing machinery.
Previous work indicated that PSII assembly occurs in a step-wise
fashion and requires a number of facilitating factors, which
interact transiently with nascent PSII complexes. Earlier studies
of one of those assembly factors, the cyanobacterial PratA protein,
suggested that PSII biogenesis does not only underlie a temporal
order but is also organized at the spatial level, as PratA was
shown to mark a special intermediate membrane subfraction (PDMs),
hypothesized to represent regions for initial steps of PSII
biogenesis. The presented work focused on a more detailed
characterization of PDMs, clearly supporting their significance not
only with regard to early protein assembly, but also concerning
pigment synthesis and integration into the PSII precomplexes. The
PDMs could further be allocated to special membrane regions, named
biogenesis centers, at sites where thylakoid membranes converge to
the plasma membrane, thus demonstrating the spatial organization of
PSII assembly at the cellular level. Moreover, a novel function of
PratA in preloading of PSII with Mn2+ ions, necessary for
construction of the water-splitting complex, was discovered.
Concomitantly with progression of the assembly, the nascent PSII
complexes are transported from the PDMs to the thylakoid membrane
system, where Sll0933 – a novel PSII assembly factor identified in
this thesis – mediates the integration of the PSII inner antenna
proteins followed by completion of the assembly process.
Additionally, it could be shown that many of the so far identified
facilitating factors interact with each other and, thus, form a
complex network for PSII assembly. Especially the interaction
between the two assembly factors YCF48 and Sll0933 was
characterized in more detail, revealing a successive mode of action
with YCF48 operating upstream of Sll0933. Taken together, the
presented results enable the development of an extended and
elaborated model of PSII assembly, which is a concerted process
connecting protein and cofactor synthesis/integration in a
spatiotemporal manner, and thus contribute to a more profound
understanding of photosynthesis itself.