POSTERS - BLAST X - University of Utah

BLAST X Mon. Evening Session
DYNAMIC ASSEMBLY AND DISASSEMBLY OF THE TYPE IV MOLECULAR MACHINE
Iryna Bulyha and Lotte Søgaard-Andersen
Max Planck Institute for Terrestrial Microbiology, D-35043 Marburg, Germany
Myxococcus xanthus harbors two gliding motility systems, A and S. The S(ocial)-system
depends on type IV pili (T4P) and is generally active only when the cells are within contact
distance of each other. T4P undergo cycles of assembly and retraction. The force for S-motility
is generated by retraction of T4P. T4P are localized in a unipolar pattern and are present only at
the leading pole of the rod-shaped cells. M. xanthus cells periodically undergo cellular reversals
in which the old leading pole (which harbors T4P) becomes the new lagging pole (which does
not harbor T4P). These observations suggest that in parallel with a cellular reversal, the pole at
which T4P assemble switches. The molecular mechanisms regulating the T4P
extension/retraction cycle and underlying T4P pole switching remain unknown.
To investigate these mechanisms, we focused on the cellular localization of five highly
conserved T4P biogenesis proteins (PilB, PilT, PilM, PilC and PilQ), which are present in all
known T4P systems. PilB and PilT are cytoplasmic proteins and members of the secretion
ATPase superfamily of proteins; PilB is required for assembly of T4P, while PilT is necessary for
T4P retraction. PilM shows similarity to MreB/FtsA and is indispensable for T4P assembly. PilC
is an inner membrane protein and is necessary for T4P assembly. The PilQ secretin forms a
gated oligomeric channel for the pilus in the outer membrane. Using immuno-fluorescence
microscopy, and time-lapse fluorescence microscopy with functional YFP-fusions, we show that
PilQ, PilC and PilM are localized in clusters at both cell poles. These clusters have equal
intensities and they do not oscillate between the two poles during reversals. The analysis of PilB
and PilT localization revealed that both proteins are localized in polar clusters. PilB is
predominantly localized in a cluster at the leading pole and PilT is predominantly localized in a
cluster at the lagging cell pole. This localization is dynamic and the two proteins oscillate
between the poles during reversals. These observations show that T4P function depends on
two sets of proteins: one set is statically localized at both cell poles, and the other set is
dynamically localized. Based on these findings we suggest that during cellular reversals, the
T4P machinery is disassembled at the old leading pole and reassembled at the new leading
pole.
Moreover, we will present the data which suggest that the T4P assembly/retraction cycle
relies on a PilB/PilT competition-based mechanism. According to this model, PilB at the leading
cell pole stimulates T4P assembly, and the occasional accumulation of PilT at the leading cell
pole results in retraction. Therefore, the two dynamically localized ATPases determine whether
assembly or disassembly of pilus takes place.
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