Medicina y Microbiologia medica

PNAS PLUS

Dynamic gradients of an intermediate filament-like
cytoskeleton are recruited by a polarity landmark
during apical growth
Katsuya Fuchinoa, Sonchita Bagchib,1, Stuart Cantlaya,1, Linda Sandbladc, Di Wud, Jessica Bergmanb,
Masood Kamali-Moghaddamd, Klas Flärdha, and Nora Ausmeesa,2
a

Department of Biology, Lund University, 22362 Lund, Sweden; bDepartment of Cell and MolecularBiology, Uppsala University, 751 24 Uppsala, Sweden;
Department of Molecular Biology, Umeå University, 901 87 Umeå, Sweden; and dDepartment of Immunology, Genetics and Pathology, Science for Life
Laboratory, Uppsala University, 751 85 Uppsala, Sweden
c

Intermediate filament (IF)-like cytoskeleton emerges as a versatile
tool for cellular organization in all kingdoms of life, underscoring theimportance of mechanistically understanding its diverse manifestations. We showed previously that, in Streptomyces (a bacterium
with a mycelial lifestyle similar to that of filamentous fungi, including extreme cell and growth polarity), the IF protein FilP confers
rigidity to the hyphae by an unknown mechanism. Here, we provide
a possible explanation for the IF-like function of FilP bydemonstrating its ability to self-assemble into a cis-interconnected regular network in vitro and its localization into structures consistent with
a cytoskeletal network in vivo. Furthermore, we reveal that a spatially restricted interaction between FilP and DivIVA, the main component of the Streptomyces polarisome complex, leads to formation
of apical gradients of FilP in hyphae undergoing active tipextension.
We propose that the coupling between the mechanism driving polar
growth and the assembly of an IF cytoskeleton provides each new
hypha with an additional stress-bearing structure at its tip, where
the nascent cell wall is inevitably more flexible and compliant while
it is being assembled and matured. Our data suggest that recruitment of cytoskeleton around a cell polarity landmark isa broadly
conserved strategy in tip-growing cells.

C

oiled coil-rich proteins are emerging as wide-spread and important determinants of cell architecture in bacteria. A subclass
of such proteins possesses a segmented coiled-coil architecture,
resembling that of metazoan intermediate filament (IF) proteins.
Recently, an increasing number of reports have shown that
these putatively IF-likeproteins assemble into cytoskeletons in
evolutionarily diverse bacteria, of which several examples are
given below.
Crescentin, which determines the characteristic curved cell morphology in the ubiquitous aquatic bacterium Caulobacter crescentus,
was the first and is now the best characterized bacterial IF-like
protein (1, 2). Crescentin forms a long cable-like cytoskeletal
structure, whichis characterized by stability and very low turnover
rates (2, 3). Because it is attached to the cell membrane parallel to
the long cell axis in a stretched-out state, it applies physical force
to the underlying cell envelope, thereby locally reducing the rate
of new cell wall incorporation. The opposite sides of the cylindrical cell wall will therefore grow with different rates, resulting
ina crescent-shaped curved sacculus (1, 2). Thus, crescentin
functions via mechanical interference with the building of the
bacterial cell wall. Although less studied, filament-forming coiled
coil-rich proteins are important determinants also of the characteristic helical cell shape of Helicobacter species (4, 5). In Corynebacterium glutamicum, an essential coiled-coil protein, rod
shapemorphology protein (RsmP) forms a cytoskeletal element
involved in determination of the rod shape of the cells (6). Another
example of a coiled-coil cytoskeleton in bacteria is presented by
the conspicuous cytoplasmic filaments of Treponema (7). We have
previously shown that IF-like proteins are present and conserved

www.pnas.org/cgi/doi/10.1073/pnas.1305358110

in several species of the...