Coiled-coil protein Scy is a key component of a multiprotein assembly controlling polarized growth inStreptomyces

Abstract

Polarized growth in eukaryotes requires polar multiprotein complexes. Here, we establish that selection and maintenance of cell polarity for growth also requires a dedicated multiprotein assembly in the filamentous bacterium, Streptomyces coelicolor. We present evidence for a tip organizing center and confirm two of its main components: Scy (Streptomyces cytoskeletal element), a unique bacterial coiled-coil protein with an unusual repeat periodicity, and the known polarity determinant DivIVA. We also establish a link between the tip organizing center and the filamentforming protein FilP. Interestingly, both deletion and overproduction of Scy generated multiple polarity centers, suggesting a mechanism wherein Scy can both promote and limit the number of emerging polarity centers via the organization of the Scy-DivIVA assemblies. We propose that Scy is a molecular "assembler," which, by sequestering DivIVA, promotes the establishment of new polarity centers for de novo tip formation during branching, as well as supporting polarized growth at existing hyphal tips.bacterial polarized growth | polar multiprotein assembly | cell division | modified hendecad coiled coil H ow organisms, cells, or tissues establish polarity is one of the fundamental questions in developmental biology. In eukaryotes, from unicellular organisms to multicellular plants and animals, some of the core mechanisms for generating polarity are conserved (1). Sites for polarization must be selected using positional markers, followed by the recruitment of a complex assembly, which, in turn, orients cytoskeletal filaments that deliver vesicles to the particular sites. A specific example of polarity is polarized growth, during which cells select a single polarization site and generate elongated, cylindrical shapes, such as neuronal dendrites in animals, root hairs and pollen tubes in plants, hyphal growth of filamentous fungi, elongation of Schizosaccharomyces pombe, or the short period of polarized growth during budding in Saccharomyces cerevisiae. However, the presumed earliest examples of polarized growth can be found in bacteria. These include the actinomycete Streptomyces coelicolor, which is used as a model organism for studying morphological differentiation and filamentous growth.The complex life cycle of S. coelicolor begins with an ovoid spore that contains a single chromosome. During germination, long, multigenomic filaments (germ tubes) are formed, which branch regularly to generate a network of hyphal filaments. Branching is a necessity for exponential growth because the rate of tip elongation cannot exceed a certain maximum; hence, there is an exponential increase in the number of new tips. New tips develop on the lateral wall well behind the existing tip, a phenomenon also observed and described as apical dominance in eukaryotic filamentous fungi. When grown on semisolid agar medium, these hyphal filaments first grow across and into the solid medium, generating the vegetative mycelium, followed by the formation of an aerial mycelium by h...

Taxonomy, Physiology, and Natural Products of Actinobacteria

“…The protein FilP forms intermediate filament-like structures that contribute to mechanical stress resistance (322). In addition, the Scy protein, encoded by a gene immediately adjacent to filP, apparently functions as a "molecular assembler" and sequesters DivIVA (323). DivIVA is essential for growth in streptomycetes and localizes to tips to drive apical growth, although the molecular mechanism of this process is still unclear (325,326).…”

“…It also interacts with the chromosome-partitioning protein ParA (327) and the intermediate filament-like protein FilP, which in turn interacts with DivIVA. The apical assembly that drives tip growth was termed the tip organizing complex (TIPOC) by Gabriella Kelemen and colleagues (323,328,329). SsgA (330) and the polysaccharide synthase CslA (323,328,329) are other proteins that are part of this TIPOC.…”

“…The apical assembly that drives tip growth was termed the tip organizing complex (TIPOC) by Gabriella Kelemen and colleagues (323,328,329). SsgA (330) and the polysaccharide synthase CslA (323,328,329) are other proteins that are part of this TIPOC. Clearly, we can only see the tip of the iceberg at present, and future discoveries will undoubtedly shed new light on these processes.…”

“…Besides the tubulin homolog FtsZ and the actin-like proteins MreB and Mbl (320,321), a large number of proteins with coiled-coil structural elements occur in these bacteria, and evidence is accumulating regarding their important role in growth, cell shape, and morphogenesis (319,(322)(323)(324). The protein FilP forms intermediate filament-like structures that contribute to mechanical stress resistance (322).…”

How do bacteria localize proteins to the cell pole?

“…Experiments with B. subtilis cells in which cytokinesis is artificially blocked support this idea: as cells become filamentous and do not form new septa, DivIVA progressively relocalizes to the cell poles ). In Streptomyces coelicolor hyphae, the DivIVA homolog mainly localizes at future and emerging hyphal tips, probably by sensing negatively curved surfaces (Flärdh, 2003;Hempel et al, 2008;Holmes et al, 2013). These hyphal tips are formed de novo along the cell without a division event.…”