Bacterial Birth Scar Proteins Mark Future Flagellum Assembly Site

Abstract

Many prokaryotic protein complexes underlie polar asymmetry. In Caulobacter crescentus, a flagellum is built exclusively at the pole that arose from the previous cell division. The basis for this pole specificity is unclear but could involve a cytokinetic birth scar that marks the newborn pole as the flagellum assembly site. We identified two developmental proteins, TipN and TipF, which localize to the division septum and the newborn pole after division. We show that septal localization of TipN/F depends on cytokinesis. Moreover, TipF, a c-di-GMP phosphodiesterase homolog, is a flagellum assembly factor that relies on TipN for proper positioning. In the absence of TipN, flagella are assembled at ectopic locations, and TipF is mislocalized to such sites. Thus TipN and TipF establish a link between bacterial cytokinesis and polar asymmetry, demonstrating that division does indeed leave a positional mark in its wake to direct the biogenesis of a polar organelle.

PflI, a Protein Involved in Flagellar Positioning in Caulobacter crescentus

“…When PflI is overproduced, it localizes ectopically to the stalked pole, and this correlates with a considerable increase in ectopic flagellar synthesis at that site. C. crescentus populations carrying tipN, podJ, pleC, or pleD mutations also have a proportion of cells with a flagellum at the stalked pole or at the stalk itself, but these mutations also alter many aspects of cell polarity and polar morphogenesis (1,20,21,27,47,49). On the other hand, we found no evidence of defects in cell polarization or polar morphogenesis in the ⌬pflI or pflI-overexpressing strains, apart from a high frequency of flagellum misplacement.…”

“…6C). TipN is a cell polarity determinant that marks the new pole (where the flagellum assembles) for most of the cell cycle and relocates to the division site during the predivisional cell stage (21,27). TipN-GFP showed similar localization profiles in wild-type (CJW1406), ⌬pflI (CJW2186), and pflI-overexpressing (CJW2187 plus xylose) backgrounds (Fig.…”

“…Our data argue that the presence and proper localization of PflI are important for positioning the flagellum at the correct pole. It is known that mutations in genes involved in multiple aspects of cell polarity or polar morphogenesis can result in flagellar placement defects (1,20,21,27,47,49). Unlike some of these genes with pleiotropic functions, pflI does not affect cell division or stalk biogenesis, as judged by the normal cell and stalk morphology of ⌬pflI and pflI-overexpressing cells (Fig.…”

“…The sessile stalked cells then begin expressing flagellar genes in a stepwise, hierarchical manner, and a new flagellum is formed at the pole opposite the stalk (28,35,52). Division yields a flagellated swarmer progeny for dispersal and a sessile stalked progeny for local colonization.Defects in flagellar placement have been observed in several C. crescentus mutants, but the mutations lie in genes that have pleiotropic functions in cell polarization and polar morphogenesis (1,20,21,27,47,49). In this study, we identify CC_2060, renamed pflI (the polar-flagellum-linked gene), as one of the genes specifically associated with flagellar positioning.…”

“…Defects in flagellar placement have been observed in several C. crescentus mutants, but the mutations lie in genes that have pleiotropic functions in cell polarization and polar morphogenesis (1,20,21,27,47,49). In this study, we identify CC_2060, renamed pflI (the polar-flagellum-linked gene), as one of the genes specifically associated with flagellar positioning.…”