Advantages and mechanisms of polarity and cell shape determination in Caulobacter crescentus

Abstract

The tremendous diversity of bacterial cell shapes and the targeting of proteins and macromolecular complexes to specific subcellular sites strongly suggest that cellular organization provides important advantages to bacteria in their environment. Key advances have been made in the understanding of the mechanism and function of polarity and cell shape by studying the aquatic bacterium Caulobacter crescentus, whose cell cycle progression involves the ordered synthesis of different polar structures, and culminates in the biosynthesis of a thin polar cell envelope extension called the stalk. Recent results indicate that the important function of polar development is to maximize cell attachment to surfaces and to improve nutrient uptake by nonmotile and attached cells. Major progress has been made in understanding the regulatory network that coordinates polar development and morphogenesis and the role of polar localization of regulatory proteins.

Figure 1

Cell shape variation in stalked bacteria. Caulobacter crescentus (top left) has a flagellum at one pole of the predivisional cell and a stalk tipped by an adhesive holdfast at the other pole. Asticcacaulis biprosthecum (top right) has two stalks symmetrically placed on each side of its stalked and predivisional cells, with its flagellum and holdfast at the same respective poles as C. crescentus. In budding stalked bacteria such as Hyphomonas neptunium (bottom left), the daughter swarmer cell grows from the tip of the stalk and possesses a single flagellum at its distal pole. Ancalomicrobium adetum (bottom right), which may well be the most extreme case of stalked bacteria, appears to have neither flagella nor motility, grows by budding from the cell body, and produces up to a dozen prosthecae around the periphery of the predivisional cell. Reproduced from Molecular Microbiology [4] with permission.

Figure 2

Life cycle of stalked bacteria. A) Caulobacter crescentus life cycle. A motile swarmer cell and a sessile stalked cell are produced at every cell division. The older pole of the swarmer cell has pili and a single polar flagellum. After a defined period in the swarmer phase of the life cycle, during which the cell is not competent for DNA replication, the cell differentiates into a stalked cell by retracting the pili, ejecting the flagellum, and synthesizing an adhesive holdfast at the previously flagellated pole. A stalk is synthesized at the holdfast-bearing pole, resulting in the extension of the holdfast away from the cell as the stalk grows. The stalked cell initiates DNA replication and elongates in preparation for division. The division septum is placed off center in the predivisional cell, and a new flagellum is formed at the pole opposite the stalk. After cell compartmentalization, the flagellum is activated, and the force generated by the flagellum helps to separate the daughter cells. After cell separation is complete, pili are synthesized at the same pole as the flagellum in the swarmer cell. The swarmer cell swims away, and the stalked cell remains attached to immediately initiate a new cell cycle. B) Asticcacaulis biprosthecum life cycle. A. biprosthecum synthesizes two stalks on the side of cells instead of one, but still has a flagellum and a holdfast at the same poles as C. crescentus. C) Hyphomonas neptunium life cycle. H. neptunium is a budding bacterium in which the daughter swarmer cell grows from the tip of the stalk. Therefore, a copy of the chromosome must traverse the stalk at every cell cycle to be inherited by the swarmer cell.

Figure 3

Localization of several C. crescentus developmental regulatory proteins during the cell cycle. Large dots indicate distinct protein localization at a particular location, while small dots represent diffuse cytoplasmic protein throughout the cell compartment. As PleC is a membrane bound protein, delocalization of PleC is illustrated as a ring around the cell body. In swarmer cells, CtrA is present throughout the cytoplasm. During swarmer to stalked cell differentiation, CtrA localizes to the incipient stalked pole, together with CpdR, RcdA, and ClpXP, resulting in CtrA degradation and allowing the initiation of DNA replication. CtrA is synthesized again in late stalked cells, and after compartmentalization, localizes to the stalked pole of the stalked cell, where it is degraded by the same complex described above. Therefore, CtrA is present in the daughter swarmer cell but not the stalked cell, allowing the stalked cell to immediately initiate a new round of DNA replication. The DivK protein is diffuse in swarmer cells, then localizes to the stalked pole after differentiation, where it is phosphorylated by the DivJ protein. Upon phosphorylation, DivK is released from the stalked pole and diffuses toward the nascent flagellar pole, where it is dephosphorylated by PleC. This shuttling of DivK between the two poles of the cell continues until the time of cell compartmentalization, when DivK remains phosphorylated in the stalked cell compartment and dephosphorylated in the swarmer cell compartment. DivJ localizes to the stalked pole in cells with a stalk. PodJ is present in two forms – a full-length form (PodJ_L) produced in stalked cells and a shortened form (PodJ_S) that is the result of proteolytic cleavage of PodJ_L late in the cell cycle. Newly-synthesized PodJ_L localizes to the nascent flagellar pole in stalked cells and is required for the localization of PleC at the same pole. Processing of PodJ_L and cell division results in the production of a swarmer cell with PodJ_S and PleC co-localized at the flagellar pole. During swarmer cell differentiation, PodJ_S is degraded, resulting in delocalization of PleC. The TipN protein is localized at the pole opposite the flagellum in swarmer cells, and remains there until cell division is initiated, which targets its localization to the division site. TipN is consequently inherited by both daughter cells after cell division, localized at the newly formed pole in both cell types.