Archaeal tubulin-like proteins CetZ1 and CetZ2 have opposing effects on cell morphology during the growth cycle of Haloferax volcanii.

内容摘要

CetZs are archaeal tubulin superfamily cytoskeletal proteins implicated in the control of cell shape and motility. In the pleomorphic archaeon Haloferax volcanii, CetZ1 is required for the transformation of a discoid, or plate-like, cell morphology to the rod shape in early log-phase cultures, and for the development of swimming motility. In this study, we found that the paralog CetZ2 is not required for rod development or reversion to plates in log phase but was strongly upregulated later in stationary phase, where it participates in the maintenance of plate cell shape. Overproduction of CetZ2 or a functional fluorescently tagged CetZ2 promoted maintenance of plate cell shape specifically in the stationary phase. The tagged CetZ2 localized in patchy subcellular assemblies at the cell edges that were most dynamic in mid-stationary phase, where they showed directional movement around the cell edge and other complex behaviors. The stationary phase dynamics of CetZ1 and CetZ2 localization patterns were dependent on their individual GTPase activities and on the presence of one another, despite the much higher cellular concentration of CetZ1 over CetZ2 (ratios of over 40:1 in log and 5:1 in stationary phase). Together, the results suggest that CetZ2 counteracts the CetZ1-based rod development pathway to maintain plate shape in the stationary phase and imply that additional stationary-phase factors are required. CetZ1 and CetZ2 are co-conserved in many haloarchaea, suggesting that their coordinated functions at different stages of the growth cycle may be widely utilized to control cell shapeshifting. This study describes the first defined function of a CetZ2 protein, which represents a distinct haloarchaeal protein family within the near-universal tubulin superfamily of cytoskeletal proteins. Using the model archaeal organism Haloferax volcanii, we showed that CetZ2 is upregulated as cultures enter the stationary phase, where it dynamically localizes in patches at the cell edges and maintains the plate- or disk-like cell morphology, thus counteracting the CetZ1-based rod cell development. This CetZ1-CetZ2 interplay might represent a useful paradigm for understanding the evolution of antagonistic cytoskeletal functions, such as those that led to a reliance on active and inactive subunits during the early evolution of microtubules in eukaryotes. Our findings also suggest that diversification and specialization of tubulin homologs have occurred multiple times in archaeal evolution.