Halofilins as Emerging Bactofilin Families of Archaeal Cell Shape Plasticity Orchestrators

Abstract

Bactofilins are rigid, non-polar bacterial cytoskeletal filaments that link cellular processes to specific curvatures of the cytoplasmic membrane. Although homologs of bactofilins have been identified in archaea and eukaryotes, functional studies have remained confined to bacterial systems. Here, we characterize representatives of two new families of archaeal bactofilins from the pleomorphic archaeonHaloferax volcanii, halofilin A (HalA) and halofilin B (HalB). HalA and HalB polymerizein vitro, assembling into straight bundles. HalA polymers are highly dynamic and accumulate at positive membrane curvaturesin vivo, whereas HalB forms more static foci that localize in areas of local negative curvatures on the outer cell surface. Gene deletions and live-cell imaging show that halofilins are critical in maintaining morphological integrity during shape transition from disk (sessile) to rod (motile). Morphological defects in ΔhalAresult in accumulation of highly positive curvatures in rods but not in disks. Conversely, disk-shaped cells are exclusively affected byhalBdeletion, resulting in flatter cells. Furthermore, while ΔhalAand ΔhalBcells imprecisely determine the future division plane, defects arise predominantly during the disk-to-rod shape remodeling. In fact, the deletion ofhalAin the haloarchaeonHalobacterium salinarum, whose cells are consistently rod-shaped, impacted morphogenesis but not cell division. Increased levels of halofilins enforced drastic deformations in cells devoid of S-layer, suggesting that HalB polymers are more stable at defective S-layer lattice regions. Our results set halofilins apart from their bacterial correlate, where they provide mechanical scaffolding instead of directing envelope synthesis.