Abstract
Polymerizing and filament-forming proteins are instrumental for numerous cellular processes such as cell division and growth. Their function in stabilization and localization of protein complexes and replicons is achieved by a filamentous structure. Known filamentous proteins assemble into homopolymers consisting of single subunits - for example, MreB and FtsZ in bacteria - or heteropolymers that are composed of two subunits, for example, keratin and α/β tubulin in eukaryotes. Here, we describe two novel coiled-coil-rich proteins (CCRPs) in the filament-forming cyanobacterium Anabaena sp. PCC 7120 (hereafter Anabaena) that assemble into a heteropolymer and function in the maintenance of the Anabaena multicellular shape (termed trichome). The two CCRPs - Alr4504 and Alr4505 (named ZicK and ZacK) - are strictly interdependent for the assembly of protein filaments invivo and polymerize nucleotide independently invitro, similar to known intermediate filament (IF) proteins. A ΔzicKΔzacK double mutant is characterized by a zigzagged cell arrangement and hence a loss of the typical linear Anabaena trichome shape. ZicK and ZacK interact with themselves, with each other, with the elongasome protein MreB, the septal junction protein SepJ and the divisome associate septal protein SepI. Our results suggest that ZicK and ZacK function in cooperation with SepJ and MreB to stabilize the Anabaena trichome and are likely essential for the manifestation of the multicellular shape in Anabaena. Our study reveals the presence of filament-forming IF-like proteins whose function is achieved through the formation of heteropolymers in cyanobacteria.
Highlights
Cytoskeletal proteins that polymerize to form protein filaments are paramount in bacterial cell biology where they play a role in cell division, alignment of bacterial microcompartments (BMCs), chromosome and plasmid segregation, organization of cell polarity and the determination of cell shape (Wagstaff and Löwe, 2018)
134 coiled-coil-rich proteins (CCRPs) ZicK and ZacK from Anabaena are conserved in Cyanobacteria
Further annotation using the NCBI conserved domain search (Marchler-Bauer et al, 2016) showed that ZicK and ZacK contain “structural maintenance of chromosomes” (SMC) domains (Fig. 1B), to what we previously identified in other self-polymerizing cyanobacterial CCRPs (Springstein et al, 2020b)
Summary
Cytoskeletal proteins that polymerize to form protein filaments are paramount in bacterial cell biology where they play a role in cell division, alignment of bacterial microcompartments (BMCs), chromosome and plasmid segregation, organization of cell polarity and the determination of cell shape (Wagstaff and Löwe, 2018). FtsZ (Van De Putte et al., 1964; de Boer et al, 1992), the prokaryotic homolog to the eukaryotic tubulin (Löwe and Amos, 1998; Nogales et al, 1998), is a main component of the divisome (den Blaauwen et al., 2017), a multiprotein complex that governs cell division in bacteria and self-assembles into a proteinaceous ring (called Z-ring) at the midcell position (Bi and Lutkenhaus, 1991) Another key bacterial cytoskeletal protein is MreB (Jones et al, 2001), which is a homolog of the eukaryotic actin (de Boer et al, 1992; Ent et al, 2001) and a crucial component of the multi protein complexes termed the elongasome. Eukaryotic IF proteins, despite sharing substantially the same building blocks and a high degree of coiled coil (CC) domains (Fuchs and Weber, 1994), which are considered excellent mediators of protein-protein interactions (Mason and Arndt, 2004), only form heteropolymers with a subset of other IF proteins within their same assembly group but otherwise form obligate homopolymers (Herrmann and Aebi, 2000)
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