Tat-fimbriae (“tafi”) – novel type of haloarchaeal surface structures

Abstract

SUMMARYIn present study, we describe “tat-fimbriae (tafi)” – a novel type of archaeal surface appendages isolated from haloarchaeonHaloarcula hispanica. Thesefilamental structures are unique because they are formed of protein subunits secreted through thetwin-arginine translocation pathway (Tat-pathway), in contrast to well-known archaeal surface filamentous structures secreted by the general secretory pathway (Sec-pathway). No cases of the role of Tat-pathway in the assembly of archaeal and bacterial filamentous structures have been described to date. “Tafi” are the first example of such structures. The precursor of the major tafi protein subunit TafA contains the N-terminal signal peptide carrying a twin-arginine consensus motif and fimbria-forming mature TafA lacks this signal peptide. We analyzed the gene neighborhood of thetafAhomologues in the known haloarchaeal genomes and found a conservative cluster of seven associated genestafA, B, C, D, E, F, G. We assume that all of them take part in the tafi synthesis. TafC and TafE proteins, whose precursor sequences also contain twin-arginine motifs, were detected as minor components of tafi. TafE protein is structurally similar to TafA, while TafC contains a TafA-like N-terminal domain and a C-terminal “laminin G-like” domain capable of functioning as an adhesin. TafD is annotated as a signal peptidase I. The functions of TafB, TafF and TafG are not known yet. This study demonstrated that ΔtafAand ΔtafDdeletion mutant strains synthesized archaella and not tafi, and only tafi were detected in ΔarlK(gene of common archaellin/pilin signal peptidase) deletion strain. It was shown that the expression of completeHar. hispanica taf-gene cluster in a heterologous hostHaloferax volcaniithat does not have similar genes leads to synthesis of recombinant tafi structures similar to the native ones. The tafi function remains elusive, but our preliminary data suggest that these structures may be involved in cell adhesion to different surfaces or substrates.