Type IV pili play important roles in a wide array of processes, including surface adhesion and twitching motility. Although archaeal genomes encode a diverse set of type IV pilus subunits, the functions for most remain unknown. We have now characterized six Haloferax volcanii pilins, PilA[1-6], each containing an identical 30-amino-acid N-terminal hydrophobic motif that is part of a larger highly conserved domain of unknown function (Duf1628). Deletion mutants lacking up to five of the six pilin genes display no significant adhesion defects; however, H. volcanii lacking all six pilins (⌬pilA[1-6]) does not adhere to glass or plastic. Consistent with these results, the expression of any one of these pilins in trans is sufficient to produce functional pili in the ⌬pilA[1-6] strain. PilA1His and PilA2His only partially rescue this phenotype, whereas ⌬pilA[1-6] strains expressing PilA3His or PilA4His adhere even more strongly than the parental strain. Most surprisingly, expressing either PilA5His or PilA6His in the ⌬pilA[1-6] strain results in microcolony formation. A hybrid protein in which the conserved N terminus of the mature PilA1His is replaced with the corresponding N domain of FlgA1 is processed by the prepilin peptidase, but it does not assemble functional pili, leading us to conclude that Duf1628 can be annotated as the N terminus of archaeal PilA adhesion pilins. Finally, the pilin prediction program, FlaFind, which was trained primarily on archaeal flagellin sequences, was successfully refined to more accurately predict pilins based on the in vivo verification of PilA[1-6].

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