Ar­chaeal Nin­jas

by Elio

We con­fess to hav­ing missed a re­port of one of the mi­crobial world's most un­usual struc­tures. These are pili-like fil­a­ments lo­cated on an ar­chaeon that look for all the world like strands of barbed wire with a three-pronged grap­pling hook at the end. The ar­ti­cle ap­peared in 2005 and has not had any fol­low-up that we could find. On the as­sump­tion that oth­ers may also have missed it, we share this bit of ex­cit­ing old news.

Grap­pling hook with 2 more barbs than a hamus.

The or­gan­ism first. Called SM1, this is a pre­vi­ously un­de­scribed ar­chaeon found in a sul­furous marsh in Bavaria, grow­ing at about 10°C. It can­not yet be cul­tivated in the lab­o­ra­tory, but it can be re­trieved by plac­ing poly­eth­yl­ene nets in the wa­ter and wait­ing about a week. The or­gan­ism has been found only in close as­so­ci­a­tion with a fil­a­men­tous sul­fur-ox­i­diz­ing gamma-pro­teobac­terium, Thio­thrix. To­gether, they form char­ac­ter­is­tic, eas­ily iden­ti­fi­able, macro­scopic string-of-pearls struc­tures. The SM1 cocci make up the in­ner por­tion of each pearl. Each SM1 cell has about 100 pili-like fibers dis­trib­uted all over its sur­face. Each fiber con­sist of a long fil­a­ment dec­o­rated by strands that give it the ap­pear­ance of barbed wire, termi­nating in a struc­ture that looks like a Ninja grap­pling hook. The au­thors call this struc­ture a hamus (pl. hami), which is Latin for prickle, claw, hook, barb, or fish­ing rod.

Cryo-elec­tron to­mog­ra­phy of an SM1 hamus. A and B. Lon­gi­tu­di­nal sec­tion through the hook/prickle re­gion of the three-di­men­sion­ally re­con­structed hamus (A) be­fore and (B) af­ter de­nois­ing of the to­mo­graph­i­cal data. The widths of the sec­tions are 110 nm each. C. 3D model of the hamus struc­ture as vi­su­al­ized by sur­face ren­der­ing of the de­noised data set.
D. Model of the hamus with the char­ac­ter­is­tic di­men­sions in­di­cated. E. Se­ries of cross-sec­tions through the orig­i­nal to­mo­gram per­pen­dic­u­lar to the hamus axis. The sec­tions are dis­tant by 1.36 nm from each other and il­lus­trate the on­sets and the num­ber of the prick­les of the prickle site in­di­cated in (A). The se­ries is dis­played in the or­der from the bot­tom close to the cen­tral fil­a­ment to­wards the tips of the prick­les ori­ented to­wards the hook re­gion. F. Pro­jec­tion through the last three sec­tions in (E), il­lus­trat­ing the po­si­tions of the fil­a­ment and the prick­les more clearly.G. Cor­re­spond­ing pro­jec­tion through four con­sec­u­tive sec­tions in the hook re­gion. Im­age size of cross-sec­tions: 67 nm. Source

Us­ing cryo-elec­tron to­mog­ra­phy, the au­thors present struc­tural de­tails of this com­plex struc­ture. They also pu­ri­fied the hami and showed that they con­sist of one ma­jor protein‑a very tough, 120 kD pro­tein, but small amounts of other com­po­nents might be present. How are these struc­tures with their char­ac­ter­is­tic base-to-tip ar­chi­tec­ture as­sem­bled? Since they found no ev­i­dence of a cen­tral chan­nel, the au­thors sug­gest that new sub­units are added at the base, but they can not imag­ine how the prick­les and the hooks are at­tached.

Jus­ti­fi­ably, the au­thors sug­gest that hami are ad­he­sive struc­tures for at­tach­ment or an­chor­ing of the or­gan­ism. In ex­per­i­ments, the SM1 were seen to ad­here strongly to all sur­faces tested and to each other.

You will read in this ar­ti­cle that: "It ap­pears that one of the most ba­sic forms of the mi­cro­bial world long ago de­vel­oped a tool that mankind now uses in tech­nol­ogy world­wide, a par­al­lel of di­rect in­ter­est to the fields of bion­ics and bio­mi­met­rics." What's next?