Com­plex Sim­plic­ity

We trans­late and of­fer a post from the Span­ish blog Esos Pe­queños Bi­chi­tos (Those Small Bugs).

by Miguel Vi­cente

The text­books we used in the 20th cen­tury de­scribed bac­te­r­ial di­vi­sion with one term, bi­nary fis­sion, im­ply­ing a sim­ple process. Noth­ing could be far­ther from the truth. To­day we know that more than a dozen pro­teins are em­ployed to make the di­vi­sion sep­tum in Es­cherichia coli. They as­sem­ble in a com­plex se­quence and all are need­ed. In the ab­sence of any one, sub­se­quent as­sem­bly is not pos­si­ble and di­vi­sion can­not take place. Be­fore this ma­chin­ery can be as­sem­bled, an ini­tia­tor pro­tein, FtsZ, be­comes lo­cal­ized in the mid­dle of the cell, where it makes a con­stric­tion ring.

Bernini's bal­dachin (aka canopy of state) in the Vatican's Basil­ica of Saint Pe­ter. The he­li­cal columns re­mind us of the he­li­cal rails that trans­port pro­teins along the mem­brane of bac­te­r­ial cells. Credit: Thi­ago.

The Search for the Cen­ter

There are at least two mech­a­nisms that en­sure the pre­cise lo­ca­tion of FtsZ. One is that this pro­tein does not func­tion in the vicin­ity of the nu­cleoid (termed "nu­cleoid oc­clu­sion" by Con­rad Woldringh.) The other way is inde­pendent of the nu­cleoid and re­lies on the lo­cal concen­tration of an FtsZ in­hibitor called MinC (a mech­a­nism pro­posed by Larry Roth­field.)

Like­wise, there are two mech­a­nisms for the es­tab­lish­ment of this MinC gra­di­ent, with the great­est con­cen­tra­tion be­ing found at the cell poles. MinC is as­so­ci­ated with a mem­brane-bound pro­tein, MinD. The MinCD com­plex os­cil­lates from pole to pole in E. coli with the help of a third pro­tein, MinE. In Bacil­lus sub­tilis, on the other hand, the MinCD com­plex is at­tracted to a pro­tein called Di­vIVA that is lo­cal­ized at the cell pole.

Rid­ing the Rails

Two im­ages of the same B. sub­tilis cell with stained lipid he­lices taken at di­fferent fo­cal depths. Source

In ei­ther case, how do these pro­teins man­age to travel the length of the cell? In­ves­ti­ga­tors from the Slo­vak Aca­demy of Sci­ences and the Uni­ver­sity of York found that lipids in the mem­brane in B. sub­tilis make he­li­cal "rails" that bind MinD and its as­so­ci­ated MinC. To de­tect these rails, the au­thors used flu­o­res­cent dyes that preferen­tially stain the par­tic­u­lar phos­pho­lipid to which MinD binds.

B. sub­tilis di­vi­sion pro­teins dis­trib­uted along the cell mem­brane by the he­li­cal "rails." Source

There are other pro­teins that form he­li­cal bun­dles along the in­te­rior of the mem­brane of rod-shaped bac­te­ria. One of them, MreB, is re­spon­si­ble for main­te­nance of the cell's shape and cells be­come rounded in its ab­sence. Even FtsZ and its ac­com­pa­ny­ing pro­tein FtsA, two pro­teins needed for mak­ing the di­vi­sion sep­tum, make in­tracellular he­lices.