Structure of bacterial tubulin BtubA/B: Evidence for horizontal gene transfer

Abstract

␣␤-Tubulin heterodimers, from which the microtubules of the cytoskeleton are built, have a complex chaperone-dependent folding pathway. They are thought to be unique to eukaryotes, whereas the homologue FtsZ can be found in bacteria. The exceptions are BtubA and BtubB from Prosthecobacter, which have higher sequence homology to eukaryotic tubulin than to FtsZ. Here we show that some of their properties are different from tubulin, such as weak dimerization and chaperone-independent folding. However, their structure is strikingly similar to tubulin including surface loops, and BtubA͞B form tubulin-like protofilaments. Presumably, BtubA͞B were transferred from a eukaryotic cell by horizontal gene transfer because their high degree of similarity to eukaryotic genes is unique within the Prosthecobacter genome.cytoskeleton ͉ tubulin family ͉ Prosthecobacter dejongeii ͉ Verrucomicrobia ͉ polymerization G enomic sequencing of the bacterium Prosthecobacter dejongeii has revealed the genes btubA and btubB that share higher sequence similarity with eukaryotic ␣␤-tubulin than with the bacterial tubulin-homologue FtsZ, raising questions about the evolutionary origins of these genes (1). Historically, bacteria were thought not to contain a cytoskeleton, a view that has only recently been overturned (2, 3). Tubulin and the bacterial tubulinhomologue FtsZ are thought to have evolved from a common ancestor as is indicated by a similar structure of the monomer and protofilament. In addition, they share a polymerization-dependent GTPase activation mechanism (4). It has been argued, however, that the evolutionary distance between these proteins (and also actin and the bacterial actin homologue MreB) seems extraordinarily large considering that tubulin is amongst the most conserved proteins in eukaryotes (5, 6). ␣␤-Tubulin differ from the single subunit bacterial protein FtsZ in that they form stable heterodimers with nonexchangeable GTP being trapped in the interface. ␣␤-Tubulin contain a C-terminal domain, absent in FtsZ, that forms the outside of microtubules (7), structures that have never been reported for FtsZ.There have been several reports showing microtubule-like structures in bacteria (8). The best examples are so-called ''epixenosomes,'' bacteria growing as ectosymbionts on Euplotidium ciliates (9). These organisms belong to the Verrucomicrobia, a phylum of bacteria of uncertain lineage (10). Recently, two genes, called btubA and btubB, sharing Ϸ35% sequence identity with ␣-and ␤-tubulin have been found in the free-living species Prosthecobacter dejongeii (1), which are also part of Verrucomicrobia (11). These genes are cotranscribed from one operon, but on the basis of theoretical modeling, Prothecobacter tubulin BtubA and BtubB were predicted not to form heterodimers (1). However, a recent study pelleting N-terminally His-tagged BtubA and -B showed that equimolar amounts of the two proteins assembled into protofilaments in a cooperative manner (12).Here we show that, contrary to eukaryotic tubulin, soluble and functiona...

The lattice as allosteric effector: Structural studies of αβ- and γ-tubulin clarify the role of GTP in microtubule assembly

“…This lack of nucleotide-linked domain rearrangements in momomeric tubulins is consistent with the behavior of prokaryotic tubulin homologs (12)(13)(14)(15), and distinguishes this family of proteins from signaling GTPases that are known to switch conformation in response to nucleotide state. This distinction likely arises because the tubulin fold is quite different from that of the switching GTPases, lacking cognates of the P-loop and switch regions that respond to nucleotide state (21,22).…”

“…Our recently reported crystal structure of ␥-tubulin:GTP (10) unexpectedly adopted this curved domain arrangement. Multiple crystal structures of more distantly related monomeric bacterial tubulin homologs also showed a curved domain arrangement independent of the nucleotide bound (12,14,15) (reviewed in ref. 4).…”

Evolution of the cytoskeleton

“…Several arguments favor the hypothesis that these bacterial tubulins were acquired by horizontal gene transfer from a eukaryotic host. (56,57) Following this transfer, the tubulins would suddenly be removed from the presence of any of the eukaryotic motors and microtubule-binding proteins, and would be free to rapidly diverge in the bacterial host. Indeed, the bacterial tubulins have lost the ability to make cylindrical microtubules, instead forming pairs and bundles of protofilaments.…”

Evolution and persistence of the cilium

“…Although bacterial a and b tubulin could have arisen by horizontal gene transfer from eukaryotes [Schlieper et al, 2005], they differ from eukaryotic a and b tubulins in enough respects to have either been retained in these bacteria from an early time or to have been transferred from another bacterium or a primitive eukaryotic cell relatively early in evolution. For example, while they heterodimerize, hydrolyze GTP, and form protofilaments, they can be synthesized in E.coli and their production is not chaperone-dependent.…”