Archaeal origin of tubulin

This article was reviewed by Gáspár Jékely and J. Peter Gogarten.Tubulins comprise a distinct family of GTPases that are highly conserved among eukaryotes and are the major components of microtubules, an essential part of the eukaryotic cytoskeleton [1,2]. All eukaryotes encode multiple, paralogous tubulins that evolved through a series of gene duplications at early stages of eukaryote evolution as well as many subsequent, lineage-specific duplications [3]. Among prokaryotes, the only bona fide tubulins have been identified in several bacteria of the genus Prosthecobacter [4] in which they form microtubule-like sturctures closely resembling eukaryotic microtubulues [5]. The tubulins of Prosthecobacteria show high sequence and structural similarity to eukaryotic homologs, and given their extremely narrow distribution among prokaryotes, are thought to have evolved via horizontal transfer of a eukaryotic tubulin gene to an ancestor of this group of bacteria [6,7]. The great majority of bacteria and many Archaea encode the FtsZ protein which plays a central role in cell division of most bacteria and many archaea and is a prokaryotic homolog of tubulin [8]. Both FtsZ and tubulin undergo GTP- hydrolysis-dependent cycles of polymerization and depolymerization, and are mechanistically analogous [9,10]. However, FtsZ and tubulin share extremely weak sequence similarity, so that the homology has become apparent only through comparison of crystal structures of these proteins [11]. Recent progress in genome sequencing and comparative genomics has revealed numerous previously unrecognized members of the FtsZ-tubulin protein superfamily [12,13]. These proteins considerably expand the range of sequence divergence adoptable by the FtsZ-tubulin fold but none of them are candidates for the role of direct prokaryotic ancestors of tubulins. In the absence of such candidates, it is generally assumed that tubulin evolved from FtsZ at the onset of eukaryote evolution, and this evolution e