Flagellin and Tubulin Genes

newsmgr at merrimack.edu newsmgr at merrimack.edu
Thu Oct 9 16:52:42 EST 1997


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Subject: Re: Flagellin and Tubulin Genes
Message-ID: <343CBCF9.5AE4 at evol5.mbl.edu>
From: "Andrew J. Roger" <roger at evol5.mbl.edu>
Date: Thu, 09 Oct 1997 11:16:08 +0000
Reply-To: roger at evol5.mbl.edu
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Brian Foley wrote:
> 
> Bob Cooper wrote:
> >
> > Hi
> >
> > I had a student question about genes that code for tubulin and
> > flagellin.  The flagella of prokaryotes and eukaryotes are analogous
> > structures.  Prokaryotic flagella are composed of flagellin subunits
> > while the microtubules that make up the eukaryotic flagellum
> > (undulopodium) are composed of tubulin.  Does anybody know if the genes
> > that code for these protein subunits (i.e., flagellin and tubulin) are
> > at all homologous?
> >
> > Bob Cooper
> > rac7 at erols.co
[snip]
>         There is a huge grey area where the % similarity is
> too low to prove that a pair of genes shared a common ancestor.
> Any two genes should be about 25% similar by chance (there
> are only 4 bases to choose from at each position) and if
> we allow some gaps, it is easy to increase that value a bit.
> 
>         If one compares the prokaryotic EF-G protein or gene to
> the eukaryotic EF-2 protein or gene, the degree of conservation
> is great enough to infer that they are homologous as well as
> similar.
> 
>         I do not see enough similarity between tubulin and
> flagellin to infer a common ancestor:

Yes I think you are right. They are unlikely to be homologs. Plus
the actual rotary motor part of the prokaryotic flagellum is buried in
the bacterial membrane, whereas bacterial flagellins are structural
elements that extend outside the cell.  Eukaryotic flagella is
enclosed within the cell membrane and tubulins are actually directly
involved in the generation of force.

Tubulins are proposed to be homologous to the bacterial septation
protein ftsZ. This protein forms a ring around the dividing bacterium
and constricts during septation.  Although sequence similarity
to tubulin is quite low, the GTP-binding region has a similar
motif and both sets of proteins have similar physical properties.
Its a little surprising that ftsZ and tubulins are so dissimilar.
But it suggests that there was a huge acceleration in the rate 
of evolution on the eukaryotic line. This is probably due to 
the change in function of ftsZ as it became a general cytoskeletal
element and as it duplicated to form alpha-, beta- and gamma-
subunits.  

Simiarly, actin appears to be homologous to a whole suite
of bacterial proteins such as the N-terminal domain of hsp70, MreB,
ftsA and hexokinase.  Once again, the similarity is very low
and probably reflects hugely accelerated evolution in the
eukaryotic lineage.

A major problem regarding the origin and evolution of eukaryotic
properties is the huge discontinuity between living prokaryotes
and eukaryotes. There don't seem to be eukaryotes around that
have only some of the cytoskeletal elements or prokaryotes that
show some eukaryotic cytoskeletal features.....Grrrr.....
its going to be difficult to work out what order all of these
things evolved in.

Cheers
Andrew J. Roger




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