Flagellin and Tubulin Genes

Julie Theriot theriot at cmgm.stanford.edu
Mon Oct 13 18:11:23 EST 1997

In article <61gvf8$5u7 at net.bio.net>
Bob Cooper <rac7 at erols.com> writes:

> 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.com

Hi Bob,

   As Anthony Pelletier correctly pointed out, the mechanics of the
eukaryotic and bacterial flagellae are quite different.  But it is
still interesting to compare the properties of the tubulin subunit and
the bacterial flagellin subunit.  Both form linear polymers by
head-to-tail self-assembly, and will assemble in vitro in the absence
of any other proteins.  This property is shared by a number of other
proteins as well, including actin, sickle-cell hemoglobin, bacterial
FtsZ, and the major sperm protein of nematodes.  Actin and tubulin link
polymerization to hydrolysis of ATP or GTP respectively, and the energy
consumption associated with polymerization in these two cases allows
the polymers certain types of nonequilibrium behaviors such as dynamic
instability and steady-state treadmilling, that equilibrium polymers
such as flagellin cannot exhibit.  There is an excellent comprehensive
treatment of this type of protein polymerization by Fumio Oosawa and
Sho Asakura, "Thermodynamics of the Polymerization of Protein,"
Academic Press, 1975, that uses actin and flagellin as major examples
throughout; in the thermodynamic treatment, tubulin is similar to actin
but very different from flagellin because of the energy coupling. 

   Andrew Roger mentioned FtsZ.  The assembly of FtsZ, the bacterial
septation ring protein, does involve GTP hydrolysis and is probably
very similar to tubulin assembly; indeed the FtsZ filaments and rings
that are formed by pure protein in vitro look a heck of a lot like
microtubules.  FtsZ is now generally considered to be a true bacterial
homolog of tubulin.  I have heard through the grapevine that the X-ray
crystal structures of both FtsZ and tubulin have recently been solved. 
I expect that comparison of their three-dimensional structures will
shed light on the question of homology (as did the recent finding that
myosin and kinesin have similar core folds).  

   Some archaea (archaebacteria) have rotary flagellae that look at
first glance much like the flagellae of eubacteria, but the subunit
used to construct the flagella has no sequence similarity to flagellin.
 Instead, it seems to be homologous to pilin, which many bacterial
species use to make (nonrotary) adhesive pili.  I have not been able to
find out whether the rotary motor in the membrane is homologous between
bacteria and archaea.  Recent evidence suggests that the bacterial
rotary motor is derived from the F1 ATP synthase.  Both couple proton
transport to physical rotation, but the flagellar motor runs backwards
with respect to the ATP synthase.

   I know this is more detail than you probably needed to know, but I
am very fond of all forms of flagellae.  And it is a very good
question.  Happy rotating,

Julie Theriot.

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