question as to where the actin gene came from has been a contentious
one. The ever increasing number of complete genomes is now
beginning to answer it however, and it turns out that just as tubulin
has an identifiable prokaryote ancestor (FtsZ, which is involved in bacterial
cytokinesis), so does actin. Actin is a
notoriously conserved protein there being trivial differences between
the actin sequence from amoeba to man. It was as it actin had been
"invented" once and immediately spread throughout organisms
like life jackets on the Titanic when the iceberg struck. There
had been spurious reports of actin-like proteins in bacteria but these
reports were not really taken seriously (Niemark,
et al, 1978). Two things happened to
prepare the ground for the discovery of prokaryotic actin analogs.
The solution of the structure of actin (Kabsch
et al, 1990)
made it clear that actin belonged not in a class of its own but that it
shared structural homology with heat shock proteins and hexokinases (Bork
et al, 1992).
Also, as more genes were sequenced it became clear that a large family
of actin-related proteins existed that made sense of at least some of
these "weird" actins in the literature. More recently,
it was appreciated that FszA certainly had actin-like properties and
den Ent & Lowe, 2000),
however FtzA lacks an equivalent to sub-domain 2 in actin. Another
even more actin-like protein has come to light that is complete through
database searches. This protein MreB is involved with bacterial
shape and so this protein even seems to share one of the basic functions
of actin. MreB polymerizes into filamentous bundles forming a
cortex under the membrane of Bacillus subtilis (Jones
et al, 2001).
A line-up of the sequences shows that actin contains a set of extra
loops lacking in MreB one of these is the so called DNAse1 binding
loop. Taken together it seems that actin was a prokaryotic
invention like most things in life! With hind sight we should
really have suspected the existence of a bacterial cytoskeleton since
some bacteria adopt some spectacularly complex shapes!
Egelman, E. H.
(2001) Molecular evolution: Actin's long lost relative found. Curr.
Biol. 11, R1022-R1024.
Jones, L. J. F., Carballido-Lopez, R.
& Errington, J. (2001) Control of cell shape in bacteria: Helical
actin-like filaments in Bacillus subtilis. Cell. 104,
Moller-Jensen, J., Jensen, R. B.,
Lowe, J. & Gerdes, K. (2002) Prokaryotic DNA segregation by an
actin-like filament. EMBO J. 21, 3119-3127.
Nakamura, K., Takahashi, K. &
Watanabe, S. (1978) Myosin and actin from Escherichia coli K12
C600. J.Biochem. (Tokyo). 84, 1453-1458.
Niemark, H. C. (1977) Extraction of
actin-like protein from the prokaryote Mycoplasma pneumoniae. PNAS.
van den Ent, F., Amos, L. A. &
Lowe, J. (2001) Prokaryotic origin of the actin cytoskeleton, Nature.
van den Ent, F. & Lowe, J. (2000)
Crystal structure of the cell division protein FtsA from Thermotoga
maritima. EMBO J. 19, 5300-5307.