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Prokaryote Ancestors of Actin

Page updated 12/2/03

This 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, 1977; Nakamura 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 structure (van 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!

 

Protein name Source Comment References
       
FtsA Thermotoga maritima   van den Ent & Lowe, 2000
       
       
ParM Escherichia coli   Moller-Jensen et al, 2002
MreB Bacillus subtilis   Jones et al, 2001
Table 1   Prokaryotic Actin -like proteins
References:-

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, 913-922.

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. 74, 4041-4045.

van den Ent, F., Amos, L. A. & Lowe, J. (2001) Prokaryotic origin of the actin cytoskeleton, Nature. 413, 39-44.

van den Ent, F. & Lowe, J. (2000) Crystal structure of the cell division protein FtsA from Thermotoga maritima. EMBO J. 19, 5300-5307.

 
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