the 1970s and 1980s saw extensive research on microtubules and actin . during this 
 period , the consensus developed that these cytoskeletal elements were unique to 
 eukaryotes and that nothing related to tubulin or actin existed in bacteria or 
 archaeans .
this consensus was overthrown in the 1990s when a series of discoveries 
 revealed that prokaryotes actually did have homologues of tubulin and actin and that 
 these assembled into cytoskeletal filaments .
it is now generally accepted that 
 eukaryotic microtubules and actin filaments originated from these prokaryotic 
 homologues .
the key discoveries of bacterial tubulin and actin were published in 1992 
 and are reviewed here on their 25th anniversary .
the discovery of ftsz as a bacterial homologue of tubulin came first and was made 
 independently by three groups ( de boer 
 et  al .
, 1992 ; raychaudhuri and park , 1992 ; mukherjee 
 et  al . , 1993 ) .
these independent studies each 
 purified ftsz protein from an escherichia coli expression system and 
 demonstrated that it bound and hydrolyzed gtp .
they noted that bacterial ftszs were 
 missing the walker sequence motifs characteristic of g proteins but that they had a 
 conserved short sequence , gggtgtg , that was almost identical to the g / aggtgsg 
 sequence conserved in all - , - , and -tubulins .
that sequence , 
 known as the tubulin signature sequence , was believed to be involved in the binding 
 of gtp in the tubulins .
the three groups all concluded that the gtp - binding site of 
 ftsz appeared to be related to that of tubulins .
a year earlier , before any link to tubulin was known , bi and lutkenhaus ( 1991 ) were the first to propose that ftsz might be a 
 cytoskeletal protein .
electron microscopy to show that ftsz 
 localized to the invaginating septum in dividing e. coli :  in 
 our model the role of ftsz is to form a cytoskeletal element that is functionally 
 analogous to the role of actin in cytokinesis in animal cells .
( 1992 ) 
 noted that the gtpase activity showed a dependence on ftsz concentration 
 characteristic of the self - assembly of tubulin and actin . a subsequent study by mukherjee and lutkenhaus 
 ( 1994 ) provided two major advances .
first , they demonstrated that purified 
 ftsz could assemble in vitro into filamentous polymers .
second , they extended the sequence alignment to 
 identify > 20 amino acids that were highly conserved in all tubulins and ftsz .
they 
 did this by starting the alignment at the gggtgtg sequence and inserting gaps in the 
 ftsz to maximize identity .
remarkably most of the gaps coincided with gaps already in 
 the alignment of - , - , and -tubulins .
i considered this 
 alignment to be compelling evidence for full homology , but the editors of 
 cell were more cautious , insisting on adding a question mark to 
 my 1995 minireview , 
our laboratory took up the question of in vitro assembly and showed that ftsz 
 assembled in vitro into sheets of protofilaments and mini - rings that were similar to 
 tubulin polymers ( erickson 
 et  al . , 1996 ) .
any question of homology was 
 dramatically resolved when the structures of ftsz ( lwe and amos , 1998 ) and tubulin ( nogales et  al .
they had an identical complex fold , which is the ultimate 
 test of homology .
many 
 archaeans have up to five ftsz homologues , some with very divergent sequences that 
 likely serve functions other than cell division .
the discovery of bacterial actins was complicated by the homology of actin to other 
 protein families .
sometimes , this is 
 indicated by amino acid sequence identity , but often this sequence identity is too 
 weak to recognize .
the most definitive demonstration of homology is from protein 
 structure . when the x - ray structure of actin was determined ( kabsch et  al . ,
1990 ) , it was seen to have 
 a complex fold that was identical to that of hexokinase and also to hsp70 , a 
 chaperonin . because this fold is so complex yet was shared so precisely , it was 
 concluded that the three shared a common ancestry .
actin , hexokinase , and hsp70 are 
 homologues and are considered to be members of an actin superfamily .
probably the original protein in this family 
 was the sugar kinase , which underwent gene duplications that evolved into a 
 chaperonin and separately into a protein that could assemble cytoskeletal filaments , 
 actin .
the first strong suggestion for actin homologues in bacteria was a theoretical study 
 by bork et  al .
they used the recent x - ray structures of actin , hexokinase , and hsp70 to do a 
 structure - based sequence alignment , which identified four short signature sequences 
 that were conserved across the superfamily .
they then looked for these signatures in 
 bacterial proteins and found them in three : ftsa , mreb , and parm ( stba ) .
these 
 bacterial proteins were closest in sequence to hsp70 and actin rather than the sugar 
 kinases and were therefore candidates for bacterial actin .
( 2001 ) studied the localization of mreb in bacteria by light microscopy , 
 and found helical filaments running through the cell under the membrane .
( 2001 ) 
 isolated mreb protein and showed by electron microscopy that it assembled thin 
 filaments .
their major discovery was x - ray crystallography , which showed that mreb 
 had the actin fold and was assembled in the crystals into actin - like filaments .
later 
 work has discovered multiple prokaryotic actins with a variety of cytoskeletal 
 functions , although the functions of most are unknown .
it now seems clear that both tubulin and actin were invented in bacteria and/or 
 archaeans and proliferated into diverse families of cytoskeletal filaments well 
 before the emergence of eukaryotes ; for various perspectives on the evolution , see 
 erickson ( 2007 ) , lwe and amos ( 2009 ) , and wickstead and gull ( 2011 ) .
an interesting irony is that roles of tubulin 
 and actin have somewhat switched from bacteria to eukaryotes .
ftsz forms the 
 cytokinetic ring in bacteria , whereas actin provides the major cytoskeletal framework 
 in eukaryotes .
some bacterial ( plasmid ) actins function for nucleoid segregation , a 
 role performed by microtubules in eukaryotes .
a global conclusion would be that once a protein 
 has evolved the ability to assemble cytoskeletal filaments , these can be modified to 
 perform a wide range of useful and sometimes overlapping cellular functions .
the discovery of ftsz as a bacterial homologue of tubulin came first and was made 
 independently by three groups ( de boer 
 et  al . , 1992 ; raychaudhuri and park , 1992 ; mukherjee 
 et  al . , 1993 ) . these independent studies
each 
 purified ftsz protein from an escherichia coli expression system and 
 demonstrated that it bound and hydrolyzed gtp .
they noted that bacterial ftszs were 
 missing the walker sequence motifs characteristic of g proteins but that they had a 
 conserved short sequence , gggtgtg , that was almost identical to the g / aggtgsg 
 sequence conserved in all - , - , and -tubulins .
that sequence , 
 known as the tubulin signature sequence , was believed to be involved in the binding 
 of gtp in the tubulins .
the three groups all concluded that the gtp - binding site of 
 ftsz appeared to be related to that of tubulins .
a year earlier , before any link to tubulin was known , bi and lutkenhaus ( 1991 ) were the first to propose that ftsz might be a 
 cytoskeletal protein .
electron microscopy to show that ftsz 
 localized to the invaginating septum in dividing e. coli :  in 
 our model the role of ftsz is to form a cytoskeletal element that is functionally 
 analogous to the role of actin in cytokinesis in animal cells .
( 1992 ) 
 noted that the gtpase activity showed a dependence on ftsz concentration 
 characteristic of the self - assembly of tubulin and actin .
a subsequent study by mukherjee and lutkenhaus 
 ( 1994 ) provided two major advances .
first , they demonstrated that purified 
 ftsz could assemble in vitro into filamentous polymers .
second , they extended the sequence alignment to 
 identify > 20 amino acids that were highly conserved in all tubulins and ftsz .
they 
 did this by starting the alignment at the gggtgtg sequence and inserting gaps in the 
 ftsz to maximize identity .
remarkably most of the gaps coincided with gaps already in 
 the alignment of - , - , and -tubulins .
i considered this 
 alignment to be compelling evidence for full homology , but the editors of 
 cell were more cautious , insisting on adding a question mark to 
 my 1995 minireview , 
our laboratory took up the question of in vitro assembly and showed that ftsz 
 assembled in vitro into sheets of protofilaments and mini - rings that were similar to 
 tubulin polymers ( erickson 
 et  al . , 1996 ) .
any question of homology was 
 dramatically resolved when the structures of ftsz ( lwe and amos , 1998 ) and tubulin ( nogales et  al . , 1998 )
they had an identical complex fold , which is the ultimate 
 test of homology .
many 
 archaeans have up to five ftsz homologues , some with very divergent sequences that 
 likely serve functions other than cell division .
the discovery of bacterial actins was complicated by the homology of actin to other 
 protein families .
sometimes , this is 
 indicated by amino acid sequence identity , but often this sequence identity is too 
 weak to recognize .
the most definitive demonstration of homology is from protein 
 structure . when the x - ray structure of actin was determined ( kabsch et  al . ,
1990 ) , it was seen to have 
 a complex fold that was identical to that of hexokinase and also to hsp70 , a 
 chaperonin . because this fold is so complex yet was shared so precisely , it was 
 concluded that the three shared a common ancestry .
actin , hexokinase , and hsp70 are 
 homologues and are considered to be members of an actin superfamily .
probably the original protein in this family 
 was the sugar kinase , which underwent gene duplications that evolved into a 
 chaperonin and separately into a protein that could assemble cytoskeletal filaments , 
 actin .
the first strong suggestion for actin homologues in bacteria was a theoretical study 
 by bork et  al .
they used the recent x - ray structures of actin , hexokinase , and hsp70 to do a 
 structure - based sequence alignment , which identified four short signature sequences 
 that were conserved across the superfamily .
they then looked for these signatures in 
 bacterial proteins and found them in three : ftsa , mreb , and parm ( stba ) .
these 
 bacterial proteins were closest in sequence to hsp70 and actin rather than the sugar 
 kinases and were therefore candidates for bacterial actin .
( 2001 ) studied the localization of mreb in bacteria by light microscopy , 
 and found helical filaments running through the cell under the membrane .
( 2001 ) 
 isolated mreb protein and showed by electron microscopy that it assembled thin 
 filaments .
their major discovery was x - ray crystallography , which showed that mreb 
 had the actin fold and was assembled in the crystals into actin - like filaments .
later 
 work has discovered multiple prokaryotic actins with a variety of cytoskeletal 
 functions , although the functions of most are unknown .
it now seems clear that both tubulin and actin were invented in bacteria and/or 
 archaeans and proliferated into diverse families of cytoskeletal filaments well 
 before the emergence of eukaryotes ; for various perspectives on the evolution , see 
 erickson ( 2007 ) , lwe and amos ( 2009 ) , and wickstead and gull ( 2011 ) .
an interesting irony is that roles of tubulin 
 and actin have somewhat switched from bacteria to eukaryotes .
ftsz forms the 
 cytokinetic ring in bacteria , whereas actin provides the major cytoskeletal framework 
 in eukaryotes .
some bacterial ( plasmid ) actins function for nucleoid segregation , a 
 role performed by microtubules in eukaryotes . however , some tubz filaments also 
 function for plasmid segregation .
a global conclusion would be that once a protein 
 has evolved the ability to assemble cytoskeletal filaments , these can be modified to 
 perform a wide range of useful and sometimes overlapping cellular functions .
 the year 2017 marks the 25th anniversary of the discovery of homologues of tubulin 
 and actin in prokaryotes . before 1992 , it was largely accepted that tubulin and actin 
 were unique to eukaryotes . 
 then three laboratories independently discovered that 
 ftsz , a protein already known as a key player in bacterial cytokinesis , had the 
  tubulin signature sequence  
 present in all - , - , and 
 -tubulins . 
 that same year , three candidates for bacterial actins were 
 discovered in silico . 
 x - ray crystal structures have since confirmed multiple 
 bacterial proteins to be homologues of eukaryotic tubulin and actin . 
 tubulin and 
 actin were apparently derived from bacterial precursors that had already evolved a 
 wide range of cytoskeletal functions . 
INTRODUCTION
Prokaryotic homologues of tubulin
Prokaryotic homologues of actin