{PDOC00199}
{PS00227; TUBULIN}
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* Tubulin subunits alpha, beta, and gamma signature *
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Tubulins [1,2], the major  constituent of  microtubules  are dimeric  proteins
which consist of two closely related subunits (alpha and  beta). Tubulin binds
two molecules of GTP at two different sites (N and E). At the E (Exchangeable)
site, GTP is  hydrolyzed during incorporation into the microtubule. Near the E
site is an invariant region rich in glycines which is found in both chains and
which is now [3]  said to control the access of the nucleotide  to its binding
site. We developed a signature pattern from this region.

With the exception of the simple eukaryotes, most species express a variety of
closely related alpha and beta isotypes.

In most species there is a third member  of the tubulin family: gamma tubulin.
Gamma tubulin is  found at  microtubule  organizing centers (MTOC) such as the
spindle poles or the centrosome, suggesting that it  is involved in the minus-
end nucleation of microtubule assembly [4].

-Consensus pattern: [SAG]-G-G-T-G-[SA]-G
-Sequences known to belong to this class detected by the pattern: ALL,  except
 for maize tubulin beta-2 which has Leu in the first position of the pattern.
-Other sequence(s) detected in Swiss-Prot: 13.

-Note: The first residue in the pattern is Gly in all alpha and beta tubulins,
 and is Ala or Ser in gamma-tubulin.
-Note: This  pattern  is  almost  identical  to  the  GTP-binding  site of the
 bacterial protein  ftsZ  (see  <PDOC00873>)  whose  role  in  prokaryotes  is
 probably similar to that of tubulins.

-Last update: November 1995 / Text revised.

[ 1] Cleveland D.W., Sullivan K.F.
     "Molecular biology and genetics of tubulin."
     Annu. Rev. Biochem. 54:331-365(1985).
     PubMed=3896122; DOI=10.1146/annurev.bi.54.070185.001555;
[ 2] Joshi H.C., Cleveland D.W.
     "Diversity among tubulin subunits: toward what functional end?"
     Cell Motil. Cytoskeleton 16:159-163(1990).
     PubMed=2194680
[ 3] Hesse J., Thierauf M., Ponstingl H.
     "Tubulin sequence region beta 155-174 is involved in binding
     exchangeable guanosine triphosphate."
     J. Biol. Chem. 262:15472-15475(1987).
     PubMed=3680207
[ 4] Joshi H.C.
     "Gamma-tubulin: the hub of cellular microtubule assemblies."
     BioEssays 15:637-643(1993).
     PubMed=8274140

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