{PDOC00272}
{PS00300; SRP54}
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* SRP54-type proteins GTP-binding domain signature *
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The signal recognition particle (SRP) is  an oligomeric  complex that mediates
targeting  and  insertion of  the signal  sequence of  exported  proteins into
the membrane  of the  endoplasmic reticulum.  SRP consists of a 7S RNA and six
protein subunits.  One of these subunits, the 54 Kd protein (SRP54), is a GTP-
binding protein that interacts with the signal sequence  when  it emerges from
the ribosome.  The  N-terminal 300  residues of  SRP54 include the GTP-binding
site (G-domain)    and  are  evolutionary  related to similar domains in other
proteins which are listed below [1].

 - Escherichia  coli and Bacillus subtilis ffh  protein (P48), a protein which
   seems to be the prokaryotic counterpart of SRP54.  Ffh is associated with a
   4.5S RNA in the prokaryotic SRP complex.
 - Signal recognition particle  receptor  alpha  subunit (docking protein), an
   integral membrane GTP-binding protein which ensures,  in  conjunction  with
   SRP, the correct targeting of nascent secretory proteins to the endoplasmic
   reticulum membrane.  The G-domain is located at the C-terminal extremity of
   the protein.
 - Bacterial  ftsY protein, a protein which is believed to play a similar role
   to that  of  the  docking protein in eukaryotes. The G-domain is located at
   the C-terminal extremity of the protein.
 - The pilA protein from Neisseria gonorrhoeae  which  seems to be the homolog
   of ftsY.
 - A protein from the archaebacteria Sulfolobus solfataricus. This protein  is
   also believed  to  be  a  docking  protein.  The G-domain is also at the C-
   terminus.
 - Bacterial flagellar biosynthesis protein flhF.

The best conserved  regions in  those domains are the sequence motifs that are
part of the GTP-binding site, but as those regions are not specific  to  these
proteins, we did not use them as a signature pattern.   Instead, we selected a
conserved region located at the C-terminal end of the domain.

-Consensus pattern: P-[LIVM]-x-[FYL]-[LIVMAT]-[GS]-{Q}-[GS]-[EQ]-x-{K}-x(2)-
                    [LIVMF]
-Sequences known to belong to this class detected by the pattern: ALL,  except
 for flhF.
-Other sequence(s) detected in Swiss-Prot: 9.
-Last update: December 2004 / Pattern and text revised.

[ 1] Althoff S., Selinger D., Wise J.A.
     "Molecular evolution of SRP cycle components: functional
     implications."
     Nucleic Acids Res. 22:1933-1947(1994).
     PubMed=7518075

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