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From sequence comparisons and crystallographic data analysis it has been shown
[1,2,3,4,5,6] that an appreciable proportion of proteins that bind ATP or GTP
share a number of more or less conserved sequence motifs. The best conserved
of these motifs is a glycine-rich region, which typically forms a flexible
loop between a β-strand and an α-helix. This loop interacts with one of
the phosphate groups of the nucleotide. This sequence motif is generally
referred to as the 'A' consensus sequence  or the 'P-loop' .
There are numerous ATP- or GTP-binding proteins in which the P-loop is found.
We list below a number of protein families for which the relevance of the
presence of such motif has been noted:
DNA mismatch repair proteins mutS family (See <PDOC00388>).
Bacterial type II secretion system protein E (see <PDOC00567>).
Not all ATP- or GTP-binding proteins are picked-up by this motif. A number of
proteins escape detection because the structure of their ATP-binding site is
completely different from that of the P-loop. Examples of such proteins are
the E1-E2 ATPases or the glycolytic kinases. In other ATP- or GTP-binding
proteins the flexible loop exists in a slightly different form; this is the
case for tubulins or protein kinases. A special mention must be reserved for
adenylate kinase, in which there is a single deviation from the P-loop
pattern: in the last position Gly is found instead of Ser or Thr.
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