Walker motifs

The Walker A and Walker B motifs are protein sequence motifs, known to have highly conserved three-dimensional structures. These were first reported in ATP-binding proteins by Walker and co-workers in 1982. Of the two motifs, the A motif is the main "P-loop" responsible for binding phosphate, while the B motif is a much less conserved downstream region. The P-loop is best known for its presence in ATP- and GTP-binding proteins, and is also found in a variety of proteins with phosphorylated substrates. Major lineages include: RecA and rotor ATP synthase / ATPases (α and β subunits). Nucleic acid-dependent ATPases: helicases, Swi2, and PhoH () AAA proteins STAND NTPases including MJ, PH, AP, and NACHT ATPases ABC-PilT ATPases Nucleotide kinases () G domain proteins: G-proteins (transducin), myosin. Walker A motif, also known as the Walker loop, or P-loop, or phosphate-binding loop, is a motif in proteins that is associated with phosphate binding. The motif has the pattern G-x(4)-GK-[TS], where G, K, T and S denote glycine, lysine, threonine and serine residues respectively, and x denotes any amino acid. It is present in many ATP or GTP utilizing proteins; it is the β phosphate of the nucleotide that is bound. The lysine (K) residue in the Walker A motif, together with the main chain NH atoms, are crucial for nucleotide-binding. It is a glycine-rich loop preceded by a beta strand and followed by an alpha helix; these features are typically part of an α/β domain with four strands sandwiched between two helices on each side. The phosphate groups of the nucleotide are also coordinated to a divalent cation such as a magnesium, calcium, or manganese(II) ion. Apart from the conserved lysine, a feature of the P-loop used in phosphate binding is a compound LRLR nest comprising the four residues xxGK, as above, whose main chain atoms form a phosphate-sized concavity with the NH groups pointing inwards.