The IQ calmodulin-binding motif is an

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

sequence motif In biology, a sequence motif is a nucleotide or amino-acid sequence pattern that is widespread and usually assumed to be related to biological function of the macromolecule. For example, an ''N''-glycosylation site motif can be defined as ''A ...

containing the following sequence: * ILVxxx Kxxx Kx ILVWY The term "IQ" refers to the first two amino acids of the motif:

isoleucine Isoleucine (symbol Ile or I) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...

(commonly) and

glutamine Glutamine (symbol Gln or Q) is an α-amino acid that is used in the biosynthesis of proteins. Its side chain is similar to that of glutamic acid, except the carboxylic acid group is replaced by an amide. It is classified as a charge-neutral ...

(invariably).

Function

Calmodulin Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all Eukaryote, eukaryotic cells. It is an intracellular target of the Second messenger system, sec ...

(CaM) is recognized as a major

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

(Ca²⁺) sensor and orchestrator of regulatory events through its interaction with a diverse group of cellular proteins. Three classes of recognition motifs exist for many of the known CaM binding proteins; the IQ motif as a consensus for Ca²⁺-independent binding and two related motifs for Ca²⁺-dependent binding, termed 1-14 and 1-5-10 based on the position of conserved

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

residues.

Example

The regulatory domain of scallop

myosin Myosins () are a Protein family, family of motor proteins (though most often protein complexes) best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are adenosine triphosphate, ATP- ...

is a three-chain protein complex that switches on this motor in response to Ca²⁺ binding. Side-chain interactions link the two light chains in tandem to adjacent segments of the heavy chain bearing the IQ-sequence motif. The Ca²⁺-binding site is a novel

EF hand The EF hand is a helix–loop–helix structural domain or ''motif'' found in a large family of calcium-binding proteins. The EF-hand motif contains a helix–loop–helix topology, much like the spread thumb and forefinger of the human hand, in ...

motif on the essential light chain and is stabilized by linkages involving the heavy chain and both light chains, accounting for the requirement of all three chains for Ca²⁺binding and regulation in the intact myosin molecule.

References

{{InterPro content, IPR000048 Protein domains