= Protein structural motif
A \b[protein structural motif] is a recurring spatial arrangement of elements within a <protein structure>, such as interacting <alpha helices> and a connecting segment of chain. It describes local organization rather than the entire <protein tertiary structure>. A motif can form part of a <protein domain>; recognizing the motif alone does not show that it folds independently or has a specified biological function.
The <EF hand> is a concrete example: two <alpha helices> flank a calcium-binding loop in canonical members. Its characteristic geometry can recur despite differences in the detailed <protein sequences>.<EF-hand calcium-binding domain signature and profile in PROSITE>[Description, helix-loop-helix organization]{ref} In deposited <calmodulin> structure PDB 1CLL, <X-ray crystallography> resolved two EF-hand loops per lobe and four calcium ions in the model, linking this description to experimental structural coordinates.<Calmodulin structure 1CLL in the Protein Data Bank>[Experimental Data Snapshot and Primary Citation abstract]{ref}
An original geometric example explains why absolute coordinates are not the motif. Mark three illustrative anchor points in arbitrary units:
$$
A=(0,0,0),\quad B=(1,0,0),\quad C=(1,1,0).
$$
Rotate them 90 degrees about the third axis and translate by $(5,-2,1)$. Their new coordinates are
$$
A'=(5,-2,1),\quad B'=(5,-1,1),\quad C'=(4,-1,1).
$$
The corresponding distances remain $AB=BC=1$ and $AC=\sqrt2$. This <rigid transformation> changes the coordinates while preserving the arrangement. The three points are a schematic geometry exercise, not actual atomic coordinates or a sufficient definition of an EF hand.
A structural comparison must also specify which chain segments or atoms correspond, and how their orientations and connectivity relate. A matching <protein sequence motif> and a recurring spatial arrangement are complementary clues; neither alone supplies every structural or functional conclusion.
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