Final answer:
Protein motifs are recognizable regions of protein structure defined by a unique function. The PH domain is an example of an interacting domain/motif that allows proteins to bind to phosphatidyl-inositol triphosphate.
Step-by-step explanation:
Protein motifs are small regions of protein three-dimensional structure or amino acid sequence shared among different proteins. They are recognizable regions of protein structure that may (or may not) be defined by a unique chemical or biological function.
In the context of the question, the PH (Pleckstrin Homology) domain is an example of an interacting domain/motif. This domain is found in two different proteins and allows them to bind a molecule of phosphatidyl-inositol triphosphate, which is part of a common cell-signaling pathway.