Do calmodulin binding IQ motifs have built-in capping domains?

Abstract

Most calmodulin (CaM) targets are alpha-helices. It is not clear if CaM induces the adoption of an alpha-helix configuration to its targets or if those targets are selected as they spontaneously adopt an alpha-helical conformation. Other than an alpha-helix propensity, there is a great variety of CaM targets with little more in common. One exception to this rule is the IQ site that can be recognized in a number of targets, such as those ion channels belonging to the KCNQ family. Although there is negligible sequence similarity between the IQ motif and the docking site on SK2 channels, both adopt a similar three-dimensional disposition. The isolated SK2 target presents a pre-folded core region that becomes fully alpha-helical upon binding to CaM. The existence of this pre-folded state suggests the occurrence of capping within CaM targets. In this review, we examine the capping properties within the residues flanking this core domain, and relate known IQ motifs and capping.