The extremely low solubility product constant of calcium phosphate (Ksp~10-33) provides the chemical basis for calcium as a universal second messenger across membranes that maintain steep calcium concentration gradients. Calcium signaling plays a prominent role in plants for coordinating numerous developmental processes and responses to environmental cues. Generation of stimulus-specific calcium signatures, decoding of the encrypted information, and mounting specific cellular responses are integral phases of the transduction process, with differential interactions between calcium sensor proteins, such as calmodulin (CaM) and closely related CaM-like proteins (CML), and their molecular targets at its nexus.
We previously identified a novel class of CaM/CML target proteins in Arabidopsis, the 33-member IQD (IQ67-DOMAIN) family of largely unknown functions. The plant-specific IQD family represents one of the largest classes of putative CaM/CML targets. Its founding member, IQD1, stimulates plant defense, and reverse genetic characterization of the entire Arabidopsis IQD family implicates IQD proteins in the regulation of plant development and various stress responses. Molecular, biochemical and histochemical analysis of IQD1 suggests that IQD family members provide an assortment of microtubule-associated scaffold proteins with functions in kinesin-dependent transport processes. Very little is known in plants about the regulation of kinesin motor activity, cargo selection, and the underlying biological roles. Our studies of the IQD family, together with a detailed characterization of IQD1, provide a unique opportunity to address some of these important questions.