ORCID IDs: 0000-0002-3493-4800 (K.B.); 0000-0002-7146-043X (B.M.).Calcium (Ca 2+ ) signaling and dynamic reorganization of the cytoskeleton are essential processes for the coordination and control of plant cell shape and cell growth. Calmodulin (CaM) and closely related calmodulin-like (CML) polypeptides are principal sensors of Ca 2+ signals. CaM/CMLs decode and relay information encrypted by the second messenger via differential interactions with a wide spectrum of targets to modulate their diverse biochemical activities. The plant-specific IQ67 DOMAIN (IQD) family emerged as possibly the largest class of CaM-interacting proteins with undefined molecular functions and biological roles. Here, we show that the 33 members of the IQD family in Arabidopsis (Arabidopsis thaliana) differentially localize, using green fluorescent protein (GFP)-tagged proteins, to multiple and distinct subcellular sites, including microtubule (MT) arrays, plasma membrane subdomains, and nuclear compartments. Intriguingly, the various IQD-specific localization patterns coincide with the subcellular patterns of IQD-dependent recruitment of CaM, suggesting that the diverse IQD members sequester Ca 2+ -CaM signaling modules to specific subcellular sites for precise regulation of Ca 2+ -dependent processes. Because MT localization is a hallmark of most IQD family members, we quantitatively analyzed GFP-labeled MT arrays in Nicotiana benthamiana cells transiently expressing GFP-IQD fusions and observed IQD-specific MT patterns, which point to a role of IQDs in MT organization and dynamics. Indeed, stable overexpression of select IQD proteins in Arabidopsis altered cellular MT orientation, cell shape, and organ morphology. Because IQDs share biochemical properties with scaffold proteins, we propose that IQD families provide an assortment of platform proteins for integrating CaM-dependent Ca 2+ signaling at multiple cellular sites to regulate cell function, shape, and growth.

Background: Plant-specific IQD genes encode putative CaM targets of unknown functions. Results: IQD1 interacts with KLCR1, binds to Arabidopsis CaM/CMLs, and localizes to microtubules. Conclusion: IQD1 may act as a scaffold protein recruiting cargo to kinesin motors for directional transport along microtubules. Significance: This work provides novel insight into IQD function and a framework to study plant kinesin regulation.

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Microtubule-localized Arabidopsis IQ67 DOMAIN5 regulates pavement cell morphogenesis in the leaf epidermis and links calcium–calmodulin signaling to anisotropic expansion during early phases of pavement cell growth.

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