The emerging function of IQD proteins as scaffolds in cellular signaling and trafficking

Abstract

Calcium (Ca(2+)) signaling modules are essential for adjusting plant growth and performance to environmental constraints. Differential interactions between sensors of Ca(2+) dynamics and their molecular targets are at the center of the transduction process. Calmodulin (CaM) and CaM-like (CML) proteins are principal Ca(2+)-sensors in plants that govern the activities of numerous downstream proteins with regulatory properties. The families of IQ67-Domain (IQD) proteins are a large class of plant-specific CaM/CML-targets (e.g., 33 members in A. thaliana) which share a unique domain of multiple varied CaM retention motifs in tandem orientation. Genetic studies in Arabidopsis and tomato revealed first roles for IQD proteins related to basal defense response and plant development. Molecular, biochemical and histochemical analysis of Arabidopsis IQD1 demonstrated association with microtubules as well as targeting to the cell nucleus and nucleolus. In vivo binding to CaM and kinesin light chain-related protein-1 (KLCR1) suggests a Ca(2+)-regulated scaffolding function of IQD1 in kinesin motor-dependent transport of multiprotein complexes. Furthermore, because IQD1 interacts in vitro with single-stranded nucleic acids, the prospect arises that IQD1 and other IQD family members facilitate cellular RNA localization as one mechanism to control and fine-tune gene expression and protein sorting.

Keywords: IQ motif; calcium; calmodulin-binding; cellular signaling; cytoskeleton; kinesin; microtubules; scaffold proteins.

Figure 1. Subcellular localization and molecular interactors of Arabidopsis IQD1. (A) Transient expression of CaMV 35S_Pro:IQD1~GFP in tobacco leaves (N. benthamiana) reveals association of GFP fluorescence with the microtubular network and cell nucleus (epidermal cell in the center). Lower IQD1~GFP expression level (upper cell) or lower photomultiplier gain (inset, lower right corner) indicates targeting of IQD1~GFP to the nucleolus (see ref. for original report and controls). (B) In addition to its recruitment to microtubules, IQD1 interacts in planta with Arabidopsis CaM2 and Arabidopsis KLCR1. A possible interaction of IQD1 in planta with GSTU26 (reproducibly identified in a yeast two-hybrid screen²⁷) and with single nucleic acid substrates (demonstrated in vitro), such as cellular RNAs, remains to be tested. Querying the PhosPhAt4.0 database retrieved evidence for in vivo phosphorylation (-P) of IQD1on a site near to its N-terminus. To date, there is no evidence for binding of KLCR1 to kinesin motor proteins, which facilitate cellular transport of specific cargo along microtubular tracks.

Figure 2. Current model of IQD1-regulated cellular processes. IQD1 is hypothesized to function as a scaffold protein that recruits in a Ca²⁺-CaM-dependent manner various cargos to kinesin (KHCs) motor complexes via KLCR1. Proteins such as GSTU26 or as yet unidentified RNAs are putative cargo molecules. Ribonucleoprotein complexes containing IQD1 may be assembled in the cell nucleolus/nucleus, exported and directionally transported along microtubule tracks to specific cellular sites related to plant defense response for local mRNA translation, or via plasmodesmata to adjoining cells. A generalized model may apply to other IQD family members.