Poster: Signaling, cell-to-cell

Abs # 420: Differential regulation of a microtubule motor protein by Ca²⁺-sensors: Role of KIC and KCBP/ZWI in trichome morphogenesis

Presenter:	Reddy, Vaka S., vaka@lamar.colostate.edu
Authors	Reddy, Vaka S. (A)   Reddy, Anireddy S. N. (A)
Affiliations:	(A): Colorado State University

Kinesin-like calmodulin-binding protein/ZWICHEL (KCBP/ZWI), a microtubule (MT) motor, was used as a model to study Ca²⁺-sensor specificity. Two distinct Ca²⁺-sensors, calmodulin (CaM) with four EF-hands and KCBP-interacting Ca²⁺-binding protein (KIC) with one EF-hand, were found to negatively regulate the activity of KCBP in a Ca²⁺-dependent manner. Among the members of AtCaM subfamily, CaM2 showed two-fold higher affinity as compared to CaM4 and -6. Although both CaM and KIC inhibit MT-binding activity of KCBP, the concentration of Ca²⁺ required to inhibit MT-stimulated ATPase activity of motor by KIC was three-fold less than that required for CaM. However, other Ca²⁺-binding proteins including KIC-related proteins with one EF-hand and centrin with four EF-hands did not affect motor-MT interaction, indicating the specificity of Ca²⁺ sensors. Interestingly, the Ca²⁺-sensor-binding domain (CBD) of KCBP acts as a regulatory module in conferring Ca²⁺-sesnor regulation when fused to non-CaM-binding animal kinesins, supporting the hypothesis that CaM-binding kinesins are evolved by fusion of CBD to a kinesin protomotor. Overexpression of KIC in Arabidopsis resulted in trichomes with reduced branch number resembling the zwi phenotype. In addition, transgenic lines expressing the N terminus or full-length KCBP under CaMV 35S promoter showed mostly (~90%) two-branched trichomes with reduced stalk-length compared to transgenics expressing the C terminus motor domain (~28% two-branched trichomes with ~30% reduced stalk-length). These results suggest that misexpression of either KCBP or its regulator KIC alters trichome morphogenesis. Consistent with KCBP functions, its promoter activity is strong in dividing cells, trichomes and other cells exhibiting polar growth.