Interactions of calmodulin-binding helix of KCBP with the hydrophobic pocket of KIC. KCBP is in orange; KIC is in green. The selected interacting residues are shown as stick models and specified. The calmodulin-binding helix is shown as a ribbon. The secondary structure elements of KIC are shown schematically.

The conformational change in KCBP accompanying ATP hydrolysis. (A) The structure of KCBP-KIC complex is superposed with the structure of KCBP alone (PDB ID code 3cob, chain A). Only the switch II helix α4 and the regulatory elements (in pink) of the solo KCBP structure are shown. The analogous elements of KCBP in the structure of the complex are highlighted in orange. For our previous crystallographic studies of KCBP (PDB ID codes 1sdm, 3cob, 3cnz), we used KCBP from potato (amino acids 884-1252) that is 80% identical to Arabidopsis KCBP. (B) The hydrophobic pocket (marked in yellow) on the kinesin surface (gray) is occupied by Ile-1210 in the ATP-like conformation. (C) The shifted hydrophobic pocket (marked in yellow) on the kinesin surface (gray) is occupied by Ile-890 of the β1 strand as found in the ADP state. Ile-1210 is expelled from the hydrophobic pocket on the kinesin surface and interacts with KIC (Fig. 3). Hydrophobic residues are conserved at positions 890 and 1210 in all kinesins. Helices α4 and α6 and the β1 strand of KCBP (indicated) are shown schematically and are visible through the translucent surface. The neck mimic is shown as a coil and is colored according to the conformational state.

Three-dimensional organization of complex between kinesin motor KCBP and regulatory Ca²⁺-binding protein KIC. (A) Schematic illustration of complex components. The fragments of KCBP and KIC visualized in the crystal structure are traced in orange for KCBP and in green for KIC. The domain structure of KCBP is depicted. (B) Crystal structure of the complex of kinesin KCBP and KIC solved at a resolution of 2.4 Å. The motor (light orange) and KIC (green) are shown schematically. ADP (light blue) and Ca²⁺ ion (black) are space-filling models. The regulatory domain of KCBP is highlighted in orange. (C and D) Two views of the structure of KIC in the complex. In C, KIC is shown in orientation allowing visualization of the part corresponding to the central helix of the Ca²⁺ sensors (calmodulin, troponin C). The C-terminal part of the “central” helix in KIC is less ordered than its N-terminal half. For alignment of KIC with troponin C and calmodulin, please see Table S2. In D, the EF-hands helices are indicated (A, B, C, D) as the similar elements in the structures of Ca²⁺ sensors. The β-sheet stabilizing the EF-hand pair is shown in cyan.

Interactions formed between the residues of the neck mimic and KIC and between the EF-hands loops of KIC and KCBP. KCBP is in orange; KIC is in green. Ca²⁺ is shown as a black sphere. The secondary structure elements of both proteins are shown schematically and labeled as in Fig. 1. Principal residues interacting at the interface are shown as stick models and labeled. For better figure presentation, we chose not to show Gln96 of KIC.