Is found in the crystals of potato KCBP, we analyzed the previously solved structures (Fig. 3). We found that in crystal structures of potato KCBP the regulatory helix always interacted with its counterpart from either a molecule related by crystallographic symmetry (structure 1SDM.pdb, Fig. 3A) or a second molecule in the crystal asymmetric unit (structures 3COB.pdb and 3CNZ.pdb, Fig. 3B and Fig. 3C). These interfaces were not highlighted since the interfaces are less extensive, however, the nature of the interactions supporting dimerization via the regulatory helices was always hydrophobic. Thus, in crystals, two orthologs of KCBP form dimers via the hydrophobic interactions between the regulatory helices of each monomer.Dimerization of KCBP at C-Terminus?Figure 3. Dimeric assemblies in the crystals of potato KCBP. Crystal structures of potato KCBP solved at resolution of A- 2.3 A, PDB code ??1SDM.pdb; B- 2.2 A, PDB code 3COB.pdb; 11967625 C- 2.9 A, PDB code 3CNZ.pdb. Structural elements of KCBP are shown in cartoon model. Motor core is in grey. Regulatory domain is highlighted in red. ADP is shown as a space-filling model in blue. doi:10.1371/journal.pone.0066669.gThere are two microtubule-binding surfaces for each dimer of KCBP. The two microtubule-binding surfaces in the dimer are oriented such that bound microtubules would be orthogonal.The Regulatory Helix Enables KCBP to Form Dimers in SolutionTo determine whether dimerization of KCBP takes place in solution, we prepared a truncated construct of Arabidopsis KCBP (a.a. 884?225), lacking the regulatory helix and the Calyculin A negative coil. 23148522 Then, we get TA-01 compared the Arabidopsis KCBP (a.a. 884?253) with an intact regulatory helix and the truncated KCBP using size exclusion chromatography (Fig. 4). We observed that the molecular weight of the truncated construct was 2-fold less than the molecular weight of the KCBP construct with an intact regulatory helix (Table 2). The calculated values of the molecular weight for KCBP and the truncated KCBP were 72 kDa and 35 kDa, respectively. These values were lower than the predicted values of 84 kDa and 42 kDa. Nonetheless, our findings indicate that KCBP forms stable dimers in solution and that the C-terminal peptide encompassing the regulatory helix and the negative coil enables dimerization.produce a stable dimer in solution. To determine whether the negative coil contributes to stability of the observed dimers, we prepared truncated KCBP (884?244) lacking most of the negatively charged amino acids at the C-terminus but long enough, by 3 amino acids, to support the helical conformation of the preceding residues of the regulatory helix. The molecular weight of this truncated construct was compared with the molecular weight of KCBP (884?261) with the intact regulatory domain by analytical ultracentrifugation using the method of sedimentation equilibrium. We found that a 1-component model best described sedimentation equilibrium for both constructs (Figure S1). The molecular weight for KCBP (884?261) was estimated to be 8961 kDa and corresponded to a dimer. The molecular weight for KCBP (884?244) was estimated to be 4961 kDa and corresponded to a monomer. This molecular weight of KCBP (884?244) is close to the molecular weight estimated from gel-filtration (41 kDa, Table 2). These findings indicate that regulatory domain of KCBP is engaged in dimerization and that the negative coil strengthens dimerization dramatically.KIC Forms a Dimer in SolutionThe observed dime.Is found in the crystals of potato KCBP, we analyzed the previously solved structures (Fig. 3). We found that in crystal structures of potato KCBP the regulatory helix always interacted with its counterpart from either a molecule related by crystallographic symmetry (structure 1SDM.pdb, Fig. 3A) or a second molecule in the crystal asymmetric unit (structures 3COB.pdb and 3CNZ.pdb, Fig. 3B and Fig. 3C). These interfaces were not highlighted since the interfaces are less extensive, however, the nature of the interactions supporting dimerization via the regulatory helices was always hydrophobic. Thus, in crystals, two orthologs of KCBP form dimers via the hydrophobic interactions between the regulatory helices of each monomer.Dimerization of KCBP at C-Terminus?Figure 3. Dimeric assemblies in the crystals of potato KCBP. Crystal structures of potato KCBP solved at resolution of A- 2.3 A, PDB code ??1SDM.pdb; B- 2.2 A, PDB code 3COB.pdb; 11967625 C- 2.9 A, PDB code 3CNZ.pdb. Structural elements of KCBP are shown in cartoon model. Motor core is in grey. Regulatory domain is highlighted in red. ADP is shown as a space-filling model in blue. doi:10.1371/journal.pone.0066669.gThere are two microtubule-binding surfaces for each dimer of KCBP. The two microtubule-binding surfaces in the dimer are oriented such that bound microtubules would be orthogonal.The Regulatory Helix Enables KCBP to Form Dimers in SolutionTo determine whether dimerization of KCBP takes place in solution, we prepared a truncated construct of Arabidopsis KCBP (a.a. 884?225), lacking the regulatory helix and the negative coil. 23148522 Then, we compared the Arabidopsis KCBP (a.a. 884?253) with an intact regulatory helix and the truncated KCBP using size exclusion chromatography (Fig. 4). We observed that the molecular weight of the truncated construct was 2-fold less than the molecular weight of the KCBP construct with an intact regulatory helix (Table 2). The calculated values of the molecular weight for KCBP and the truncated KCBP were 72 kDa and 35 kDa, respectively. These values were lower than the predicted values of 84 kDa and 42 kDa. Nonetheless, our findings indicate that KCBP forms stable dimers in solution and that the C-terminal peptide encompassing the regulatory helix and the negative coil enables dimerization.produce a stable dimer in solution. To determine whether the negative coil contributes to stability of the observed dimers, we prepared truncated KCBP (884?244) lacking most of the negatively charged amino acids at the C-terminus but long enough, by 3 amino acids, to support the helical conformation of the preceding residues of the regulatory helix. The molecular weight of this truncated construct was compared with the molecular weight of KCBP (884?261) with the intact regulatory domain by analytical ultracentrifugation using the method of sedimentation equilibrium. We found that a 1-component model best described sedimentation equilibrium for both constructs (Figure S1). The molecular weight for KCBP (884?261) was estimated to be 8961 kDa and corresponded to a dimer. The molecular weight for KCBP (884?244) was estimated to be 4961 kDa and corresponded to a monomer. This molecular weight of KCBP (884?244) is close to the molecular weight estimated from gel-filtration (41 kDa, Table 2). These findings indicate that regulatory domain of KCBP is engaged in dimerization and that the negative coil strengthens dimerization dramatically.KIC Forms a Dimer in SolutionThe observed dime.
