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![[Figure 2]](id5005fig2mag.jpg)
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Figure 2
GMPCPP MT end structure visualized by cryo-electron tomography. (a) Top, longitudinal slice through the tomographic reconstruction, showing an MT in which both plus and minus ends (as indicated and assigned using the moiré pattern) are visible. Bottom, a series of ∼5 nm transverse sections through the tomographic reconstruction showing the transition from lattice to end. At both ends the cylindrical organization of the MT is retained even as PFs are lost, until a critical point (see below) when the PFs curve gently away from the MT axis in a sheet, often remaining laterally connected until the very end of the MT. (b) Segmented and three-dimensional surface-rendered volumes of a plus (purple) and minus (yellow) MT end and a model minus-end volume constructed from straight (PDB entry 3jat; Zhang et al., 2015  ) and curved (PDB entry 3ryh; Nawrotek et al., 2011) tubulin conformations; the arrow indicates the beginning of tapering along the MT axis in each case. PDB entry 3ryh at 9.5° curvature was selected because it is a curved GTP-bound structure of four monomers in length that is useful for model building, but numerous structures of curved tubulin have been determined that show a range of curvatures (∼9.5–13°; for a review and calculation of curvature, see Brouhard & Rice, 2016). (c) Schematic representation of the PF position at MT ends relative to the MT wall. (d) Graphical representation from subtomograms of five plus (purple) and five minus (yellow) ends, plotting three-dimensional PF trajectories as they transition from MT lattice to MT end. These plots show that while there is a range of PF curvature and length at MT ends, PFs at plus and minus ends are not as curved as a PDB-based model PF; for the plus end n = 62 PFs and for the minus end n = 63 PFs. (e) Graphical representation from five subtomogram(s) of the loss of PFs from MT ends showing that at both ends, after approximately five PFs are lost from the 14PF architecture, the remaining PFs abruptly start curving outward as an MT sheet, i.e. in our data GMPCPP MTs need at least ∼9 PFs to maintain their cylindrical structure. The model end (grey) is built such that curved and lattice-constrained PFs superimpose on each other, leading to the data spread in the plot. In each case, whisker plots show the mean ± standard deviation, with individual measurements for all PFs shown as a scatter plot. |
![[Figure 2]](id5005fig2.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
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