Figure 1: MT lattice packing of tubulin heterodimers.

Adjacent PFs within a MT B-lattice have a relative axial shift of 0.9 nm forming major α–α and β-β lateral contacts between tubulin heterodimers (a). The flat sheet of PFs can close to form a tube. The MT lattice geometry is determined by the number of PFs and the start number of helices that can be drawn through the tubulin subunits around the tube (red line). In a three-start helix three independent spirals of tubulin subunits are required to fill the MT lattice. For a B-lattice 13 PF three-start MT (13–3) tube closure is only possible if a seam forms containing A-lattice contacts with a 4.9-nm axial shift between adjacent PFs and α–β lateral connections (b). For 13 PF MTs helically symmetric tubes without any seams can form from 13–2 B-lattice or 13–3 A-lattice MTs. The 13–3 B-lattice with a single A-lattice seam is the only B-lattice MT where the PFs are parallel with the longitudinal MT axis. In the other types of B-lattice a tilt of the PFs axes relative to the MT axis is required to achieve registration of the tubulin subunits.