3-DIMENSIONAL STRUCTURE OF NCD-DECORATED MICROTUBULES OBTAINED BY A BACK-PROJECTION METHOD

We have used cryo-electron microscopy and image analysis to obtain the three-dimensional (3D) structure of 11, 12, 14 and 15 protofilament m icrotubules decorated with the motor domain of ncd. To obtain the 3D m aps, we developed a back-projection method that does not require a hel ical arrangement of the tubulin heterodimers. This method allows the c alculation of 3D maps even when lattice discontinuities (seams) are pr esent. The maps show that the microtubules we studied conform to a B-t ype lattice with one or more seams. In the presence of 5'-adenylimidod iphosphate (AMP-PNP), the motor domain of ncd binds to the microtubule protofilament crest interacting with only one protofilament. Viewing the structures along the microtubule axis shows that the ncd motor dom ain and the tubulin are tilted in opposite directions. We determined t hat a clockwise tilt of the tubulin subunits corresponds to a view fro m the minus end towards the plus end of the microtubule. (C) 1996 Acad emic Press Limited