Lattice structure of cytoplasmic microtubules in a cultured Mammalian cell.
Tubulin can polymerize in two distinct arrangements: "B-lattices," in which the alpha-tubulins of one protofilament lie next to alpha-tubulins in the neighboring protofilaments, or the "A" configuration, where alpha-tubulins lie beside beta-tubulins. Microtubules (MTs) in flagellar axonemes and those assembled from pure tubulin in vitro display only B-lattices, but recent work shows that A-lattices are found when tubulin co-polymerizes in vitro with an allele of end-binding protein 1 that lacks C-terminal sequences. This observation suggests that cytoplasmic MTs, which form in the presence of this "tip-associating protein," may have A-lattices. To test this hypothesis, we have decorated interphase MTs in 3T3 cells with monomeric motor domains from the kinesin-like protein Eg5. These MTs show only B-lattices, as confirmed by visual inspection of electron cryo-tomograms and power spectra of single projection views, imaged at higher electron dose. This result is significant because 13 protofilament MTs with B-lattices must include a "seam," one lateral domain where adjacent dimers are in the A-configuration. It follows that cytoplasmic MTs are not cylindrically symmetric; they have two distinct faces, which may influence the binding patterns of functionally significant MT-interacting proteins.
McIntosh
J Richard
JR
Laboratory for 3D Structure of Cells and Molecules, Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, CO 80309-0347, USA. Richard.McIntosh@colorado.edu
Morphew
Mary K
MK
Grissom
Paula M
PM
Gilbert
Susan P
SP
Hoenger
Andreas
A
D016475:3T3 Cells; D000818:Animals; D002478:Cells, Cultured; D003593:Cytoplasm / Q000648:ultrastructure*; D051379:Mice; D008870:Microtubules / Q000648:ultrastructure*; D014404:Tubulin / Q000648:ultrastructure*
D014404:Tubulin