Tubulin can polymerize in two distinct arrangements: “B-lattices” in which the α-tubulins of one protofilament lie next to α's in the neighboring protofilaments; or the “A” configuration, where α's lie beside β's. Microtubules 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 EB1 that lacks C-terminal sequences. This observation suggests that cytoplasmic microtubules, which form in the presence of this “tip-associating protein,” may have A-lattices. To test this hypothesis we have decorated interphase microtubules in 3T3 cells with monomeric motor domains from the kinesin-like protein, Eg5. These microtubules 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 microtubules with B-lattices must include a “seam”, one lateral domain where adjacent dimers are in the A-configuration. It follows that cytoplasmic microtubules are not cylindrically symmetric; they have two distinct faces, which may influence the binding patterns of functionally significant microtubule-interacting proteins.
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