Mal3, the Schizosaccharomyces pombe homolog of EB1, changes the microtubule lattice

• Published in: Nature structural & molecular biology Vol. 15; no. 10; pp. 1102 - 1108
• Main Authors: des Georges, Amédée, Cross, Robert A, Amos, Linda A, Katsuki, Miho, Drummond, Douglas R, Osei, Michael
• Format: Journal Article
• Language: English
• Published: New York Nature Publishing Group US 01-10-2008; Nature Publishing Group
• Subjects: Amino Acid Sequence; Biochemistry; Biological Microscopy; Biomedical and Life Sciences; Cellular biology; Cryoelectron Microscopy; Gene Deletion; Genetic aspects; Life Sciences; Membrane Biology; Microtubule-Associated Proteins - chemistry; Microtubule-Associated Proteins - genetics; Microtubule-Associated Proteins - metabolism; Microtubule-Associated Proteins - ultrastructure; Microtubules; Microtubules - chemistry; Microtubules - metabolism; Microtubules - ultrastructure; Models, Molecular; Molecular biology; Molecular structure; Physiological aspects; Polymers; Protein Binding; Protein Structure; Proteins; Saccharomyces; Schizosaccharomyces - chemistry; Schizosaccharomyces - genetics; Schizosaccharomyces - metabolism; Schizosaccharomyces - ultrastructure; Schizosaccharomyces pombe; Schizosaccharomyces pombe Proteins - chemistry; Schizosaccharomyces pombe Proteins - genetics; Schizosaccharomyces pombe Proteins - metabolism; Schizosaccharomyces pombe Proteins - ultrastructure; Structure; Tubulins; United Kingdom
• ISSN: 1545-9993; 1545-9985
• DOI: 10.1038/nsmb.1482

In vitro
, pure tubulin assembles into B-lattice microtubules, in which lateral αα and ββ contacts between tubulin heterodimers predominate. Mal3, a homolog of the plus end–tracking protein EB1, is now shown to promote microtubule assembly into an A-lattice arrangement, forcing reconsideration of
in vivo
microtubule structure.
In vitro
studies of pure tubulin have suggested that tubulin heterodimers in cells assemble into B-lattice microtubules, where the 8-nm dimers in adjacent protofilaments are staggered by 0.9 nm. This arrangement requires the tube to close by forming a seam with an A-lattice, in which the protofilaments are staggered by 4.9 nm. Here we show that Mal3, an EB1 family tip-tracking protein, drives tubulin to assemble
in vitro
into exclusively 13-protofilament microtubules with a high proportion of A-lattice protofilament contacts. We present a three-dimensional cryo-EM reconstruction of a purely A-lattice microtubule decorated with Mal3, in which Mal3 occupies the groove between protofilaments and associates closely with one tubulin monomer. We propose that Mal3 promotes assembly by binding to freshly formed tubulin polymer and particularly favors any with A-lattice arrangement. These results reopen the question of microtubule structure in cells.