title: XMAP215 is a processive microtubule polymerase.
abstract: Fast growth of microtubules is essential for rapid assembly of the microtubule cytoskeleton during cell proliferation and differentiation. XMAP215 belongs to a conserved family of proteins that promote microtubule growth. To determine how XMAP215 accelerates growth, we developed a single-molecule assay to visualize directly XMAP215-GFP interacting with dynamic microtubules. XMAP215 binds free tubulin in a 1:1 complex that interacts with the microtubule lattice and targets the ends by a diffusion-facilitated mechanism. XMAP215 persists at the plus end for many rounds of tubulin subunit addition in a form of "tip tracking." These results show that XMAP215 is a processive polymerase that directly catalyzes the addition of up to 25 tubulin dimers to the growing plus end. Under some circumstances XMAP215 can also catalyze the reverse reaction, namely microtubule shrinkage. The similarities between XMAP215 and formins, actin polymerases, suggest that processive tip tracking is a common mechanism for stimulating the growth of cytoskeletal polymers.
authors.affiliation: Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany.
authors.forename: Gary J
authors.initials: GJ
authors.lastname: Brouhard
authors.forename: Jeffrey H
authors.initials: JH
authors.lastname: Stear
authors.forename: Tim L
authors.initials: TL
authors.lastname: Noetzel
authors.forename: Jawdat
authors.initials: J
authors.lastname: Al-Bassam
authors.forename: Kazuhisa
authors.initials: K
authors.lastname: Kinoshita
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Harrison
authors.forename: Jonathon
authors.initials: J
authors.lastname: Howard
authors.forename: Anthony A
authors.initials: AA
authors.lastname: Hyman
mesh_terms: D000818:Animals; D001665:Binding Sites / Q000502:physiology; D001681:Biological Assay / Q000379:methods; D020134:Catalytic Domain / Q000502:physiology; D002454:Cell Differentiation / Q000502:physiology; D049108:Cell Enlargement; D002460:Cell Line; D003599:Cytoskeleton / Q000378:metabolism / Q000648:ultrastructure; D004058:Diffusion; D019281:Dimerization; D005326:Fetal Proteins / Q000378:metabolism; D000080865:Formins; D049452:Green Fluorescent Proteins / Q000378:metabolism; D008840:Microfilament Proteins / Q000378:metabolism; D008856:Microscopy, Fluorescence; D008869:Microtubule-Associated Proteins / Q000235:genetics / Q000378:metabolism*; D008870:Microtubules / Q000378:metabolism* / Q000648:ultrastructure; D009687:Nuclear Proteins / Q000378:metabolism; D011108:Polymers / Q000378:metabolism; D011485:Protein Binding / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology; D021381:Protein Transport / Q000502:physiology; D018411:Spodoptera; D034421:Sus scrofa; D014404:Tubulin / Q000378:metabolism*; D029867:Xenopus Proteins / Q000235:genetics / Q000378:metabolism*; D014982:Xenopus laevis
chemical_list: C486738:CKAP5 protein, Xenopus; D005326:Fetal Proteins; D000080865:Formins; D008840:Microfilament Proteins; D008869:Microtubule-Associated Proteins; D009687:Nuclear Proteins; D011108:Polymers; D014404:Tubulin; D029867:Xenopus Proteins; D049452:Green Fluorescent Proteins