The Journal of Cell Biology, Vol 104, 1553-1561, Copyright © 1987 by The Rockefeller University Press

ARTICLES

"Buttonin," a unique button-shaped microtubule-associated protein (75 kD) that decorates spindle microtubule surface hexagonally

N Hirokawa and S Hisanaga

A 75-kD protein was purified from sea urchin egg microtubule proteins through gel filtration. It enhanced the polymerization of porcine brain tubulin, but was not heat-stable and did not bind to calmodulin in the presence of calcium as demonstrated by calmodulin affinity column chromatography. Rotary shadowing of the freeze-etched 75-kD protein adsorbed on mica revealed the protein to be a spherical molecule (approximately 9 nm in diameter). Quick-freeze deep-etch electron microscopy revealed that the surface of microtubules polymerized with 75-kD protein was entirely covered with hexagonally packed, round, button-like structures that were quite uniform in shape and size (approximately 9 nm) and similar to the buttons observed on microtubules of mitotic spindles in vivo or microtubules isolated from mitotic spindles. Judging from calibration studies of molecular mass by gel filtration, the 75-kD protein probably exists in a dimeric form (approximately 150 kD) in its native condition. The stoichiometry of tubulin (dimer) versus 75-kD protein (dimer) in the polymerized pellet was 3-3.4:1. Hence, we concluded that the 75-kD protein was a unique microtubule-associated protein that formed the microtubule button in vivo and in vitro. We propose to name this protein "buttonin".
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This article has been cited by other articles:

• Hamill, D. R., Howell, B., Cassimeris, L., Suprenant, K. A. (1998). Purification of a WD Repeat Protein, EMAP, That Promotes Microtubule Dynamics through an Inhibition of Rescue. J. Biol. Chem. 273: 9285-9291 [Abstract] [Full Text]  
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• Suprenant, K., Dean, K, McKee, J, Hake, S (1993). EMAP, an echinoderm microtubule-associated protein found in microtubule-ribosome complexes. J. Cell Sci. 104: 445-450 [Abstract]  

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