THE PLANT CELL, Vol 5, Issue 9 1063-1080, Copyright © 1993 by American Society of Plant Biologists
Unique Functional Characteristics of the Polymerization and MAP Binding Regulatory Domains of Plant Tubulin
J. D. Hugdahl, C. L. Bokros, V. R. Hanesworth, G. R. Aalund and L. C. Morejohn
Department of Botany, University of Texas at Austin, Austin, Texas 78713
An understanding of the regulation of microtubule polymerization and
dynamics in plant cells requires biochemical information on the structures,
functions, and molecular interactions of plant tubulin and
microtubule-associated proteins (MAPs) that regulate microtubule function.
We have probed the regulatory domain and polymerization domain of purified
maize tubulin using MAP2, an extensively characterized mammalian neuronal
MAP. MAP2 bound to the surface of preformed, taxol-stabilized maize
microtubules, with binding saturation occurring with one MAP2 molecule per
five to six tubulin dimers, as it does with mammalian microtubules. MAP2
binding and dissociation analyses revealed two affinity classes of binding
sites on maize microtubules: a high-affinity site 12 dimers apart that may
be homologous to the cognate mammalian MAP2 binding site and an additional
low-affinity site also 12 dimers apart that may be homologous to the
mammalian tau binding site. MAP2 corrected in vitro folding errors in
taxol-stabilized maize microtubules and reduced the critical concentration
of maize tubulin polymerization eightfold, from 8.3 to 1.0 [mu]M. However,
MAP2 dissociated much more readily from maize microtubules than from
mammalian microtubules and induced the assembly of maize tubulin into
aberrant helical ribbon polymers that remained stable for prolonged
periods. Our results indicated that MAP2 binds to maize tubulin via a
partially specific, low-fidelity interaction that reflects unique
structural and functional properties of the polymerization and regulatory
domains of plant tubulin and possibly of the tubulin binding domains of
undocumented MAPs that regulate microtubule function in plant cells.