The 65-kDa carrot microtubule-associated protein forms regularly arranged filamentous cross-bridges between microtubules.
In plants, cortical microtubules (MTs) occur in characteristically parallel groups maintained up to one microtubule diameter apart by fine filamentous cross-bridges. However, none of the plant microtubule-associated proteins (MAPs) so far purified accounts for the observed separation between MTs in cells. We previously isolated from carrot cytoskeletons a MAP fraction including 120- and 65-kDa MAPs and have now separated the 65-kDa carrot MAP by sucrose density centrifugation. MAP65 does not induce tubulin polymerization but induces the formation of bundles of parallel MTs in a nucleotide-insensitive manner. The bundling effect is inhibited by porcine MAP2, but, unlike MAP2, MAP65 is heat-labile. In the electron microscope, MAP65 appears as filamentous cross-bridges, maintaining an intermicrotubule spacing of 25-30 nm. Microdensitometer-computer correlation analysis reveals that the cross-bridges are regularly spaced, showing a regular axial spacing that is compatible with a symmetrical helical superlattice for 13 protofilament MTs. Because MAP65 maintains in vitro the inter-MT spacing observed in plants and is shown to decorate cortical MTs, it is proposed that this MAP is important for the organization of the cortical array in vivo.
Chan
J
J
Department of Cell Biology, John Innes Centre, Norwich NR4 7UH, United Kingdom.
Jensen
C G
CG
Jensen
L C
LC
Bush
M
M
Lloyd
C W
CW
D002462:Cell Membrane / Q000648:ultrastructure; D018552:Daucus carota; D008869:Microtubule-Associated Proteins / Q000494:pharmacology / Q000648:ultrastructure*; D008870:Microtubules / Q000648:ultrastructure*; D010940:Plant Proteins / Q000494:pharmacology / Q000648:ultrastructure*; D014404:Tubulin / Q000187:drug effects
D008869:Microtubule-Associated Proteins; D010940:Plant Proteins; D014404:Tubulin