A microtubule. Plus end is is at top, with
beta tubulin exposed. This image, without rings
attached, is new - the best diagram of a microtubule I know of.
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These images are from
Consult the article for details and discussion of the relation of
tubulin and FtsZ rings. The 3-D models were made using the program
"showcase" on a silicon graphics computer. Here we show 2-D images of
the 3-D models, which provide a detailed and accurate depiction of
the microtubule and ring lattice. Click on the image to get a
larger JPEG file.
A microtubule. Plus end is is at top, with
beta tubulin exposed. This image, without rings
attached, is new - the best diagram of a microtubule I know of.
A microtubule with rings. The microtubule
lattice and the relation of rings to protofilaments. Longitudinal
bonds connect alternating alpha and beta subunits into
protofilaments, and lateral bonds connect subunits into the 3-start
helix, one of which is highlighted. The 3-start helix connects
primarily alpha to alpha and beta to beta, but there is a
discontinuity or seam where alpha connects to beta. A partial ring
and a spiral are shown continuous with protofilaments at the top end.
The curvature of the ring is perpendicular to the plane of the
microtubule wall, which means that the outside of the ring
corresponds to the inside of the microtubule.
Tubulin and FtsZ rings. Subunits in the curved
conformation are modeled with a 20-23 degree tilt at the upper
interface forming the longitudinal bond. The 4 nm spacing is
maintained along the outer circumference. For FtsZ and gamma tubulin
all subunits are in the curved conformation. For the alpha/beta ring,
the alpha subunit is assumed to be in the straight conformation and
the tilt is only applied to the beta subunit. This would explain why
the alpha/beta rings are 42 nm in diameter, approximately twice the
23 nm diameter of gamma and FtsZ rings.
Nucleation of microtubule assembly by a gamma
tubulin spiral. This spiral extends a short length of straight
protofilament, which serves as a stable seed for nucleation of a
second protofilament. Alpha/beta subunits form lateral bonds to the
gamma tubulin protofilament, and longitudinal bonds to each other.
When this second protofilament has achieved three consecutive
alpha/beta subunits, growth is more favorable than disassembly and
the microtubule should be nucleated.