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Fig. 5. Model for alignment of the MT array and the transport of the dorsalizing
activity. (A) The sperm centriole (-) organizes microtubules (MTs) near the
sperm entry point in the animal hemisphere. As cortical rotation initiates,
disorganized MTs populate the vegetal shear zone, arising from both deep and
peripheral sources. MTs are shown as red lines, and plus-end directed
kinesin-related motor proteins (KRPs) attached to the cortex are shown as dark
blue circles. (D-F) Enlargements of boxed regions shown in A-C. (D) Early in
the first cell cycle, dorsalizing particles nucleate in the vegetal shear zone
of the embryo. GBP, which is perhaps associated with other dorsalizing
proteins such as Dishevelled (Dsh), interacts with the plus-end-directed motor
protein kinesin by binding its cargo-carrying subunit, kinesin light chain
(KLC). (B) As cortical rotation progresses, plus-end-directed KRP motor
proteins tethered to the moving cortex associate with the MTs and move along
their length, which serves to align them so that their plus-ends grow in the
same direction that the cortex is moving. (E) As the MT array aligns, kinesin
carries particles quickly towards the MT plus-ends, which are directed towards
the future dorsal region near the equator. Some particles are transported more
slowly in the same direction by associating with the rotating cortex. (C)
Towards the end of the first cell cycle, the MT array depolymerizes, and
cortical rotation and kinesin-based transport cease. (F) In the dorsal region,
GBP dissociates from kinesin in favor of binding to GSK3. The interaction of
GBP and GSK3 may be facilitated by Dsh binding to Axin, which would bring GBP
to the destruction complex. GBP removes GSK3 from Axin, allowing
ß-catenin (ß) to be stabilized in the dorsal region (also indicated
in C). Stabilized ß-catenin enters dorsal nuclei and activates
transcription of dorsal organizer genes, ultimately resulting in the formation
of the DV and AP axes. D, dorsal side; KHC, kinesin heavy chain; NT,
normalized time; V, ventral.
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