Worm Breeder's Gazette 12(2): 91 (January 1, 1992)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
When a cell divides, each new daughter cell inherits a single centrosome associated with the nucleus. The centrosome duplicates and each one then migrates around the nucleus until diametrically apposed to the other. Thus, when the cell divides again, the division axis will be orthogonal to the preceding one. In C. elegans embryos the nucleus and centrosomes in certain cells (P0, P1 and most of the early P1 derived cells) undergo a rotation after this migration. This rotation places the centrosomes along the same axis as the previous division and allows repeated division along the anterior posterior axis.
Previous work has shown that rotation depends upon a discrete cortical site that pulls on the astral microtubules (mTs) nucleated by each centrosome (1, 2). By a random process, the pulling action becomes greater for one set of astral mTs, and torque is generated. Laser irradiation studies have shown that in P1 the site is localized to the anterior conex (i.e the region of P1 in contact with AB).
It is possible that a cortically anchored motor protein could be responsible for generating the pulling force. As astral mTs have their plus ends growing towards the cortex, such a motor would need to be minus end directed, either a dynein, or a kinesin like fly ncd. To investigate the latter possibility, we examined the distribution of kinesin in the early embryo using antibodies kindly provided by Linda Wordeman at UCSF.
One of these antibodies, against the conserved motor domain sequence HIPYRESKLT, stains centrosomes in all cells from prometaphase onwards. It also stains a discrete region (<1µm wide) on the cortex of specific cells. In the two cell embryo, this region is between AB and P1 ,probably in the anterior cortex of P1 .Following rotation in P1 ,the cortical staining is often aligned along the same axis as the centrosomes and is sometimes closely associated with the anterior one.
Occasionally, a single, small invagination is seen on the anterior cortex of P1 during rotation, indicative of tension at this point. Laser irradiation at these invaginations blocks rotation, suggesting that they are the regions of the cortex where the mT pulling force is generated (2). When embryos with these invaginations are stained, the kinesin localizes to exactly this position.
As the spindle forms and grows in P1 ,the centrosomes remain aligned with the cortical staining even as the cell is pushed by the cleaving AB cell. The staining persists in EMS, and a new spot forms between P2 and EMS. P2 and EMS subsequently rotate with one of the centrosomes in P2 closely approaching the new cortical kinesin staining. There appears to be no staining between ABa and ABp, so it is likely that the kinesin staining is specific to particular cells in the four cell embryo.
We have previously proposed that actin microfilaments (mFs) are needed to localize (and keep localized) the components of the rotational site. In the absence of mFs, P1 rotation fails, but it appears that the pulling force is still being exerted on the centrosomes. However, in this case the pulling force seems to be coming from all over the anterior cortex, so the necessary torque cannot be generated. The kinesin staining appears soon after the first division as a faint, extended line (about 6µm long and 4µm deep) between AB and P1 .The staining legion becomes progressively smaller until, by prometaphase, it appears as the small, bright dot described above. It will be interesting to see whether this apparent localization event requires the presence of intact mFs.
(2) Hyman, A. A, JCB 109, 1185-1193 (1989)