|
|
|
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
|
Fig. 5. A model of polarity establishment in one-cell C. elegans
embryos. Illustrations of C. elegans embryos from fertilization
(left) through to the initial phase of polarity establishment (right). Thin
arrows indicate the temporal hierarchy of events. The identity of the proteins
or complexes depicted is indicated in the key. The approximate timing of
events relative to entry into the first cell cycle is shown in the upper bar.
The acto-myosin network is indicated from a surface view of the embryo cortex;
centrosome localization and the distribution of other proteins is shown in the
mid-plane of embryos. Anterior, or the meiotic pole, is to the left. The
events indicated are as follows, with numbers corresponding to those in the
figure. (1) Fertilization, indicated by the thick black arrow, contributes
centrosomes and RhoGAP CYK-4 to the oocyte. (2) CYK-4 localizes around the
centrosomes and to a local cortical region. (3) Following entry into the cell
cycle, RhoGEF ECT-2 becomes active throughout the cortex, leading to myosin
activation via Rho•GTP and thus uniform contractility. (4) CDC-42 helps
link the anterior PAR complex to the contractile cortex. (5) Centrosome
assembly and RhoGAP CYK-4 activation provide a polarity establishment signal.
(6) RhoGEF ECT-2 is eliminated from the cortex overlying the centrosomes. (7)
Rho activity is downregulated in the local region, leading to downregulation
of myosin activity and cessation of contractility. (8) The local region of
non-contractility breaks the symmetry of the cortex and promotes a partial
collapse of the acto-myosin network. (9) CDC-42 shrinks with the contractile
domain, causing the anterior PAR complex to follow; RhoGEF ECT-2 and Rho
shrink with the contractile domain. (10) RhoGEF ECT-2, RHO-1, CDC-42 and PAR-6
in the anterior domain promote contractility of the acto-myosin network,
allowing for continued contraction of the anterior domain through a feedback
loop.
|
