Biology Reference
In-Depth Information
FIGURE 18.13
The current view of the pathway leading from ecdysone reception to contraction of the apical
ends of leg disc cells (adapted from the discussions of Bayer et al.
41
and Ward et al.
42
). The dotted line may include
very indirect action, possibly by the modulation of other signalling pathways.
Activation of the Rho pathway in the discs seems to be under the control of amultifunctional
protein Stubbled-stubbloid. When disc cells receive the metamorphosis-inducing hormone
ecdysone, they activate the expression of the stubble-stubbloid (sb-sbd) gene, which encodes
a transmembrane serine protease with an extracellular protease domain and receptor
activity.
40
The receptor activity seems, through genetic evidence at least, to control Rho acti-
vation and therefore to control the formation of a contractile actin-myosin system (
Figure
18.13
), although the ligand that acts on Stubbled-stubbloid has not yet been identified.
41
The proteolytic activity of the Stubbled-stubbloid may play a direct role in morphogenesis
as well. Cells of the imaginal disc are attached by their basal surfaces to extracellular matrix,
which will resist the deformation of the disc and the exchange of neighbours. Cleavage of
basement membrane components such as collagen IV is a feature of disc eversion
43
and inhibi-
tors of serine proteases block disc eversion.
44
It is not clear whether this reflects an essential
requirement for matrix degradation or for serine protease activity in signalling events. The
Stubbled-stubbloid protein is a protease that can, in principle at least, activate itself by proteo-
lytic cleavage and, once activated, be involved in the degradation of extracellular matrix and
a consequent increase in the flexibility of the disc. It should be borne in mind, though, that the
protein is located mainly on the apices of cells, at which place it is arguably an unlikely candi-
date for being a major modifier of the basement membrane. Other proteases may therefore be
involved.
