Biology Reference
In-Depth Information
Progress has been made in defining the molecules involved in adhesion
during the growth phase as well as the early (i.e., during aggregation or
streaming) and late (i.e., morphogenesis) developmental phases of Dictyoste-
lium. The adhesion mechanism(s) employed during the growth phase and early
development that are characterized by high levels of single cell migration will
be highlighted here.
Adhesion receptors in Dictyostelium
The original studies of adhesion in Dictyostelium focused on molecules
required for cell-cell contact during early development. Isolated antibodies
directed against Dictyostelium cells were observed to block either early
calcium-dependent or later calcium-independent cell-cell adhesion (Beug et
al., 1973). The molecules responsible for both of these activities have been
isolated and characterized. Calcium-dependent adhesion is mediated by a
24 000mw molecule DdCAD-1/gp24 (Knecht et al., 1987; Brar and Siu, 1993)
so named because it binds Ca
2+
, it has some homology to the first and second
extracellular repeats of E-cadherin, and forms homophilic interactions (Brar
and Siu, 1993; Wong et al., 1996). Antibodies specific for this molecule block
calcium-dependent cell-cell adhesion during early development (Knecht et al.,
1987; Brar and Siu, 1993) and the null mutant also shows reduced calcium-
dependent adhesion (Wong et al., 2002). The DdCAD-1 null forms normal
streams but completes the streaming process early, forming mounds 1-2 h
ahead of the control cells. The resulting mounds are larger than normal and
often form multiple finger structures yet the mutants do complete the
developmental cycle and form normal fruiting bodies (Wong et al., 2002).
These findings indicate that another molecule must participate in adhesion
during the initial aggregation process. Perhaps the as yet uncharacterized
Mg
2+
-dependent adhesions (Fontana, 1993) can account for the ability of the
mutant cells to stream normally. It should be noted that a second DdCAD
gene has been found in the Dictyostelium genome and this could encode the
protein responsible for this second class of divalent cation dependent adhesion
molecule (Coates and Harwood, 2001). However, it is clear that DdCAD-1 is
required for correct mound formation and normal morphogenesis.
DdCAD-1 neither has a signal sequence nor a transmembrane domain and
all available data indicate that it can exist both as soluble in the cytoplasm as
well as on the surface of cells (Sesaki and Siu, 1996; Wong et al., 1996). It
appears to be exported out to the cell surface via an unusual transport
pathway through the contractile vacuole (Sesaki et al., 1997). Nothing is
known about the mechanism by which this protein is linked to the external
plasma membrane.
