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The second major adhesion molecule is contact site A (csA), also referred to
as gp80, an 80 000 mw protein expressed late in the streaming process (8 h
post-starvation). Surprisingly, csA/gp80 null mutants do not exhibit any
developmental defects under standard laboratory conditions (Harloff et al.,
1989). This may due to the precocious expression of gp150/lagC (Wang et al.,
2000), a third adhesion molecule normally expressed during early mound
formation (10 h post-starvation). However, when the csA/gp80 null cells are
developed on soil plates that more closely mimic the native environment of
Dictyostelium, the mutants do not develop eciently and, interestingly, appear
to adhere more avidly to the substratum (Ponte et al., 1998). csA/gp80 is a
GPI-anchored protein and has been found in association with lipid raft-like
complexes in developed cells (12 h post-starvation), along with cytoskeletal
proteins such as ponticulin, a major link between the plasma membrane and
actin cytoskeleton (Hitt et al., 1994; Harris and Siu, 2002; Harris et al., 2003).
While csA/gp80 is required for recruiting ponticulin into the lipid rafts, there
is no evidence for a direct association of these two proteins and the molecular
details of their (likely indirect) interaction remain to be elucidated (Harris
et al., 2003).
The adhesion receptors for vegetative cells are less well-described. Screens
for phagocytosis mutants have provided clues about their properties and have
resulted in the identification of at least one. Careful phenotypic characteriza-
tion of the early phagocytosis mutants revealed that Dictyostelium bind
particles non-specifically through a combination of glucose and hydrophobic
receptors (Vogel et al., 1980). The genes mutated in these early studies have
not been cloned. However, Dictyostelium plasma membranes possess a major
130 kD glycoprotein that appears to be altered in one of these adhesion
mutants, HV29 (Chia, 1996). Interestingly, the original antibodies made
against gp130 recognize a carbohydrate epitope and these block particle
uptake, presumably due to inhibiting adhesion (Chia and Luna, 1989). The
130 kD molecule has not yet been cloned. A later phagocytosis screen resulted
in the identification of Phg1, an integral membrane protein with nine
transmembrane domains (Cornillon et al., 2000). Phg1 homologues are found
throughout phylogeny but their function in other cell types is not yet known.
The phg1 mutant is defective in the uptake of latex beads and E. coli but not
Klebsiella aerogenes bacteria, indicating that this molecule plays a role in
adhesion to a subset of surfaces (Cornillon et al., 2000).
A deceptively simple and direct screen for molecules required for cell-
substrate adhesion resulted in the identification of an adhesion receptor with
attributes of receptors found higher eukaryotes. A population of cells
mutagenized through the random insertion of a plasmid was plated and
then the dish subjected to gentle shaking. The non-adherent cells were
collected and these cells were subjected to the same screening process for
several rounds. Cloning of one mutant locus identified in this collection, sadA
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