VLA family with the ~1 subunit includes various
receptors for collagens, laminin and fibronectin;
the LEU CAM family with the ~2 subunit is made
up of different leukocyte adhesion molecules;
and the cytoadhesin family with ~3 includes,
amongst others, the vitronectin receptor.
Sequence comparisons indicate the great age of
the three ~-chains. The ~1 subunits of man, the
mouse, chicken and Xenopus agree 82-86 %, but
human ~1, ~2 and ~3 are only 40-48 % similar
[170]. With the recent discovery of a further three
~ subunits, a still larger variety is now recognizable in the integrin super-family [142, 168]. The
most unusual of these newly discovered ~-chains
is the ~4 subunit, which was sequenced via cDNA
clones from epithelial cells. With 1778 amino
acids, it is more than twice as long as the other ~chains; the extracellular domain (683 amino
acids) lacks eight of the otherwise conserved 56
cysteine residues. The cytoplasmic domain (1072
amino acids) of ~4 is much longer than that of the
other ~-chains (50 amino acids), shows no homology to the latter and, furthermore, contains four
type-III fibronectin repeats [72, 182]. In addition
to the integrins, further types of matrix receptor
proteins are the collagen-binding protein anchorin CII [46] and the membrane-bound proteoglycan syndecan; this is approximately 300 amino
acids long and has an extracellular domain that
binds glycosamineglycans [105].
6.9.2 Cell-Binding Proteins of Invertebrates
Only very recently have adhesion molecules from
insects been described in any detail. Quite surprisingly, amongst them are members of the Ig
super-family: amalgam, fasciclin II and neuroglian. Amalgam is the product of a gene in the
antennipedia complex of Drosophila melanogaster; during embryo development it is expressed
on the surface of various mesodermal and neural
cells. In addition to the signal sequence and a
short hydrophobic C-terminal region, the
encoded sequence of 333 amino acids includes
three Ig homology units [164]. Fasciclin II and
neuroglian belong to the four membrane-bound
glycoproteins found on the growing axons and
glial cells of various insects. Fasciclin II is composed of an extracellular domain of 742 amino
acids with five Ig-like domains and two type-III
fibronectin repeats, a transmembrane segment of
25 amino acids and a cytoplasmic portion of 108
amino acids [68]. Neuroglian contains six Ig
homology units and five type-III fibronectin
6.9.2 Cell-Binding Proteins of Invertebrates
239
repeats and has a great similarity to L1 of mammals [14]. On growing axon membranes, the glycoproteins known as fasciclin I and III are not
homologous to fasciclin II or to any other known
protein. Fasciclin I is a homophilic adhesion molecule made up of four homologous domains of
about 150 amino acids and anchored in the membrane via glycosyl-phosphatidyl-inositol. Probably as the result of hydrolytic cleavage of the
anchor, large amounts of free soluble fasciclin I
are found in Drosophila embryos [75]. The
sequence of 488 fasciclin-III amino acids was
determined from the cDNA, and has no similarity
to the other two fasciclins [172]. The chaoptin located in the rhabdomers of the Drosophila eye
contains within its sequence 41 copies of a
leucine-rich repeat, which is known in numerous
proteins from those of yeast to those of man, and
is possibly involved in specific protein-protein or
protein-membrane interactions [92]. Integrins
are also known from insects, for example in Drosophila, where two integrins with the same ~ subunits are expressed in various very different tissues and are responsible for, amongst other
things, the close juxtaposition of the dorsal and
ventral wing epithelia [18].
Fertilization in the sea urchin, and cell
aggregation in sponges are two examples of cellcell interactions in invertebrates that have been
well characterized at the molecular level. During
the first step in the fertilization of sea urchins, an
important role is played by a small body (acrosome) lying at the tip of the sperm. Interaction
between fucose sulphate in the egg jelly and
receptors in the sperm membrane leads to ejection of the acrosome granula from the sperm and
the formation of the acrosome process; this connects the sperm with the vitellin membrane of the
egg and makes possible fusion of the plasma
membranes of the egg and sperm cells. The main
component of the acrosome granula and the acrosome process is a carbohydrate-free protein
bindin, which binds to a receptor protein in the
vitellin membrane. The bindins show great
species-specific diversity; thus, the acrosome
reaction represents the first barrier to species
intercrossing. The precursor (pro-bindin), encoded by a cDNA from Stronglyocentrotus lividus, includes an unusually large pro-peptide of
245 amino acids and the mature bindin of 236
amino acids. The N-terminal 73 amino acids of
the bindin from the related species S. franciscanus
differ in at least 45 % of positions [54].
In the Porifera, one can observe a fascinating
example of biological self-organization: if the
receptors for collagens, laminin and fibronectin;
the LEU CAM family with the ~2 subunit is made
up of different leukocyte adhesion molecules;
and the cytoadhesin family with ~3 includes,
amongst others, the vitronectin receptor.
Sequence comparisons indicate the great age of
the three ~-chains. The ~1 subunits of man, the
mouse, chicken and Xenopus agree 82-86 %, but
human ~1, ~2 and ~3 are only 40-48 % similar
[170]. With the recent discovery of a further three
~ subunits, a still larger variety is now recognizable in the integrin super-family [142, 168]. The
most unusual of these newly discovered ~-chains
is the ~4 subunit, which was sequenced via cDNA
clones from epithelial cells. With 1778 amino
acids, it is more than twice as long as the other ~chains; the extracellular domain (683 amino
acids) lacks eight of the otherwise conserved 56
cysteine residues. The cytoplasmic domain (1072
amino acids) of ~4 is much longer than that of the
other ~-chains (50 amino acids), shows no homology to the latter and, furthermore, contains four
type-III fibronectin repeats [72, 182]. In addition
to the integrins, further types of matrix receptor
proteins are the collagen-binding protein anchorin CII [46] and the membrane-bound proteoglycan syndecan; this is approximately 300 amino
acids long and has an extracellular domain that
binds glycosamineglycans [105].
6.9.2 Cell-Binding Proteins of Invertebrates
Only very recently have adhesion molecules from
insects been described in any detail. Quite surprisingly, amongst them are members of the Ig
super-family: amalgam, fasciclin II and neuroglian. Amalgam is the product of a gene in the
antennipedia complex of Drosophila melanogaster; during embryo development it is expressed
on the surface of various mesodermal and neural
cells. In addition to the signal sequence and a
short hydrophobic C-terminal region, the
encoded sequence of 333 amino acids includes
three Ig homology units [164]. Fasciclin II and
neuroglian belong to the four membrane-bound
glycoproteins found on the growing axons and
glial cells of various insects. Fasciclin II is composed of an extracellular domain of 742 amino
acids with five Ig-like domains and two type-III
fibronectin repeats, a transmembrane segment of
25 amino acids and a cytoplasmic portion of 108
amino acids [68]. Neuroglian contains six Ig
homology units and five type-III fibronectin
6.9.2 Cell-Binding Proteins of Invertebrates
239
repeats and has a great similarity to L1 of mammals [14]. On growing axon membranes, the glycoproteins known as fasciclin I and III are not
homologous to fasciclin II or to any other known
protein. Fasciclin I is a homophilic adhesion molecule made up of four homologous domains of
about 150 amino acids and anchored in the membrane via glycosyl-phosphatidyl-inositol. Probably as the result of hydrolytic cleavage of the
anchor, large amounts of free soluble fasciclin I
are found in Drosophila embryos [75]. The
sequence of 488 fasciclin-III amino acids was
determined from the cDNA, and has no similarity
to the other two fasciclins [172]. The chaoptin located in the rhabdomers of the Drosophila eye
contains within its sequence 41 copies of a
leucine-rich repeat, which is known in numerous
proteins from those of yeast to those of man, and
is possibly involved in specific protein-protein or
protein-membrane interactions [92]. Integrins
are also known from insects, for example in Drosophila, where two integrins with the same ~ subunits are expressed in various very different tissues and are responsible for, amongst other
things, the close juxtaposition of the dorsal and
ventral wing epithelia [18].
Fertilization in the sea urchin, and cell
aggregation in sponges are two examples of cellcell interactions in invertebrates that have been
well characterized at the molecular level. During
the first step in the fertilization of sea urchins, an
important role is played by a small body (acrosome) lying at the tip of the sperm. Interaction
between fucose sulphate in the egg jelly and
receptors in the sperm membrane leads to ejection of the acrosome granula from the sperm and
the formation of the acrosome process; this connects the sperm with the vitellin membrane of the
egg and makes possible fusion of the plasma
membranes of the egg and sperm cells. The main
component of the acrosome granula and the acrosome process is a carbohydrate-free protein
bindin, which binds to a receptor protein in the
vitellin membrane. The bindins show great
species-specific diversity; thus, the acrosome
reaction represents the first barrier to species
intercrossing. The precursor (pro-bindin), encoded by a cDNA from Stronglyocentrotus lividus, includes an unusually large pro-peptide of
245 amino acids and the mature bindin of 236
amino acids. The N-terminal 73 amino acids of
the bindin from the related species S. franciscanus
differ in at least 45 % of positions [54].
In the Porifera, one can observe a fascinating
example of biological self-organization: if the