phate receptor which is responsible for transport
of proteins into the lysosomes [42, 100, 175]. The
best-characterized membrane-bound forms are
galactose-, mannose- and acetylgalactosaminespecific lectins on the hepatocytes of birds and
mammals. Membrane-bound lectins have also
been described in invertebrates, e.g. on the haemocytes of the oyster Crassostrea virginica and
other mussels [17]. Detailed concepts of the
molecular structure are available for only a few
lectins. The galactose-specific, soluble lectins of
fish, amphibians, birds and mammals are mostly
dimers with subunits of 13-17 kDa; the eggs of
fish and amphibians are especially lectin-rich,
with more than 1 % of the total soluble protein
falling into this category. These vertebrate lectins
form a significantly homologous group; for example, the lectins of the electric eel Electrophorus
electricus and chicken skin agree at 39 % of comparable amino acid positions. The lectins have no
similarities to other proteins and apparently constitute their own protein super-family [27, 34,
48,131,174]. The ll1-amino-acid sequence of the
sialic acid-specific lectin from the egg of the
American bullfrog Rana catesbeiana shows no
similarity to other vertebrate lectins or to any
other protein [191].
The Iimulin molecule from the haemolymph of
the xiphosuran Limulus polyphemus appears as a
hexagonal structure in electron micrographs;
each of the six 67-kDa subunits consists of three
22-kDa polypeptide chains whose sequence of
163 amino acids shows no similarity with the
known lectins of vertebrates or plants. Only a
very few invertebrate lectins have been either
fully or partially sequenced. The a-chain of the
lectin from the fly Sarcophaga peregrina and the
lectins from the barnacle Megabalanus rosea, the
sea urchin Anthocidaris crassispina and the ascidian Polyandrocarpa misakiensis are all C-type;
they have about 20-30 % sequence similarity
amongst themselves and with vertebrate C-type
lectins [89, 122, 130, 183, 200]. The lectin from
the albumen gland of the edible snail Helix pomatia is widely used as a specific reagent; it is constructed from three non-covalently bound dimers
with internal disulphide bridges. That of the fly
Sarcophaga peregrina is constructed from six
chains according to the formula a4~2 [89]. In
many invertebrates, several variants of a lectin
(isolectins) may appear together; thus, upon isoelectric focusing the albumen-gland lectin of the
edible snail separates into 12 fractions of varying
amino acid composition. It was a surprising finding that the lactose-specific haemolymph lectin of
6.7 Lectins
235
the cephalopod Octopus vulgaris is coppercontaining and both electrophoretically and
immunologically similar to the subunits of the
respiratory pigment haemocyanin, although it has
no oxygen-binding capacity [151]. The echinoidin
of the coelom fluid from the sea urchin Anthocidaris crassispina has a native molecular weight of
about 300 kDa and consists of 147-amino-acid
subunits with three internal disulphide bridges
and a serine-bound oligosaccharide. The Cterminal sequence shows significant homology to
the mannose-binding protein of rat liver and
other mammalian lectins, but also has a certain
resemblance to the central sequence of the Sarcophaga lectin [55].
The biological role of animal lectins is largely
undetermined; in all probability it is as varied as
their structures. The first animal lectin discovered, the limulin of Limulus polyphemus, was
considered to be a defence substance. Various
observations suggest a defence function for the
invertebrate lectins; the occurrence of lectins
with marked specificity for 2-keto-3-desoxyoctonate (KDO; Fig. 6.5), which is found
only on bacterial cells, can only be understood in
this context. Various insect lectins agglutinate
parasitic flagellates; the lectin of the bloodsucking bug Rhodnius prolixus is specific for the
epimastigotes of Trypanosoma cruzi, living in
insects, but not for the trypomastigotes in vertebrate blood. The phagocytosis-stimulating
(opsonic) effect of lectins has often been discussed but has seldom been demonstrated with
pure lectins and species-specific phagocytes, e.g.
in the lobster Homarus americanus, the caterpillar of the butterfly Spodoptera exigua, the oyster
Crassostrea virginica, the edible mussel Mytilus
edulis and the snail Lymnaea stagnalis [17]. The
carcinoscorpin of the Indian horseshoe crab Carcinoscorpius rotundicauda agglutinates its own
amoebocytes and could, therefore, possibly influence their defence function [176]. The lectin
bound to granulae in the egg cortex of Xenopus is
supposedly involved in the formation of the fertilization membrane between the vitellin membrane
and the egg jelly [124]. A role in tissue differentiation may be assumed for several vertebrate lectins
whose concentration changes in a tissue-specific
manner during ontogeny, e.g. the chicken CLL-I,
which makes up no less than 0.1 % of the
total protein in 16-day embryonal breast muscle
but is entirely absent from adult muscle. The
membrane-bound lectins of mammalian and bird
liver cells are responsible for the binding, pinocytosis and further degradation of circulating gly-
of proteins into the lysosomes [42, 100, 175]. The
best-characterized membrane-bound forms are
galactose-, mannose- and acetylgalactosaminespecific lectins on the hepatocytes of birds and
mammals. Membrane-bound lectins have also
been described in invertebrates, e.g. on the haemocytes of the oyster Crassostrea virginica and
other mussels [17]. Detailed concepts of the
molecular structure are available for only a few
lectins. The galactose-specific, soluble lectins of
fish, amphibians, birds and mammals are mostly
dimers with subunits of 13-17 kDa; the eggs of
fish and amphibians are especially lectin-rich,
with more than 1 % of the total soluble protein
falling into this category. These vertebrate lectins
form a significantly homologous group; for example, the lectins of the electric eel Electrophorus
electricus and chicken skin agree at 39 % of comparable amino acid positions. The lectins have no
similarities to other proteins and apparently constitute their own protein super-family [27, 34,
48,131,174]. The ll1-amino-acid sequence of the
sialic acid-specific lectin from the egg of the
American bullfrog Rana catesbeiana shows no
similarity to other vertebrate lectins or to any
other protein [191].
The Iimulin molecule from the haemolymph of
the xiphosuran Limulus polyphemus appears as a
hexagonal structure in electron micrographs;
each of the six 67-kDa subunits consists of three
22-kDa polypeptide chains whose sequence of
163 amino acids shows no similarity with the
known lectins of vertebrates or plants. Only a
very few invertebrate lectins have been either
fully or partially sequenced. The a-chain of the
lectin from the fly Sarcophaga peregrina and the
lectins from the barnacle Megabalanus rosea, the
sea urchin Anthocidaris crassispina and the ascidian Polyandrocarpa misakiensis are all C-type;
they have about 20-30 % sequence similarity
amongst themselves and with vertebrate C-type
lectins [89, 122, 130, 183, 200]. The lectin from
the albumen gland of the edible snail Helix pomatia is widely used as a specific reagent; it is constructed from three non-covalently bound dimers
with internal disulphide bridges. That of the fly
Sarcophaga peregrina is constructed from six
chains according to the formula a4~2 [89]. In
many invertebrates, several variants of a lectin
(isolectins) may appear together; thus, upon isoelectric focusing the albumen-gland lectin of the
edible snail separates into 12 fractions of varying
amino acid composition. It was a surprising finding that the lactose-specific haemolymph lectin of
6.7 Lectins
235
the cephalopod Octopus vulgaris is coppercontaining and both electrophoretically and
immunologically similar to the subunits of the
respiratory pigment haemocyanin, although it has
no oxygen-binding capacity [151]. The echinoidin
of the coelom fluid from the sea urchin Anthocidaris crassispina has a native molecular weight of
about 300 kDa and consists of 147-amino-acid
subunits with three internal disulphide bridges
and a serine-bound oligosaccharide. The Cterminal sequence shows significant homology to
the mannose-binding protein of rat liver and
other mammalian lectins, but also has a certain
resemblance to the central sequence of the Sarcophaga lectin [55].
The biological role of animal lectins is largely
undetermined; in all probability it is as varied as
their structures. The first animal lectin discovered, the limulin of Limulus polyphemus, was
considered to be a defence substance. Various
observations suggest a defence function for the
invertebrate lectins; the occurrence of lectins
with marked specificity for 2-keto-3-desoxyoctonate (KDO; Fig. 6.5), which is found
only on bacterial cells, can only be understood in
this context. Various insect lectins agglutinate
parasitic flagellates; the lectin of the bloodsucking bug Rhodnius prolixus is specific for the
epimastigotes of Trypanosoma cruzi, living in
insects, but not for the trypomastigotes in vertebrate blood. The phagocytosis-stimulating
(opsonic) effect of lectins has often been discussed but has seldom been demonstrated with
pure lectins and species-specific phagocytes, e.g.
in the lobster Homarus americanus, the caterpillar of the butterfly Spodoptera exigua, the oyster
Crassostrea virginica, the edible mussel Mytilus
edulis and the snail Lymnaea stagnalis [17]. The
carcinoscorpin of the Indian horseshoe crab Carcinoscorpius rotundicauda agglutinates its own
amoebocytes and could, therefore, possibly influence their defence function [176]. The lectin
bound to granulae in the egg cortex of Xenopus is
supposedly involved in the formation of the fertilization membrane between the vitellin membrane
and the egg jelly [124]. A role in tissue differentiation may be assumed for several vertebrate lectins
whose concentration changes in a tissue-specific
manner during ontogeny, e.g. the chicken CLL-I,
which makes up no less than 0.1 % of the
total protein in 16-day embryonal breast muscle
but is entirely absent from adult muscle. The
membrane-bound lectins of mammalian and bird
liver cells are responsible for the binding, pinocytosis and further degradation of circulating gly-