74
3 The Structural Variety and Metabolism of Proteins
of the two secondary messengers, but are mostly
not absolutely specific; there are in fact enzymes
in the insects that show equal affinity for cAMP
and cGMP. cAMP-specific protein kinases are
widely distributed from the mammals to yeast, but
appear to be missing from prokaryotes and higher
plants [274]. Their molecules consist of two each
of the regulatory (R) and catalytic (C) subunits.
On activation, which apparently requires four
cAMPs per enzyme molecule, the subunits dissociate according to the following equation
RzCz + 4 cAMP = Rz( cAMP)4 + 2 C.
Two types of cAMP-specific protein kinases
(I and II) are known in the mammals and differ
mainly in the R subunits. The ratio of the two
types is species and tissue specific: I and II occur
in the ratio 4: 1 in rabbit skeletal muscle and in
the heart muscle of the mouse and rat, but occur
in the ratio 1: 1 in the heart muscle of the rabbit;
only II is found in guinea-pig or bovine heart
muscle. The C subunits show a lower rate of
evolution; the mouse (351 aa) and bovine
(349 aa) heart C-polypeptides agree in 98 % of
their amino acids. The N-terminus of the Cpolypeptide is blocked by myristic acid; the Cchains are very similar in protein kinases I and II
but are not identical. In spite of their structural
and functional differences, the subunits Rr and
Rrr show significant sequence agreement. Their
polypeptide chains are organized in four
domains, the first of which is involved in RR
dimerization, the second in reactions with the Cpolypeptide, and the last two in cAMP binding.
cAMP reduces the R-C affinity by about 10 000fold; this involves both cAMP-binding sites. All
subunits carry phosphate residues, the significance of which is unknown [63].
Like those of the mammals, the cAMPdependent protein kinases of the invertebrates
are tetramers of two catalytic and two regulatory
subunits which dissociate on activation. Also like
the mammals, the insects Drosophila melanogaster and Manduca sexta and the nematode Caenorhabditis elegans contain two types of enzyme,
which in the case of the flies are coded by different genes, and in the nematodes arise by alternate splicing of the transcript of one gene. The Cpolypeptide of Drosophila (352 aa) shows 78 %
agreement with the mammalian enzyme, whereas
that of Caenorhabditis (374 aa) shows 82 %; the
R-polypeptide of the nematode is 69 % similar in
the C-terminal region (positions 145-375) but
much less so at the N-terminus [40, 74, 89, 161].
In the Protozoa, cAMP-regulated protein kinases
have been detected in the malarial agents Plasmodium berghei and P. chabaudi and, after initial
failures, also in the flagellate Trypanosoma cruzi
[281].
cGMP-regulated protein serine/threonine
kinases in mammals are found especially in
smooth muscle, lung, intestine and heart. They
are homodimers of about 155 kDa; unlike the
cAMP-dependent kinases, activation by cGMP
does not result in dissociation. The sequence of
670 amino acids in bovine subunits can be subdivided into six segments (A-F) or four domains
(A, B/C, DIE and F): A controls dimerization;
B/C are duplicated sequences, each with one
cGMP-binding site; and DIE contain the ATPbinding site in D and the active centre in E. The
N-terminal regulatory half shows similarity to
various small cGMP-binding proteins, whereas
the C-terminal half is homologous to the catalytic
subunit of the cAMP-dependent kinases, the
phosphorylase-b kinase, and the oncogene tyrosine kinases; the whole molecule appears to be a
chimera produced by gene fusion [63, 292].
cGMP-regulated protein serine/threonine kinases
also occur in arthropods; in fact, they were discovered in this group before they were recorded in
mammals. Here, the catalytic and regulatory
domains also lie on the same polypeptide. In
Drosophila, four cGMP protein kinase mRNAs
are produced from two genes; the encoded polypeptides agree in 55-64 % of their amino acids
with the enzyme from bovine lung; however, in
one case, the domains responsible for dimerization and inhibition are lacking. The enzyme is
also always found as a monomer in the ciliate
Paramecium, but in the roundworm Ascaris suum
it is a dimer [131, 180, 275]. The enzymes from
the Mediterranean fruit-fly, Ceratatis capitata,
and from the eggs of the silkworm, Bombyx mori,
are strictly cGMP specific, whereas a protein
kinase from the skin of the locust Melanoplus
sanguinipes shows as high an affinity for cAMP as
for cGMP [269]. Amongst the protozoans,
cAMP-regulated protein kinases have been found
in the malarial agents Plasmodium berghei and
P. chabaudi. In contrast, the trypanosomes have
an adenylate cyclase but no cAMP-stimulated
protein kinase [48].
Three classes may be distinguished amongst
the Ca 2 + -regulated protein kinases; the first includes the Ca2+ -calmodulin-dependent enzymes, of
which phosphorylase-b kinase and myosin-LC
kinase are the best characterized. A second class
is made up of the enzymes regulated by Ca z +,
phospholipid and diacylglycerol (DAG) and these
3 The Structural Variety and Metabolism of Proteins
of the two secondary messengers, but are mostly
not absolutely specific; there are in fact enzymes
in the insects that show equal affinity for cAMP
and cGMP. cAMP-specific protein kinases are
widely distributed from the mammals to yeast, but
appear to be missing from prokaryotes and higher
plants [274]. Their molecules consist of two each
of the regulatory (R) and catalytic (C) subunits.
On activation, which apparently requires four
cAMPs per enzyme molecule, the subunits dissociate according to the following equation
RzCz + 4 cAMP = Rz( cAMP)4 + 2 C.
Two types of cAMP-specific protein kinases
(I and II) are known in the mammals and differ
mainly in the R subunits. The ratio of the two
types is species and tissue specific: I and II occur
in the ratio 4: 1 in rabbit skeletal muscle and in
the heart muscle of the mouse and rat, but occur
in the ratio 1: 1 in the heart muscle of the rabbit;
only II is found in guinea-pig or bovine heart
muscle. The C subunits show a lower rate of
evolution; the mouse (351 aa) and bovine
(349 aa) heart C-polypeptides agree in 98 % of
their amino acids. The N-terminus of the Cpolypeptide is blocked by myristic acid; the Cchains are very similar in protein kinases I and II
but are not identical. In spite of their structural
and functional differences, the subunits Rr and
Rrr show significant sequence agreement. Their
polypeptide chains are organized in four
domains, the first of which is involved in RR
dimerization, the second in reactions with the Cpolypeptide, and the last two in cAMP binding.
cAMP reduces the R-C affinity by about 10 000fold; this involves both cAMP-binding sites. All
subunits carry phosphate residues, the significance of which is unknown [63].
Like those of the mammals, the cAMPdependent protein kinases of the invertebrates
are tetramers of two catalytic and two regulatory
subunits which dissociate on activation. Also like
the mammals, the insects Drosophila melanogaster and Manduca sexta and the nematode Caenorhabditis elegans contain two types of enzyme,
which in the case of the flies are coded by different genes, and in the nematodes arise by alternate splicing of the transcript of one gene. The Cpolypeptide of Drosophila (352 aa) shows 78 %
agreement with the mammalian enzyme, whereas
that of Caenorhabditis (374 aa) shows 82 %; the
R-polypeptide of the nematode is 69 % similar in
the C-terminal region (positions 145-375) but
much less so at the N-terminus [40, 74, 89, 161].
In the Protozoa, cAMP-regulated protein kinases
have been detected in the malarial agents Plasmodium berghei and P. chabaudi and, after initial
failures, also in the flagellate Trypanosoma cruzi
[281].
cGMP-regulated protein serine/threonine
kinases in mammals are found especially in
smooth muscle, lung, intestine and heart. They
are homodimers of about 155 kDa; unlike the
cAMP-dependent kinases, activation by cGMP
does not result in dissociation. The sequence of
670 amino acids in bovine subunits can be subdivided into six segments (A-F) or four domains
(A, B/C, DIE and F): A controls dimerization;
B/C are duplicated sequences, each with one
cGMP-binding site; and DIE contain the ATPbinding site in D and the active centre in E. The
N-terminal regulatory half shows similarity to
various small cGMP-binding proteins, whereas
the C-terminal half is homologous to the catalytic
subunit of the cAMP-dependent kinases, the
phosphorylase-b kinase, and the oncogene tyrosine kinases; the whole molecule appears to be a
chimera produced by gene fusion [63, 292].
cGMP-regulated protein serine/threonine kinases
also occur in arthropods; in fact, they were discovered in this group before they were recorded in
mammals. Here, the catalytic and regulatory
domains also lie on the same polypeptide. In
Drosophila, four cGMP protein kinase mRNAs
are produced from two genes; the encoded polypeptides agree in 55-64 % of their amino acids
with the enzyme from bovine lung; however, in
one case, the domains responsible for dimerization and inhibition are lacking. The enzyme is
also always found as a monomer in the ciliate
Paramecium, but in the roundworm Ascaris suum
it is a dimer [131, 180, 275]. The enzymes from
the Mediterranean fruit-fly, Ceratatis capitata,
and from the eggs of the silkworm, Bombyx mori,
are strictly cGMP specific, whereas a protein
kinase from the skin of the locust Melanoplus
sanguinipes shows as high an affinity for cAMP as
for cGMP [269]. Amongst the protozoans,
cAMP-regulated protein kinases have been found
in the malarial agents Plasmodium berghei and
P. chabaudi. In contrast, the trypanosomes have
an adenylate cyclase but no cAMP-stimulated
protein kinase [48].
Three classes may be distinguished amongst
the Ca 2 + -regulated protein kinases; the first includes the Ca2+ -calmodulin-dependent enzymes, of
which phosphorylase-b kinase and myosin-LC
kinase are the best characterized. A second class
is made up of the enzymes regulated by Ca z +,
phospholipid and diacylglycerol (DAG) and these