Biomedical Engineering Reference
In-Depth Information
recognition site are identifi able. The mature
protein has a predicted mass of
type I receptor phosphorylation in the GS
domain by the type II receptor [
14
kDa with an
].
Currently, seven type I receptors, termed
activin receptor-like kinases (ALKs)
134
observed mass of
kDa, presumably due to
glycosylation. The functional protein exists as
a homodimer that is linked by two disulfi de
bridges. There is some speculation on the exis-
tence of heterodimeric complexes in some situ-
ations, although in normal physiological
settings homodimeric complexes among the
BMPs are most common [
18
1
-
7
, have
been identifi ed in mammals. ALK-
3
(BMP type
IA) and ALK-
6
(BMP type IB) receptors share
an
% amino acid sequence identity in the
kinase domains, and both bind BMP-
85
4
, BMP-
2
,
GDF-
]. Truncated forms of
the ALK receptors are currently being used to
examine the role of BMP signaling during the
development of numerous types of tissues. On
the other hand, there are only three BMP type
II receptors that can interact with BMPs. The
BMPR-II receptor seems to bind exclusively to
BMPs, but the activin types IIA and IIB have
affi nities for specifi c BMPs (BMP-
5
, and BMP-
7
[
149
]. Consider-
able amino acid sequence similarity exists
between species for the various family members.
Approximately
183
,
217
BMPs have been character-
ized, with the majority demonstrating a high
percentage of amino acid sequence homology
among the different isotypes, in addition to a
high level of amino acid conservation between
species [
16
7
, BMP-
2
and
].
BMPs initiate their signaling at the cell
surface through interaction with two distinct
serine/threonine kinase receptors: a type I
receptor (
119
,
217
GDF-
5
), in addition to their activin binding
[
]. BMPR-II binds all BMPs weakly by itself,
with a dramatic increase in the binding
affi nity following recruitment of the type I
receptors.
BMP-
220
50
-
55
kDa) and a type II receptor
(more than
]. It appears that they
weakly interact with certain members of the
type II receptors independently of type I recep-
tors, but in the presence of both receptors their
binding affi nity is increased dramatically [
75
kDa) [
220
ligand and receptor interactions have
been carefully studied (
2
160
). During BMP-
2
receptor activation, the BMP-
protein contains
two distinct domains that facilitate receptor
interaction. The fi rst is a large, high-affi nity
binding site (termed the “wrist epitope”), which
interacts with the BMPR-IA. The second is a
low-affi nity binding site (termed the “knuckle
epitope”), which interacts with BMPR-II [
2
].
Following receptor dimerization induced by
BMP ligand binding, the type II receptor trans-
phosphorylates the type I receptor, which sub-
sequently transmits the BMP signal by
activation of intracellular Smad (Sma and Mad)
proteins. This activation is accomplished by
the directed phosphorylation of specifi c serine
or threonine residues within the Smad pro-
teins. The structures of the two receptors are
similar in that they contain N-glycosylated
extracellular domains, a single membrane-
spanning domain, and an intracellular serine/
threonine kinase domain. The extracellular
domains have several conserved cysteine resi-
dues believed to facilitate the formation of
essential three-dimensional structures involved
in BMP binding [
133
].
The wrist epitopes from monomers (BMPs are
dimeric structures) contribute to the binding of
the BMPR-IA receptor, whereas the knuckle
epitope from only one monomer binds to
BMPR-IA. The juxtapositioning of these regions
facilitates a close proximity of the receptors
and initiation of intracellular signaling from
inter-receptor type II phosphorylation to type
I. Transphosphorylation eventually leads to the
activation of Smad proteins and signal trans-
mission to target downstream responsive genes
[
59
].
Within the cell BMP signals are transduced
by the Smad molecules. To date, eight Smad
mammalian proteins have been isolated and
characterized. Smad proteins are the direct
downstream signaling molecules of BMPs and
other TGF-
149
]. One distinction between
the two receptor types is the presence of a
glycine- and serine-rich domain (GS domain)
found on the type I receptor within the intra-
cellular N-terminal to the serine/threonine
kinase domain. This region is important for the
transmission of the BMP signal to intracellular
second-messenger proteins by facilitating the
receptors' ability to interact with Smad pro-
teins. This was highlighted in an amino acid
mutagenesis study linking Smad
59
superfamily members and are
activated directly by their serine/threonine
kinase receptors. These proteins can be classi-
fi ed into three distinct groups based on their
intracellular function. The receptor-regulated
Smads (R-Smads) are the direct signal
β
7
activation to
