Download Poster

Rap1b: Wrapping up Blood Clotting
Abstract
Blood clotting is crucial for maintaining homeostasis, or
equilibrium of internal conditions. However, unintended
consequences may result if blood is unable to clot or clots
excessively. The protein Rap1b plays a key role in the
process of regulating blood clotting, which is facilitated by
platelets sending activation signals. When endothelial cells
are damaged, matrix proteins are exposed; hence, activation
signals are sent to Rap1b. In its inactive state, Rap1b is
bound to GDP. Through the replacement of GDP with GTP,
two switch regions on the now-activated Rap1b change
shape. With the help of guanine nucleotide exchange factors
(GEFs) and GAP proteins, Rap1b then binds to an effector
protein, activating integrins, which control the attachment of
cells to matrix proteins. In turn, the activated integrins located
on the cell membrane of a platelet allow platelets to stick
together, forming a blood clot. Research has shown that if a
mutation occurs at amino acid N17, the protein is permanently
set to an inactive state. However, when amino acid G12
mutates into valine, Rap1b is constitutively active. An
imbalance in the signaling cascade of blood clotting leads to
health risks, such as strokes or bleeding disorders. Studying
Rap1b brings us one step closer to understanding how the
body regulates platelet activation and clotting.
Whitefish Bay SMART Team
Students: Colleen Ackermann, Isaiah Kaplan, Lazura Krasteva, Shannon Mahony,
Andrew Phillips, Zack Serebin and Alice Xia
Teachers: Paula Krukar and Michael Krack
Mentors: Jackie Porath and Gil White, MD, Blood Research Institute, Blood Center of
Wisconsin
Cell Signaling Chain Leading to Platelet Aggregation
Step 1:
Damage to blood
vessel exposes matrix
proteins
Step 2:
Receptor on platelet detects
matrix proteins and binds
Step 3:
PLC generates messenger
molecules, Ca+2 and DAG
Ca+2
Introduction
In this graph, the wild type (WT) mice, with Rap1b, had a
significantly lower bleeding time, while the knock-out (KO) mice
had an extended bleeding time. This can be caused by either
leaky blood vessels or dysfunctional platelets, specifically due
to a lack of Rap1b. A bone marrow transplant experiment was
then conducted. The WT mice with WT bone marrow
experienced a shorter bleeding time, whereas the WT mice with
KO bone marrow experienced a longer bleeding time. Thus, the
increased bleeding time suggests that the problem was in fact
the dysfunctional platelets. Since the platelets were Rap1b
deficient, Rap1b is needed for proper platelet function.
Step 4:
Activation of Rap activator,
Cal-DAG GEF
DAG
When damage occurs to capillaries and blood
vessels, platelets within the blood clot to stop
the bleeding. In this case, the coagulation of
blood around the site of injury is beneficial to the
body. However, blood clotting can occur
unnecessarily, forming dangerous clots that can
result in heart attack or stroke. Rap1b, a protein
found inside the membrane of platelets, plays a
crucial role in facilitating this blood coagulation.
When activated, Rap1b signals platelets to stick
together, which results in a blood clot.
Rap1b: Necessary for Clotting
Step 5:
GDP is removed by Cal-DAG GEF
and GTP is added, changing
Rap1b from inactive to active.
Biological Significance
GTP
GDP
GTP
GDP
Inactive
Inactive
Active
Rap1b
Rap1b
Rap1b
Step 6:
Integrin activation
Mutations
Inactive
Integrin
Rap1b has two significant mutations. When
the Glycine 12 amino acid is exchanged for
valine, Rap1b permanently switches on.
Additionally, when the Serine 17 amino acid
is changed to asparagine, Rap1b becomes
permanently inactive. These mutations can
cause either excess clotting or lack of
clotting, respectively.
Active
Integrin
Active
Integrin
Fibrinogen
Step 7:
Integrins from two
different platelets bind
to fibrinogen
PDB File: 3BRW
Blood clotting caused by platelets
can have both life-saving and lifethreatening effects on the human
body. In platelets, Rap1b, a
membrane protein, is essential to
the clotting of blood. Rap1b
switches from an inactive to an
active state when matrix proteins
are detected. Without Rap1b,
platelets do not clot. Currently,
scientists are trying to connect the
discoveries of Rap1b to the Ras
proteins, which, when mutated to a
permanently active state, cause
over-proliferation of cells, resulting
in cancer. With the knowledge
gained from Rap1b, scientists can
hopefully prevent this mutation and
thus prevent some types of cancer.
Step 8:
Platelet aggregation causes a blood clot to form
A SMART Team project supported by the National Institutes of Health (NIH)-National Center for Research Resources
Science Education Partnership Award (NCCR-SEPA) and an NIH CTSA Award to the Medical College of Wisconsin.
.
Platelet
Platelet
© Health and Life
References: The Rap-RapGAP complex: GTP hydrolysis without catalytic glutamine and arginine residues.
Scrima, A., Thomas, C., Deaconescu, D., Wittinghofer, A. Journal: (2008) Embo J. 27: 1145-1153
Rap1b is required for normal platelet function and hemostasis in mice. Magdalena ChrzanowskaWodnicka, Susan S. Smyth, Simone M. Schoenwaelder, Thomas H. Fischer, and Gilbert C. White II.