Proteins of the Spectrin-based Membrane Skeleton 
G. Vann Bennett, M.D., Ph.D. — Investigator 
Dr. Bennett is also Professor of Biochemistry at Duke University Medical Center. He received his M.D. and 
Ph.D. degrees from the Johns Hopkins University School of Medicine. His postdoctoral training in mem- 
brane protein biochemistry was completed at Harvard University with Daniel Branton. Before joining 
Duke University, Dr. Bennett was a member of the Department of Cell Biology and Anatomy at Johns 
Hopkins. 
STRUCTURAL proteins in the cytoplasm and 
membranes of ceils provide the basis for spa- 
tial organization of the diverse components of eu- 
karyotic cells. These proteins thus are principal 
participants in fundamental activities of cells, 
such as cell motility, organization of the cyto- 
plasm, and cell-cell interactions. Our work over 
the past 10 years has focused on plasma mem- 
branes. We initiated these studies in the human 
erythrocyte. This relatively simple cell has pro- 
vided an experimentally accessible model system 
for detailed dissection of protein-protein interac- 
tions that are responsible for the structure and 
organization of the plasma membrane. 
The principal structural protein in the erythro- 
cyte membrane is the flexible rod-shaped mole- 
cule spectrin, which is organized in a two-dimen- 
sional network attached to the cytoplasmic 
surface of the plasma membrane. Spectrin mole- 
cules are attached at their ends to form a series of 
hexagons and pentagons that closely resembles a 
geodesic dome. The binding of spectrin to the 
protein ankyrin attaches the spectrin network to 
the plasma membrane. Ankyrin also interacts 
with high affinity with the cytoplasmic domain of 
an integral membrane protein (a protein that tra- 
verses the membrane and actually has portions 
exposed on both the inner and outer membrane 
surfaces) . The spectrin-based membrane network 
or skeleton is required for normal stability of 
erythrocytes in the circulation. Abnormalities in 
amounts or function of spectrin and associated 
proteins result in hemolytic anemias and are the 
basis for diseases such as hereditary spherocytosis 
and hereditary elliptocytosis. Proteins closely re- 
lated to spectrin are present in many vertebrate 
cells and are associated in most cases with the 
plasma membrane. 
Spectrin is present in especially high amounts 
in brain, where it comprises 3 percent of the total 
membrane protein. The spectrin-based mem- 
brane skeleton in brain and other tissues is likely 
to play an important role in providing organiza- 
tion of integral membrane proteins in the plasma 
membrane and for coupling membrane proteins 
to elements of the cytoskeleton. Potential physio- 
logical consequences of these activities include 
stabilization of the lipid bilayer and organization 
of membrane proteins in specialized regions on 
the cell surface in polarized cells. 
Specific aims of this laboratory are to elucidate 
the proteins in erythrocytes and other cells that 
mediate interaction of spectrin with membranes, 
determine how these protein interactions are reg- 
ulated, and understand the cellular functions of 
the spectrin skeleton. 
Ankyrin in the Nervous System 
Brain ankyrin binds to brain spectrin and to in- 
tegral membrane protein sites in brain mem- 
branes. Ankyrin appears to function as an adapter 
between these proteins and the spectrin skeleton. 
We recently discovered that brain contains multi- 
ple forms of ankyrin, which all bind to spectrin 
but are likely to associate with distinct membrane 
proteins. 
One well-characterized ankyrin-binding pro- 
tein is the voltage-dependent sodium channel. An 
isoform of ankyrin is highly concentrated along 
with the voltage-dependent sodium channel at 
the nodes of Ranvier of nerve axons. Nodes of 
Ranvier are specialized regions on the axons of 
nerves where the myelin or insulation of the axon 
is interrupted and where ions can enter or leave 
the axon through ion channels. The localization 
of the voltage-dependent sodium channel at 
nodes of Ranvier is important for normal conduc- 
tion of nerve impulses. We plan to explore the 
possibility that linkage of the sodium channel to 
ankyrin plays a role in either confining or initial 
targeting of the sodium channel to the nodes of 
Ranvier. These studies are relevant to diseases of 
neurons such as multiple sclerosis, where the my- 
elin coating of axons is lost and sodium channels 
are no longer restricted to the nodes of Ranvier. 
We recently determined the complete amino 
acid sequence of the major form of ankyrin in 
human brain. The gene encoding brain ankyrin is 
located on chromosome 4; the gene encoding 
erythrocyte ankyrin is on chromosome 8. Surpris- 
ingly, the same gene that encodes erythrocyte an- 
kyrin also is expressed in brain, with high abun- 
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