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 Medical School. He completed postdoctoral training at 
Harvard University in membrane protein biochemistry with Daniel Branton. Before joining Duke 
University, he was on the faculty in the Department of Cell Biology and Anatomy at Johns Hopkins. 
STRUCTURAL proteins in the cytoplasm and 
membranes of cells provide the basis for spa- 
tial organization of the diverse components of eu- 
karyotic cells. These proteins thus are principal 
participants in fundamental cellular activities 
such as cell motility and cell-cell interactions. 
Our work over the past 1 0 years has focused on 
plasma membranes. We initiated these studies in 
the human erythrocyte. This relatively simple 
cell has provided an experimentally accessible 
model system for detailed dissection of protein- 
protein interactions that are responsible for the 
structure and organization of the plasma 
membrane. 
The principal structural protein in the eryth- 
rocyte membrane is the flexible rod-shaped 
molecule spectrin, which is organized in a two- 
dimensional 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 related 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 mem- 
brane protein. The spectrin-based membrane 
skeleton in brain and other tissues is likely to play 
an important role in providing organization of 
integral membrane proteins in the plasma mem- 
brane and for coupling membrane proteins to ele- 
ments of the cytoskeleton. Potential physiologi- 
cal 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. 
Ankyrins in the Nervous System 
Ankyrin appears to function as an adapter be- 
tween certain membrane proteins and the spec- 
trin skeleton. We have discovered that brain con- 
tains multiple forms of ankyrin, with diversity 
due to distinct genes as well as alternative splic- 
ing of RNAs encoded by the same gene. We have 
determined the complete amino acid sequence of 
the major form of ankyrin in human brain and 
have discovered an unusual alternative form of 
this protein that contains a large inserted se- 
quence. The large form of brain ankyrin is the 
first ankyrin detected during brain development 
and is targeted to neuronal processes including 
unmyelinated axons. 
The same gene that encodes ankyrin in erythro- 
cytes also is expressed in brain. This form of an- 
kyrin is localized in the plasma membranes of 
certain neurons and is abundant in cerebellum, 
brain stem, and spinal cord. The erythrocyte form 
of ankyrin is missing in a strain of mutant mice 
developed at the Jackson Laboratory. Ankyrin- 
deficient mice experience degeneration of Pur- 
kinje cells, a major type of neuron in the cerebel- 
lum, and develop a stagger and difficulty in 
walking. Neurological problems of ankyrin-defi- 
cient mice may have counterparts in humans with 
slowly progressive diseases due to death of nerve 
cells. 
Another isoform of ankyrin is highly concen- 
trated 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 
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