ined the subcellular localization and biochemical 
properties of dystrophin-related protein (DRP), an 
autosomal gene product with high homology to dys- 
trophin. DRP is enriched in isolated sarcolemma 
from control and mdx mouse muscle but is much 
less abundant than dystrophin. Immunofluores- 
cence microscopy localized DRP almost exclusively 
to the neuromuscular junction region in adult rabbit 
and mouse skeletal muscle, as well as mdx mouse 
muscle and denervated mouse muscle. Thus DRP is 
a neuromuscular junction-specific membrane cyto- 
skeletal protein that may play an important role in 
the organization of the postsynaptic membrane of 
the neuromuscular junction. 
Dystroglycan, a Novel 
Laminin-binding Protein 
Because the abnormal expression of the DAGs 
may play a crucial role in the molecular pathogene- 
sis of DMD, it was essential for Dr. Campbell's labo- 
ratory to identify their normal function. A major em- 
phasis in the laboratory is now directed toward the 
isolation of cDNA clones for each DAP. The primary 
structures of two components of the dystrophin- 
glycoprotein complex, the 43-kDa DAG and 156- 
kDa DAG (dystroglycan), have been determined. Se- 
quence analysis of cDNAs reveals an open reading 
frame encoding a 97-kDa precursor polypeptide 
with no significant sequence similarity with any 
known proteins. The amino-terminal portion of the 
precursor polypeptide is processed into the mature 
156-kDa DAG with a putative protein core of ~56 
kDa with potential attachment sites for O-linked 
carbohydrates. The carboxyl-terminal portion of the 
precursor polypeptide is processed into the mature 
43-kDa DAG with potential A'-glycosylation sites, a 
single transmembrane domain, and a 120-amino 
acid long cytoplasmic tail. Northern and Western 
blot analyses have demonstrated that the 43- and 
156-kDa DAGs are expressed in both muscle and 
nonmuscle tissues. The specific mRNA for the 43- 
and 1 5 6 -kDa DAG is expressed at normal amounts in 
mdx and DMD skeletal muscle, whereas both 
glycoproteins are greatly reduced in dystrophin- 
deficient muscle. Functional studies have demon- 
strated that dystroglycan binds laminin, a well- 
characterized component of the extracellular matrix. 
Thus the results demonstrate that dystroglycan is a 
novel laminin-binding glycoprotein and suggest that 
the function of the dystrophin-glycoprotein com- 
plex is to link the subsarcolemmal cytoskeleton to 
the extracellular matrix. The goal for the next year is 
to clone the other DAPs in order to express the en- 
tire complex in nonmuscle cells to study its func- 
tion and laminin-binding properties. 
Molecular Pathogenesis 
of Muscular Dystrophy 
Characterization of the status of DAPs in dystro- 
phin-deficient muscle was necessary to understand 
the molecular pathogenesis of DMD. The relative 
abundance of each component of the dystrophin- 
glycoprotein complex in skeletal muscle from nor- 
mal and mdx mice, which are missing dystrophin, 
was examined last year. Immunoblot analysis using 
total muscle membranes from control and mdx 
mice (ages 1 d to 30 wk) found that all of the DAPs 
were greatly reduced (80-90%) in mdx mouse skel- 
etal muscle. The specificity of the loss of the DAPs 
was demonstrated by the finding that the major gly- 
coprotein composition of skeletal muscle mem- 
branes from normal and mdx mice is identical. 
Immunofluorescence microscopy confirmed 
these results and showed a drastically reduced den- 
sity of DAPs in mdx muscle cryosections compared 
with normal and dy/dy mouse muscle. Therefore all 
of the DAPs are significantly reduced in mdx skele- 
tal muscle, and the loss of these proteins may be due 
to the absence of dystrophin and not to secondary 
effects of muscle fiber degradation. In the next year 
these studies will be extended to DMD patients. In 
addition, DAP status will be examined in various au- 
tosomal muscular dystrophy patients. A grant from 
the Muscular Dystrophy Association provided par- 
tial support for the project described above. 
Dr. Campbell is also Professor of Physiology 
and Biophysics at the University of Iowa College 
of Medicine, Iowa City, and University of Iowa 
Foundation Distinguished Professor of Physiol- 
ogy and Biophysics. 
Articles 
Ervasti, J.M., and Campbell, K.P. 1991. Membrane 
organization of the dystrophin-glycoprotein com- 
plex. Ce// 66:1121-1 131. 
Ibraghimov-Beskrovnaya, O., Ervasti, J.M., Le- 
veille, C.J., Slaughter, C.A., Sernett, S.W., and 
Campbell, K.P. 1992. Primary structure of dys- 
trophin-associated glycoproteins linking dystro- 
phin to the extracellular matrix. Nature 
355:696-702. 
McPherson, S.M., McPherson, P.S., Mathews, L., 
Campbell, K.P., and Longo, F.J. 1992. Cortical 
localization of a calcium release channel in sea 
urchin eggs. / Cell Biol 116:1111-1121. 
Mickelson, J.R., Knudson, CM., Kennedy, C.F.H., 
Yang, D.-I., Litterer, L.A., Rempel, W.E., Camp- 
bell, K.P., and Louis, C.F. 1992. Structural and 
32 
