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Blood, Vol. 95 No. 4 (February 15), 2000:
pp. 1473-1480
Tropomyosin isoform 5b is expressed in human erythrocytes:
implications of tropomodulin-TM5 or tropomodulin-TM5b complexes in the
protofilament and hexagonal organization of membrane skeletons
Lanping Amy Sung,
Ke-Ming Gao,
Leland J. Yee,
Constance J. Temm-Grove,
David M. Helfman,
Jim
J.-C. Lin, and
Majid Mehrpouryan
From the Department of Bioengineering and Center for Molecular
Genetics, University of California, San Diego, La Jolla, CA;
the Nutritional Science Department, University of Arizona,
Tucson, AZ; Cold Spring Harbor Laboratory, Cold Spring Harbor, NY; and
the Department of Biological Sciences, University of Iowa, Iowa City,
IA.
The human erythrocyte membrane skeleton consists of hexagonal
lattices with junctional complexes containing F-actin protofilaments of
approximately 33-37 nm in length. We hypothesize that complexes formed
by tropomodulin, a globular capping protein at the pointed end of actin
filaments, and tropomyosin (TM), a rod-like molecule of approximately
33-35 nm, may contribute to the formation of protofilaments. We have
previously cloned the human tropomodulin complementary DNA and
identified human TM isoform 5 (hTM5), a product of the
 -TM gene, as one of the major TM isoforms in erythrocytes. We now identify TM5b, a product of the  -TM gene, to be the
second major TM isoform. TM5a, the alternatively spliced isoform of the -TM gene, which differs by 1 exon and has a weaker
actin-binding affinity, however, is not present. TM4, encoded by
the  -TM gene, is not present either. In sodium dodecyl
sulfate-polyacrylamide gel electrophoresis, hTM5 comigrated with the
slower TM major species in erythrocyte membranes, and hTM5b comigrated
with the faster TM major species. TM5b, like TM5, binds strongly to
tropomodulin, more so than other TM isoforms. The 2 major TM isoforms,
therefore, share several common features: They have 248 residues, are
approximately 33-35 nm long, and have high affinities toward F-actin
and tropomodulin. These common features may be the key to the mechanism
by which protofilaments are formed. Tropomodulin-TM5 or
tropomodulin-TM5b complexes may stabilize F-actin in segments of
approximately 33-37 nm during erythroid terminal differentiation and
may, therefore, function as a molecular ruler. TM5 and TM5b further
define the hexagonal geometry of the skeletal network and allow
actin-regulatory functions of TMs to be modulated by tropomodulin.
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Models for actin filament organization in the erythrocyte membrane skeleton
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