Blood, Vol. 96 No. 2 (July 15), 2000:
pp. 780-782
CORRESPONDENCE
 |
To the Editor: |
Models for actin filament organization in the erythrocyte membrane
skeleton
Sung et al recently reported the identification of TM5b as one
of the tropomyosin isoforms present in the human erythrocyte membrane skeleton.1 Therein, they also "propose a
molecular model of a short actin protofilament in erythrocytes ... in which tropomodulin is associated near the N-terminal end
of 1 TM molecule, which comprises either TM5 or TM5b, ... and is at the pointed end of the short actin filament" (see
"Results," p 1478).1 Clearly, the
identification of TM5b as one of the erythrocyte tropomyosin isoforms
is important: it specifies one of the unknown components of the short
erythrocyte actin filaments and will no doubt contribute to
deciphering how they assemble in vivo.1 It is
unfortunate, however, that the presentation of the Sung et al model
leaves the impression that this model for the organization of
the short erythrocyte actin filaments has not been proposed
before. To the contrary, a series of research articles on
tropomodulin,2-5 as well as several review articles on the
erythrocyte membrane skeleton6-9 and a
textbook,10 have discussed extensively the idea
that tropomodulin is located at the pointed ends of the short
erythrocyte actin filaments and functions with tropomyosin to restrict
their length. The misrepresentation by Sung et al is disappointing
because science advances by virtue of new ideas as well as facts. It is
a disservice to the scientific community not to place new findings in
their proper historical context.
Velia M. Fowler
Department of Cell Biology
The Scripps Research Institute
La Jolla, CA
| |
References |
1.
Sung LA, Gao K-M, Yee LJ, et al.
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.
Blood.
2000;95:1473-1480[Abstract/Free Full Text].
2.
Fowler VM.
Tropomodulin: a cytoskeletal protein that binds to the end of erythrocyte tropomyosin and inhibits tropomyosin binding to actin.
J Cell Biol.
1990;111:471-482[Abstract/Free Full Text].
3.
Fowler VM, Sussman MA, Miller PG, Flucher BE, Daniels MP.
Tropomodulin is associated with the free (pointed) ends of the thin filaments in rat skeletal muscle.
J Cell Biol.
1993;120:411-420[Abstract/Free Full Text].
4.
Weber A, Pennise CR, Babcock GG, Fowler VM.
Tropomodulin caps the pointed ends of actin filaments.
J Cell Biol.
1994;127:1627-1635[Abstract/Free Full Text].
5.
Ursitti JA, Fowler VM.
Immunolocalization of tropomodulin, tropomyosin and actin in spread human erythrocyte membrane skeletons.
J Cell Sci.
1994;107:1633-1639[Abstract].
6.
Gilligan DM, Bennett V.
The junctional complex of the membrane skeleton.
Sem Hematol.
1993;30:74-83[Medline]
[Order article via Infotrieve].
7.
Lux SE, Palek J.
Disorders of the red cell membrane. In:
Handin RI,Lux SE,Stossel TP, eds.
Blood: Principles and Practice of Hematology. Philadelphia, PA: JB Lippincott; 1995:1701-1818.
8.
Fowler VM.
Regulation of actin filament length in erythrocytes and striated muscle.
Curr Opin Cell Biol.
1996;8:86-96[Medline]
[Order article via Infotrieve].
9.
Luna EJ, Hilt AL.
Cytoskeleton-plasma membrane interactions.
Science.
1992;258:955-964[Abstract/Free Full Text].
10.
Lodish H, Berk A, Zippursky SL, Matsudaira P, Baltimore D, Darnell J.
Molecular Cell Biology. 4th edition. New York, NY: WH Freeman; 2000:758.
| |
Response: |
A view on the molecular basis of erythrocyte membrane
mechanics
We recently reported the identification of tropomyosin isoform
5b (TM5b) in human erythrocytes and the implications of
tropomodulin-TM5 or tropomodulin-TM5b complexes in the protofilament
and hexagonal organization of membrane skeletons.1 In this
report, schematic drawings/models of a tropomodulin-TM complex, a short
actin protofilament, and hexagonal lattices of the erythrocyte membrane
skeleton were presented to illustrate the proposed structure and
function of these newly characterized tropomodulin-TM complexes (shown
here in the middle 3 panels of the
Figure, labeled "Molecular ruler," "Actin protofilament," and "Hexagonal lattices," with minor
modifications). We extensively cited articles to support our statements
and/or proposals, with 9 articles authored or coauthored by Dr Fowler, including the 1996 review article.2
View larger version (27K):
[in this window]
[in a new window]
| Fig 1.
A composite illustrating a view of the possible molecular
basis of erythrocyte membrane mechanics in vitro and in vivo.
Read from bottom up. At "Tmod-binding site on TM5," residues at
a, d, f, and a in the N-terminal heptad
repeats of TM5,6 functioning as the tropomodulin-binding
site. At "Molecular ruler," a complex of tropomodulin and TM5 or
TM5b, in the form of homodimer or heterodimer, functioning to protect
actin filaments of an uniform length.1,6 At "Actin
protofilament," a short actin protofilament of about 33-37 nm
consisting of 6 G-actin per strand protected by the molecular ruler,
specifying the joining of 6 spectrin tetramers. At "Hexagonal
lattices of erythrocyte membrane skeleton," geometry of the membrane
skeleton defined mainly by spectrin teramers and actin protofilaments;
arrows point to junctional complexes. At "Elastic deformation of
erythrocyte," elastic deformation of an erythrocyte in a flow
channel,7 responding to a shear stress of 4.0 dyn/cm2. At "Blood circulation," "blue"
erythrocytes circulating in blood vessels of a mouse yolk sac. (X-gal
staining detected the expression of tropomodulin in erythrocytes
reported by a "knocked in" lacZ reporter gene under the
control of the endogenous Tmod promoter.) The
Tmod  / mutation is lethal, suffering from
arrests in heart development, vasculogenesis, and definitive lineage
hematopoiesis.8 A Tmod +/ embryo
at 9.5 days of gestation is shown.
|
|
We proposed that TM in the protofilament is composed of TM5 or TM5b in
the form of either homodimer or heterodimer based on our new findings.
There was no intention to impress the scientific community that this
was the only model ever proposed. Gilligan and Bennett
(1993),3 Lux and Palek (1995 and earlier),4 Fowler (1996),2 and others5 have in fact
proposed several models for the short actin filament in erythrocytes.
We unfortunately did not take the opportunity to discuss the variations
among these models. For example, in the 1996 Fowler model, the actin
protofilament is about 60 nm long, consisting of 18 G-actin, with 2 TM
molecules located at one (pointed) end associating with tropomodulin
and several spectrin molecules located at the other (barbed) end
associating with adducin tails. In the models of Gilligan and Bennett
and of Lux and Palek, protofilaments are about 35 nm long, consisting of about 12 G-actin. The end of TM to which tropomodulin binds and how
6 spectrin tetramers per protofilament are spaced, however, are not
specified. We proposed that it is the common properties shared by TM5
and TM5b that contribute to the formation of the actin protofilament
(Figure) and that the 6 pairs of G-actin in the double helix define the
hexagonal arrangement of spectrin in the filament. The properties
shared by TM5 and TM5b include the same number of G-actin that they
protect, the high tropomodulin and actin affinity they both
possess, and their unique ability to form both homodimers and
heterodimers with each other.
As to how tropomodulin functions with TM to restrict the actin filament
length: Fowler's 1996 review article stated that the actin filaments
in the native membrane skeleton are likely to be about 67 nm long and
that "strict control of the relative amounts of tropomyosin,
tropomodulin and adducin with respect to the amounts of actin, spectrin
and other associated components could act to limit the filaments to the
length of one tropomyosin rod plus the actin subunits required for
spectrin binding" (p 90).2 In contrast, our
model was based on the precise information provided by TM5 and TM5b,
and the article explained, step by step, why the erythrocyte
protofilament has only one TM in length (see "Discussion," p 1478). The significance of identifying TM5b, therefore,
goes beyond merely specifying one of the unknown components of the short erythrocyte actin filament.
The scientific community is invited to read the article by
Sung et al, as well as the references cited, as all new
findings or ideas need to be judged in the historical context by the
scientific community. The Figure represents a personal view in terms of
the roles of tropomodulin and TM5 or TM5b in the attempt to understand the molecular basis of erythrocyte membrane mechanics. Here I acknowledge my current and former collaborators, many
outstanding investigators, including Dr Fowler, who have
contributed to the advancement of this field, and those who have
developed ingenious technologies that made these studies possible.
Lanping Amy Sung
Department of Bioengineering and Center for Molecular
Genetics
University of California, San Diego
La Jolla, CA
| |
References |
1.
Sung LA, Gao K-M, Temm-Grove CJ, Helfman DM, Lin JJ-C, Mehrpouryan M.
Tropomyosin isoform 5b is expressed in human erythrocytes: implications of tropmodulin-TM5 or tropomodulin-TM5b complexes in the protofilament and hexagonal organization of membrane skeletons.
Blood.
2000;95:1473.
2.
Fowler VM.
Regulation of actin filament length in erythrocytes and striated muscle.
Curr Opin Cell Biol.
1996;8:86.
3.
Gilligan DM, Bennett V.
The junctional complex of the membrane skeleton.
Sem Hematol.
1993;30:74.
4.
Lux SE, Palek J.
Disorders of the red cell membrane. In:
Handin RI,Lux SE,Stossel TP, eds.
Blood: Principles and Practice of Hematology. Philadelphia, PA: J B Lippincott; 1995:1701.
5.
Alberts B, Dennis B, Lewis J, Raff M, Roberts K, Waston J.
Molecular Biology of the Cell. Garland Publishing: New York and London; 1994:493.
6.
Vera C, Sood A, Gao K-M, Yee LJ, Lin JJ-C, Sung LA.
Tropomodulin-binding site mapped to residues 7-14 at the N-terminal heptad repeats of human tropomyosin isoform 5.
Arch Biochem Biophys.
2000;378:16[Medline]
[Order article via Infotrieve].
7.
Chien S, Sung LA, Lee MML, Skalak R.
Red cell membrane elasticity as determined by flow channel technique.
Biorheology.
1992;29:467[Medline]
[Order article via Infotrieve].
8.
Chu X, Chen J, Chien KR, Vera C, Sung LA.
Tropomodulin-null mutation results in arrests of cardiac development, vasculogenesis, and hematopoiesis during embryogenesis [abstract].
Mol Cell Biol.
1999;10:153a.
