Department of Pediatrics, Division of Neonatology, Charles R. Drew University of Medicine and Science and Martin Luther King/Drew Medical Center, Los Angeles, California 90059
IN AN ARTICLE in The
Scientist recently published, Marc Vidal, a geneticist at the Dana
Farber Cancer Institute in Boston, reminds us that "...the
proteome is vast. It's like terra incognita,... we have
a few settlers, we have to explore a huge amount of space..." (7a). There is astounding complexity to protein science, and we are just starting to comprehend the magnitude of the question. Amino
acid chains, from small peptides to complex polymers, interacting with
countless cellular and molecular components in the vertiginous waltz of
metabolic pathways, each actor playing more than one part, entering and
exiting yet again, mutating midways through the process and reappearing
in a different configuration later on. ...
Thanks to the Human Genome Project, we now have all the elements for an
astronomical number of new questions to ask, and we will need many
scientific life spans to answer them. A couple of years back, many
investigators and a certain number of curious laypeople answered the
unusual appeal of a Stanford University project set out at
www.foldingathome. stanford.edu: they installed protein folding
calculations as screen saver programs on individual personal computers
so that their cumulative activity could contribute to the titanic task
of calculating protein secondary and tertiary structures. This creative
tool has piqued the curiosity of many toward the exciting surprises of
proteins and produced interesting data. However, we are barely
scratching the, yes, pun intended, surface.
About 30% of the proteins identified through the Human Genome Project
are membrane-related proteins, and most of them contain one or more
amphipathic peptides, configured as alpha-helical sequences, and
closely dependent on the lipid system with which they associate
(2). These findings suggest that a large body of
information on all aspects of protein science, structure-function relationships, protein-lipid interactions, membrane protein sorting, protein folding, and a number of others will continue to come from
interfacial studies (10, 12, 21). The use of proteins in
surface chemistry appears a bit of a heresy to the classic lipid
physicist for whom more than one component to the interface monolayer
represents a contaminant. The biologist, however, has achieved endless
entertainment from assessing the putative behavior of each amino acid
chain in such systems.
Proteins from a myriad of biological systems have been investigated
through their surface activity using several different lipid moieties.
Apolipoproteins (17), snake and bee venoms (4, 10), leader sequences (8), and viral envelope
peptides (5) are among the proteins tested on surface
monolayers to determine the molecular mechanisms by which an amino acid
chain produces biological effects on intracellular processes via
interactions with cell membranes. The results have been fascinating,
somewhat distant from mainstream investigations, possibly appealing to the biophysicist more than to the biologist, and above all, far removed
from the focus on molecular biology and therefore an avenue of renewed
interest for the current season of protein studies.
Few areas of the life sciences have contributed to the understanding of
protein behavior at an interface as much as research on lung
surfactant. Surfactant is part of the mammalian respiratory complex, a
most unusual substance for composition and metabolic dynamics. Its
synthesis, secretion, and reuptake have fascinated physiologists,
biophysicists, and neonatologists since the relatively recent
recognition of its existence (6, 18). A self-contained, exquisitely biophysical system comprising tightly regulated, vastly recycled, and unique components, such as proteins that behave like
lipids and lipids enmeshed with water, surfactant has a crucial role in
establishing and maintaining functional gas exchange units.
Aggressive research and development in surfactant studies have rapidly
produced successful contributions to the standards of care in neonatal
medicine and a decrease in infant mortality, a pivotal index of social
progress (1). The current neonatal fellows are amazed to
discover that clinical surfactant was actually introduced only a dozen
years ago. We are now perfectly comfortable with the choice of any of
the various surfactants as a matter of personal preference, and only
peripheral attention is paid to the basic science underlying any of the
Food and Drug Administration-approved formulations.
Nevertheless, what has now become standardized clinical practice comes
from a somewhat exclusive branch of investigation, and our knowledge of
protein-lipid interactions, ideal ratios, and optimal composition, far
from being established early and universally accepted, still is an
active pursuit, requiring patient, steady, precise work. In the study
by Wang and coworkers, one of the current articles in focus (Ref.
23, see p. L897 in this issue),
R. H. Notter's group illustrates an essential step in the
definition of how much native surfactant protein (SP)-B is needed to
optimize the activity of a protein-containing surfactant (23). We welcome this study on the surface activity of
incremental SP-B/lipid ratios, an area that has required tedious
testing and dedicated investigators with high tolerance for repetitious
permutations of one analysis under multiple conditions. Dr. Notter and
the many scholars from his laboratory have provided incalculable
contributions to the definition of the gold standard in surface
chemistry, and this paper belongs to a distinguished series on
surfactant lipids, surfactant proteins, and their role at the interface
(14, 15, 24). To the outsider, the significance of this
research may not be readily apparent.
The elegant structure and deceptively simple configuration of two small
hydrophobic proteins, SP-B and SP-C, have been studied in a variety of
lipid dispersions and by diverse techniques. Definition of their
molecular biology has offered critical insight into the proteins
sequence and their putative secondary structure (9, 16,
25). However, we are still grappling with the actual
understanding of how the described configurations translate into
surfactant function and all its baffling subtleties. Although the
development of synthetic surfactant peptides by solid phase method has
offered significant contributions to the understanding of the behavior of surfactant peptides in the presence of a series of lipids and an
air-water interface, the same data on native proteins have been more
difficult to obtain (7, 11).
Surface activity of a mixed dispersion is determined by several
concomitant elements, including temperature, composition of the
subphase, and amount and composition of the lipid system, further
proceeding to more intricate and challenging ones like the depth and
angle of insertion of the protein in the lipid layer or the size of the
lipid vesicle (20). Worse, intricate as the resulting
effects may be, direct observation of these paucimolecular two-dimensional structures is hardly feasible. Because this is the
magic of surface tension: defining the skin of water, observing the
events at the air-water interface, albeit physiologically critical, is
also scientifically elusive. Very creative and unusual techniques have
been developed in the attempt to examine and quantify the forces at the
air-water interface, and although their dimensions, measurements, and
related quantitative effects are derived from a rather traditional,
almost old-fashioned technology, the most sophisticated and inventive
molecular modeling has only recently offered us projections and
reconstructed images, a suggestion of possible configurations, yet
hardly any hard copy (13, 22).
Surface activity has an interesting history. Its nineteenth-century
flavor well matches its being the province of many "little women"
in science, Miss Blodgett among them (3), and a Cinderella in the overall picture of scientific discoveries. Nonetheless, the
phenomena it covers are critical to life. Cellular organization, organelle formation, and so many other aspects of cell biology are so
tightly linked to the hydrophobic effect and the complementary dimension of surface tension that, even though limited space is offered
to surface chemistry in course work, its importance is largely
vindicated by the biological reality (19). Life as a whole
depends on the integrity of interfaces (14). From the dynamic structure of a cell membrane to the first successful breath of
a mammal, biological phenomena affirm the power of this humble dimension.
The main contribution offered by the current study by Wang and
coworkers (23) to the understanding of surfactant function rests in the suggestion that in the native configuration there exists a
critical amount of protein necessary to optimally organize the lipid
array at the surface, with the resulting effect of decreasing surface
tension during the compression phase (23). The authors may
not wholly agree, but one may develop the impression that these
studies, beyond the sheer pursuit of scientific understanding, point
forward to an unresolved perspective: are there minimal components
indispensable to reproduce a surfactant's full activity? Should we
start to seriously consider novel surfactant recipes or try to modify
the existing ones, we might hope to solve some of the problems inherent
to surfactant, such as dependency on animal products, with the
suspicion of prion carriage or the batch-to-batch variability
inevitable in such products. Further data are, as always, necessary to
confirm this hypothesis.
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REFERENCES
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FOOTNOTES |
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Address for reprint requests and other correspondence: R. Bruni, Dept. of Pediatrics, Div. of Neonatology, King/Drew Medical Center, Los Angeles, CA 90059 (E-mail: rbbruni{at}hotmail.com).
10.1152/ajplung.00137.2002
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