EDITORIAL FOCUS
Focus on proteins, surfaces, et al.

Roberta Bruni

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


    ARTICLE
TOP
ARTICLE
REFERENCES

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.


    FOOTNOTES

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


    REFERENCES
TOP
ARTICLE
REFERENCES

1.   American Academy of Pediatrics, Committee on Fetus and Newborn. Surfactant replacement therapy for respiratory distress syndrome. Pediatrics 103: 684-685, 1999[Abstract/Free Full Text].

2.   Biggins, PC, and Sansom MS. Interaction of alpha helices with lipid bilayer. A review of simulation studies. Biophys Chem 76: 161-183, 1999[ISI][Medline].

3.   Blodgett, KB, and Langmuir I. Built-up films of barium stearate and their optical properties. Physical Review 51: 964-982, 1937.

4.   Bougis, P, Rochat H, Pieroni G, and Verger R. Penetration of phospholipid monolayers by cardiotoxins. Biochemistry 20: 4915-4920, 1981[ISI][Medline].

5.   Brasseur, R, Cornet B, Burny A, Vandenbranden M, and Ruysschaert JM. Mode of insertion into a lipid membrane of the N-terminal HIV gp41 peptide segment. AIDS Res Hum Retroviruses 2: 83-90, 1988.

6.   Bruni, R, Baritussio A, Quaglino D, Gabelli C, Benevento M, and Pasquali-Ronchetti I. Postnatal transformations of alveolar surfactant in the rabbit: changes in pool size, pool morphology and isoforms of the 32-38 kDa apolipoproteins. Biochim Biophys Acta 958: 255-267, 1988[ISI][Medline].

7.   Bruni, R, Taeusch HW, and Waring AJ. Surfactant protein SP-B: lipid interactions of synthetic peptides representing the amino terminal amphipathic domain. Proc Natl Acad Sci USA 88: 7451-7455, 1991[Abstract].

7a.   DeFrancesco, L. Probing protein interactions. Scientist 16: 28-33, 2002.

8.   Gierasch, LM. Signal sequences. Biochemistry 28: 923-930, 1989[ISI][Medline].

9.   Jacobs, KA, Phelps DS, Steinbrink R, Fisch J, Kriz R, Mitsock L, Dougherty JP, Taeusch HW, and Floros J. Isolation of a cDNA clone encoding a high molecular weight precursor to a 6-kDa pulmonary surfactant-associated protein. J Biol Chem 262: 9808-9811, 1987[Abstract/Free Full Text].

10.   Ladokhin, AS, and White SH. Folding of amphipathic alpha helices on membranes: energetics of helix formation by melittin. J Mol Biol 285: 1363-1369, 1999[ISI][Medline].

11.   Longo, LM, Bisagno AM, Zasadzinski JAN, Bruni R, and Waring AJ. A function of lung surfactant protein SP-B. Science 261: 453-456, 1993[ISI][Medline].

12.   MacRitchie, F. Proteins at interfaces. Adv Protein Chem 32: 283-326, 1978[Medline].

13.   Nag, K, Taneva SG, Perez-Gil J, Cruz A, and Keough KM. Combinations of fluorescently labeled pulmonary surfactant protein SP-B and SP-C in phospholipid films. Biophys J 72: 2638-2650, 1997[Abstract].

14.   Notter, RH. Lung Surfactants. Basic Science and Clinical Application. New York: Dekker, 2000, vol. 149 (Lung Biol Health Dis Ser).

15.   Notter, RH, and Finkelstein JN. Pulmonary surfactant: an interdisciplinary approach. J Appl Physiol 57: 1613-1624, 1984[Abstract/Free Full Text].

16.   Phelps, DS, Smith LM, and Taeusch HW. Characterization and partial amino acid sequence of a low molecular weight surfactant protein. Am Rev Respir Dis 135: 1112-1117, 1987[ISI][Medline].

17.   Phillips, MC, and Sparks CE. Properties of apolipoproteins at the air-water interface. Ann NY Acad Sci 382: 122-137, 1980.

18.   Tabor, B, Ikegami M, Yamada T, and Jobe A. Rapid clearance of surfactant associated palmitic acid from the lungs of developing and adult animals. Pediatr Res 27: 268-273, 1990[Abstract].

19.   Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes (2nd ed.). New York: Wiley Interscience, 1980.

20.   Van Ginkel, G, Korstanje LJ, van Langen H, and Levine YK. The correlation between molecular orientational order and reorientational dynamics of probe molecules in lipid multibilayers. Faraday Discuss 81: 49-61, 1986[ISI].

21.   Von Heijne, G. Transcending the impenetrable: how proteins come to term with membranes. Biochim Biophys Acta 947: 307-333, 1988[ISI][Medline].

22.   Walther, FJ, Gordon LM, Zasadinski JA, Sherman MA, and Waring AJ. Surfactant protein B and C analogues. Mol Genet Metab 71: 342-351, 2000[ISI][Medline].

23.   Wang, Z, Baatz JE, Holm BA, and Notter RH. Content-dependent activity of lung surfactant protein B in mixtures with lipids. Am J Physiol Lung Cell Mol Physiol 283: L897-L906, 2002[Abstract/Free Full Text].

24.   Wang, Z, Hall SB, and Notter RH. Roles of different hydrophobic constituents in the adsorption of pulmonary surfactant. J Lipid Res 37: 790-798, 1996[Abstract].

25.   Warr, RG, Hawgood S, Buckley DI, Crisp TM, Schilling J, Benson BJ, Ballard PL, Clements JA, and White RT. Low molecular weight human pulmonary surfactant proteins (SP5): isolation, characterization, and cDNA amino acid sequence. Proc Natl Acad Sci USA 84: 7915-7919, 1987[Abstract].


Am J Physiol Lung Cell Mol Physiol 283(5):L894-L896
1040-0605/02 $5.00 Copyright © 2002 the American Physiological Society




This Article
Full Text (PDF)
Alert me when this article is cited
Alert me if a correction is posted
Citation Map
Services
Email this article to a friend
Similar articles in this journal
Similar articles in PubMed
Alert me to new issues of the journal
Download to citation manager
Google Scholar
Articles by Bruni, R.
Articles citing this Article
PubMed
PubMed Citation
Articles by Bruni, R.


HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online