Chronology of a Discovery

Key dates in cell membrane research compared with events in genetics and physiology.
1855  C. Naegeli and C. Cramer describe cell membrane as barrier essential to explain osmosis in plant cells
1859 Charles Darwin publishes his 'Origin of Species' on his theory of evolution by natural selection
1865 Gregor Mendel publishes his findings on 'plant hybridization' laying out for the first time the mathematics of genetic inheritance of independent factors
1871, 1888  Hugo de Vries describes cell membrane permeability for ammonia and glycerol
1888 Walther Nernst develops a theory of electrical potentials based on diffusion of ions in solution. This theory (called Nernst potential) became and still is fundamental in modeling ion flux across biological membranes in electrophysiology
1895-99 Ernest Overton develops the following theories
- lipoid membrane enclosing animal and plant cells
- lipid-water partition theory for general anesthetic action (alcohol narcosis)
- exchange of external Na for internal K ions
- proposed active transport (uphill) requiring metabolic energy
1905 R. Hoeber demonstrates the effect of electrolytes (salt solutions) on the resting potential of frog muscles
1925  Gorter and Grendel propose lipid bilayer structure for cell membranes; surface area covered by lipids extracted from red blood cells on water surface is twice as large as original surface of red blood cells.

The electrical capacitance of cell membranes is used to estimate the thickness membranes to be 33 Å; this value is in complete agreement with modern data

1926, 1930 First accounts that proteins convey enzymatic activity (urease, pepsin) in cellular metabolism; an important step to demonstrate that proteins catalyze chemical reactions and are not only structural components of cells
1934  First successful X-ray study of the globular protein pepsin by Bernal and Crowfoot; it does not show high resolution details, but demonstrates water covered protein surface
1935  Danielli and Davson's membrane model of globular proteins on surface of lipid bilayer; this model specifically excludes transmembrane proteins based on the previously shown hydrophilic surface of globular proteins
1937 Citric Acid Cycle described by Hans Krebs; this is the central energy yielding pathway in all organisms; complete biochemical pathway reactions could be elucidated in the absence of any protein structure information (kinetic data represents macroscopic behavior of enzymes)

First demonstration by A.L. Hodgkin for electrical transmission in nerve cells

1939  Glycolysis completely described and enzymatic reactions demonstrated in vitro; although the first third of the glycolytic pathway has been described in the early 30s, the complete description has been obtained only by 1939; however, the entire pathway has been reconstituted in vitro using purified enzymes

Hodgkin together with A.F. Huxley publish the first action potentials recorded from inside a nerve fiber

1941  'One gene, one enzyme' hypothesis by Beadle and Tatum
1942  First demonstration of ion selectivity on enzymes; K+  ions stimulate pyruvate kinase, but Na+  ions do not; the property of proteins to discriminate among substrates based on charge, size and structure is essential for all membrane transport processes
1944  DNA is carrier of genetic information in bacteria (Oswald Avery)
1945  First complete amino acid content of a protein is published (not its sequence, however)

Voltage clamp technique developed to study macroscopic ion currents in neurons; Hodgkin and Huxley publish resting and action potential recordings form single nerve fibers

1949  Mitochondria are shown to be organelles responsible for oxidative phosphorylation and  containing enzymes of Krebs cycle (Lehninger)
1951  First complete amino acid sequence published of the protein hormone insulin by Fred Sanger

Proposed model for alpha helix and beta sheet and importance of so called hydrogen bonds in protein structures (Pauling and Corey)

1952  First high resolution electron micrograph of biological membranes of mitochondria, by G. Palade
1953  DNA structure at atomic resolution by Crick, Watson, and Wilkins; they propose a model for DNA replication based on the structural information; the concept of structure-function relationship has been successfully used to solve a major problem in biology
1958  Induced-fit model of substrate protein interaction proposed by Daniel Koshland addressing the importance of structural  flexibility of proteins for their function
1961  Chemiosmotic theory by Peter Mitchell postulating a proton gradient as energy source for ATP synthesis in mitochondria
1962  High resolution structure of myoglobin at 2 Angstrom confirms for the first time the existence of alpha helix structures in proteins  (Perutz and Kendrew)

Black lipid membrane; successful formation of the first artificial lipid bilayer (Mueller and Rudin) mimicking cell membranes

Kauzman and Tanford develop theory on hydrophobic effect on protein folding and stability, an important concept to understand cell membrane assembly and stability

The structure of the enzyme lysozyme with a bound inhibitor molecule solved at 2 Angstrom resolution giving the first structural insight into enzyme-substrate interaction and Koshland's induced fit theory

1963  Genetic code solved;  links DNA sequence to amino acid sequence in proteins (Holley, Khorana, Nirenberg

Allosteric mechanism described by J.Monod, F. Jacob, and Changeux to explain regulation and cooperativity in enzymes by ligands that bind to a protein on a location different from the active site; mechanistically induced-fit and allosterism are closely related phenomena

1965  Multilamellar vesicles in vitro separating aqueous compartments; because they contain multiple membrane layers, they are not suitable for transport studies
1969 Unilamellar lipid vesicles used for membrane protein reconstitution to study ion flux (Huang); this single membrane vesicles revolutionize transport studies because they can be prepared in large quantities; the kinetics of many channels, pumps and transporters has been demonstrated using these membrane system without having to know their structure; 
1970  Experimental evidence that membrane proteins can diffuse laterally in cell membranes (Frye and Edidin)

Unit channel structure for Gramicidin A peptide confirmed by Hladky and Haydon using black lipid membranes by demonstrating "the discreteness of conduction change in planar membranes  in the presence of certain antibiotics"

1972  Singer and Nicolson propose the fluid-mosaic model of cell membranes replacing the model of Danielli and Davson from 1935; the new model explicitly postulates integral membrane proteins 

Monolayer derived planar bilayers making synthetic membranes virtually solvent free (Montal and Mueller) to study intrinsic properties of ion channels; the importance of this technical advance lies in the fact that it can be used to mimic the lipid composition and distribution of cell membranes which affects the activity of proteins        ... ... ... read more

1975  Glycophorin, first integral membrane protein sequenced

First electron microscopy derived structure of a membrane protein, bacteriorhodopsin at 7 Angstrom resolution (Henderson and Unwin); structure shows seven transmembrane alpha helices supporting the idea that alpha helices but not beta sheets are the secondary structure of choice for membrane proteins; this idea strongly influences research on structure-function relationship of membrane protein; beta barrel models for bacterial porin and beta helix structure of Gramicidin are dismissed as exceptions confirming the rule
1976 Patch-clamp technique developed by Neher and Sakmann; this modification of the voltage clamp setup developed in the 1940s allows for the first time the measurement of single channel activity in living cells instead of the usual macroscopic currents; the existence of separate ion channels selective for either Na, K or Ca ions can be demonstrated at the single channel level
1979  Vesicle supported planar bilayers making synthetic membranes completely solvent free (H. Schindler); initially used to study purified nicotinic acetylcholine receptor  and bacterial porins
1980  First membrane protein crystal of bacterial porin diffracts to high resolution, phase problem, and thus structure, not solved  (Garavito and Rosenbusch); electron microscopy and circular dichroism have by now firmly established the exclusive beta sheet composition of this pore protein.  The so called beta barrel structure is proposed and later found to be a widely used structural feature in many proteins
1985  First X-ray high resolution structure of a membrane protein, bacterial reaction center (M.Diesenhofer, R. Huber, H. Michel) shows alpha helical transmembrane segments
1990 High resolution crystal structure of bacterial porin confirms beta barrel motif for a functional membrane protein channel
1995 Under the guidance of J.C. Venter The Institute for Genomics Research TIGR publishes the first full genome sequence of Mycoplasma genitalium, a member of the archaea branch of life. This landmark publication establishes Genomics as a new branch of biology changing the way molecular biologists study the genetics and evolution of organisms.
1999, 2000 High resolution structure of Kcsa, a K+ selective bacterial ion channel solved (R. MacKinnon); shows the involvement of protein backbone structures in the selectivity process rather than the expected amino acid residues. MacKinnon et al.'s work is the start of a very fruitful period in solving the structures of membrane proteins including the high resolution structure of a water selective aquaporin in 2000 (P. Agre). These channel structures show the mechanisms of high flux rate and selectivity of small polar and charged molecules across cell membranes.
2001 The first draft of the human genome is published by competing public and private efforts. Genome projects of dozens of organisms show that membrane proteins make up to 30% of the protein set of organisms, from bacteria to man.

(For an account of the history of biological membrane transport studies see Joseph D. Robinson's book 'Moving Questions'. For references regarding electrophysiology and ion channels, see Bertil Hille's 'Ionic Channels in Excitable Membranes')

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Copyright © 2001-2009 Lukas K. Buehler