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Nobel Prize in Physiology or
Medicine 1999
Utpal Tatu
The Nobel Prize in Physiology and Medicine for the year 1999 has been awarded to
Gunter Blobel for his discovery of signals
that direct proteins to their respective destinations in the cell. Gunter Blobel was born
in Germany in the year 1943 in a town called
Waltersdorf. He obtained his Ph.D. in Oncology with Van Potter in Germany in 1967.
He moved to USA for his postdoctoral work
at the Rockefeller University, New York
where he is continuing his interests in Cell
Biology at the Howard Hughes Medical Institute.
Why do we need Signals?
A eukaryotic cell has many intracellular compartments. These include the nucleus, the
endoplasmic reticulum (ER), the Golgi, the
lysosomes, the peroxisomes, and the mitochondria and in addition the choloroplast in
case of plant cells. The cell during its growth
makes
thouBlobel
sands of differen t proteins and
depending on
their individual
functions they
are located at different intracellular sites. For example proteins
Figure 1A.Schematic diagram of a eukaryotic
cell showing various intracellular organelles
in which proteins are trafficked and the route
ofprotein secretion outside (arrows). (Adapted
from Press Release Nobel Foundation.)
involved in DNA replication and gene expression are localised in the nucleus, proteins involved in energy generating, oxidative phosphorylation reactions are present in
the mitochondria, enzymes of the lipid oxidations are housed inside the peroxisomes
and proteins involved in glyco-sylation reaction localize inside the ER (Figure lA). Other
classes of proteins are secreted outside the
cell. How do these different proteins find
their respective destinations? These are the
kind of questions cell biologists were addressing when Gunter Blobel began his career in the laboratory of George Palade at the
Rockefeller University in USA.
Two major advances in cell biology in late
60's paved the way to Gunter Blobel's award
winning work on signals for protein secretion. These were 1) use of electron micros-
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copy to view ultrastructural details of a cell
and 2) fractionation methods to separate intracellular organelles. Using these approaches
George Palade and coworkers followed the
route to protein secretion outside the cell.
Palade's work highlighted the central role
played by endoplasmic reticulum in secretion of proteins. Together with Albert Claude
and Christian de Duve, Palade received the
Nobel Prize in 1974 for this contribution.
Cell biology was no longer regarded as the
science of observing cell shape, size and structure; it acquired a molecular perspective and
began to be called molecular cell biology.
The Signal Hypothesis: Zipcodes for Protein Delivery
Blobel's early experiments involved fractionating ribosomes from pancreatic acinar cells
into soluble and membrane bound pools. In
association with Dobberstein he started addressing how newly synthesized secretory
proteins enter the ER to be secreted out of
the cell. They formed a hypothesis to explain
how, from a pool of thousands of different
proteins in a cell, only a select group is secreted out. This was called the signal hypothesis. The hypothesis stated that secretory proteins must have a signal within their
amino acid sequence that identifies them for
secretion (Figure IB). Initially the hypothesis was received with skepticism. It was
looked upon as an over simplification of a
complex problem. But soon Blobel came up
with evidence to show that proteins like
preprolactin carried a sequence of amino acids that targeted them to the ER mediated
secretion pathway. Later other proteins were
also shown to have similar signals for secretion. The signal was called the signal peptide.
About the same time when Blobel was trying
to put the puzzle of protein secretion together, Ceser Milstein's group at the Medical
Research Council Laboratory in Cambridge,
UK came up with an important experiment
that lent credence to the signal hypothesis.
Milstein demonstrated that the secreted form
of immunoglobulins were somewhat smaller
in size than their intracellular forms. The
experiment suggested that during the secretion of IgG a part of its amino acid sequence
was excised. This finding helped Blobel put
the secretion puzzle together. He proposed
that the signal peptide was cleaved once the
protein was targeted to the ER for secretion.
Later the cleavage was shown to occur in the
ER and the enzyme activity responsible for
this cleavage was also identified. The scheme
of events in the process of targeting secretory
proteins to the ER for secretion was depicted
as shown in Figure 2. During Blobel's search
for the components responsible for targeting
newly formed proteins to the ER he identified signal recognition particle (SRP) which
bound to the signal peptide and targeted the
protein to the ER. The protein was shown to
be unique in being a complex of RNA and
protein (and hence called a particle) but its
presence was conserved from bacteria to man.
The work emphasized that the basic mechanisms of secretion were conserved across species barrier.
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secretion
Figure 1B. Representation of the signal hypothesis. Proteins trafficking to different organelles are shown in brown and the signal
sequences are indicated in red. (Adapted from
Press Release Nobel Foundation.)
Protein Conducting Channel
Having established the basic scheme for targeting of proteins to the ER, Blo bel turned
his attention to the problem of how a polypeptide chain is transferred across the ER membrane. It was a challenging task to explain
the movement of a hydrophilic polypeptide
chain through the hydrophobic lipid bilayer.
Once again Blobel proposed a bold but simple
hypothesis. He suggested that proteins were
translocated across the ER membrane
through a proteinaceous channel, which he
called the protein-conducting channel. The
scientific community once again expressed
reservations on accepting this seemingly
simple explanation for a difficult problem.
But together with Sandy Simon in 1991, using patch clamp methods to measure voltage
changes across membranes, Blobel provided
the first evidence for the presence of a protein-conducting channel. The biochemical
and genetic evidence from yeast later provided direct evidence for the presence of
such a channel and its role in translocating
proteins in the ER lumen.
Blobel's contributions were not restricted
only to secretory proteins but his work also
revealed how different proteins are trafficked
to different intracellular sites. The signal
hypothesis was shown to be applicable for
transport to other intracellular organelles also.
Depending on the destination of the protein,
the kind of signal varied. Thus the signal
peptide for secretory proteins was somewhat
mANA
/
Figure 2. A model showing early
events in the entry of secretory
proteins in the endoplasmic
retiCUlum. The protein-conducting channel discovered by
Blobel is labeled as membrane
• .datl"'...
channel. (Adapted from Press
Release Nobel Foundation.)
IIIIcuUft ,...."..".
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different from that of a nuclear protein or
that of a mitochondrial or a chloroplast protein. In collaboration with other scientists
Blobel probed into the components involved
in targeting of proteins to the nucleus, the
mitochondria, the chloroplast and also the
peroxisomes.
is compatible with bacterial secretion system
is commonly fused to the protein of interest
to ensure its secretion outside the cell. The
trick minimises the chances of intracellular
aggregation or inclusion of body formation
due to overexpression of the protein and
allows its easy purification.
Blobel's Associates
At the clinical front, Blobel's discovery of
signal sequences helped uncover the molecular bases of many inherited genetic disorders
in humans. The best example is that of
primary hyperoxaluria leading to kidney
stone formation. It was shown that a common cause of this disorder is mistargeting of
an aminotransferase from its normal peroxisomal location to the mitochondria due to a
mutation in its signal sequence. Furthermore a number of abnormalities have come
to light that are caused by mutations in the
signal sequences of different proteins. The
examples include hemophilia due to factor X
deficiency, familial hyperlipoproteinemia
caused by a mutation in the signal of
apolipoprotein B and an abnormal form of
albumin in the serum due to the mutation in
its signal sequence.
Blobel's impact on protein transport is not
limited to his own direct contributions but
by inspiring colleagues, postdocs and students associated with him, he had a widespread influence on the field of molecular
cell biology. Many of his postdocs later established as independent scientists and made
many significant contributions. Notable
among scientists inspired by Blobel were
Bernie Dobberstein who was associated with
Blobel in the discovery of SRP, Peter Blobel
- who studied protein secretion in yeast and
also Vishi Lingappa whose work focussed on
the role of stop transfer sequences in membrane protein insertion.
Medical and Biotechnological Ramification
In addition to its intellectual and academic
relevance, Blobel's work on signal sequence
also found commercial recognition. Biotech
companies involved in expressing proteins
of commercial interest in heterologous systems often add on appropriate signal sequence
on the protein of interest to direct it to the
desired subcellular or extracellular sites.
Similarly when eukaryotic proteins are overexpressed in bacteria, a signal sequence that
Finer Details of Blobers Discovery
Lastly, it is necessary to emphasize that the
finer details of Gunter Blobel's work on signal sequence are probably yet to be fully
appreciated. There is a growing realization
that in addition to determining the localization of proteins., signal sequences may also
play a role in aiding protein folding. One
clue in support of this notion comes from the
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observation that signal sequences of secretory proteins are not always exchangeable,
suggesting that at least for some proteins the
signal may carry information necessary for
its correct folding. Only time will tell if this
new dimension to the role of signal sequence
in protein biogenesis can be established. But
for now we must agree that Blobel's findings
are undisputedly deserving of the highest
recognition that the Nobel committee has
bestowed.
Bohrium - A New Element
in the Periodic Table
ber of trans-uranium elements by similar
experiments.
Srinivasan Natarajan
Introduction
The periodic table of elements, the basic and
most important component of research in
chemistry and physics is growing continuously. It is interesting to note that until the
16th century, only a handful of elements
have been known to mankind (10, to be precise) and during the 18th century, 10 more
elements have been identified. It is only in
the 19th century that most of the elements
have been discovered and a form for the
periodic arrangement of the elements has
been proposed (Mendeleyev's periodic table).
The modern periodic table as presented in
Figure 1 was arranged by Moseley in 1914. In
1934, Enrico Fermi proposed that newer elements could be made by bombarding the
atomic nucleus of an element by particles
such as neutrons. Thus, the first man-made
element, technetium (Tc), was discovered in
1936 by bombarding Mo by deuterons. This
was followed by the discovery of a large num-
Utpal Tatu, Department of Biochemistry, Indian
Institute of Science, Bangalore 560 012, India, Tel:
91.80.3092823, Email: [email protected]
The chemistry of the heavy elements (transuranium) requires separations that come to
equilibrium very rapidly, and these must be
valid on an atom-by-atom basis. Such atoms
are created in the laboratory by bombarding
heavy target nuclei with an accelerated beam
of projectile ions. The nuclei of interest,
which are created by the evaporation of few
nucleons are only a very small fraction of the
large number of reaction products produced.
The above process is illustrated in Figure 2.
In such a fusion-evaporation experiment, two
heavy nuclei collide at energies just above
the Coulomb barrier (the energy required to
overcome the electrostatic repulsion between
the two nuclei) forming a fused nucleus. In
Figure 2, a typical example of a reaction of
40Ca incident on 92Mo target nucleus is presented. When such a reaction occurs, the
first step is the nuclei fuse together to form a
compound nucleus with mass 132 which includes 62 protons i.e., an isotope of Ce (Z =
62). Such a system is liable to fission into two
parts (Figure 2a) very rapidly (10-22 s) but, if
it survives, it will now exist for quite a long
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