IF THE cell is a city, how do proteins pass from the suburb where they鈥檙e
made to the centre or out to the world beyond? This question intrigues
G眉nter Blobel of Rockefeller University in New York. His answers have
already won him a Nobel prize, but a full picture is still emerging.
Blobel received his Nobel last year for work done in the 1970s on how newly
made proteins pass through the membrane of the tubular network called the
endoplasmic reticulum (ER). Since then, biologists have found that the
principles he described hold for protein transfer across other cellular
membranes.
For the past 15 years, Blobel鈥檚 team has focused on the transfer of proteins
from the cell cytoplasm into the nucleus. In this process, a migrating protein
is recognised by a second protein, called a targeting factor, which escorts it
to a specialised membrane channel. This channel is in fact a huge complex of
other proteins, which ferries the migrant through the membrane.
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The movement of proteins and other molecules, such as RNAs, across the
nuclear membrane is crucial to almost all aspects of cell function, and
particularly to the expression of genes. Understanding how it is controlled will
help researchers working on cloning, AIDS, cancer and 鈥渁ll those things the
newspapers talk about鈥�, Blobel says.
In the cytoplasm, many proteins and RNA molecules are moved around on
filaments that make up the cytoskeleton. No one yet knows if a similar skeleton
exists within the nucleus. But recently, Blobel鈥檚 group identified a protein
that forms filaments extending from the nuclear membrane鈥檚 channels into the
nucleus. Hans Ris at the University of Wisconsin, Madison, has found that it
forms tubes, while Volker Cordes of the Karolinska Institute in Sweden has
discovered that the protein鈥檚 filaments reach far into the nucleus.
What these tracks or tubes do is anyone鈥檚 guess, but Blobel bets they play
some part in protein and RNA transport. 鈥淭he nucleus is like Manhattan,鈥� says
Blobel. 鈥淚t鈥檚 very crowded and the traffic needs to go smoothly and fast.鈥� If
the tracks are involved in transport, they could open up a whole new area of
cell biology, which Blobel has already named鈥攊ntranuclear transport.