The Traffic of Molecules Into and Out of the Nucleus
Posted February 1, 1998
Think about the cars that move into and out of every large city in the United States every day. Stop signs and traffic lights regulate the flow of traffic. The flow
of molecular traffic around a eukaryotic cell's nucleus must also be regulated. Several thousand gateways allow proteins and nucleic acids to move into and out of the nucleus. These
nuclear gateways are called nuclear pores. Hundreds of molecules per minute move through these pores. The cell's existence depends on the regulation of this heavy molecular traffic.
The nucleus contains genes with code for building cell proteins. But cell proteins are manufactured on the ribosomes, the cytoplasmic protein factories. Proteins
made in the cytoplasm must enter the nucleus to stimulate and regulate ribonucleic acid synthesis. In contrast, ribonucleic acid, made in the nucleus, must enter the cytoplasm in order
for proteins to be made. Both the proteins moving into the nucleus and the ribonucleic acids moving out of the nucleus must go through the nuclear pores.
What regulates this massive movement of molecules through the nuclear pores? Twenty years ago scientists discovered that some proteins carry a nuclear localization
signal, a short sequence of amino acids that serves as a passport to get into the nucleus. Other proteins, such as transportin and importin, discovered in the early 1990's, were found
to usher protein molecules with nuclear localization signals to and through the nuclear pores into the nucleus.
Although researchers discovered how molecules enter the nucleus, little was known about the export of molecules out of the nucleus. Recently, scientists found that
HIV (the virus that causes AIDS), contains a protein that makes HIV replication possible by quickly transporting viral RNA out of an infected cell's nucleus. This HIV protein has been
called Rev. Rev protein contains an RNA-binding site and a short sequence of amino acids now called a nuclear export signal. The nuclear export signal allows Rev to leave the nucleus
carrying an RNA. The discovery of this protein in the HIV virus has helped scientists recently find a protein called Crm1p. Crm1p has been shown to bind molecules with a nuclear export
signal and move them out through the nuclear pore.
References
Travis, John. "Outbound Traffic - Scientists Identify Proteins That Move Stuff Out of the Nucleus." Science News, November 15, 1997, Vol. 152, No. 20,
pp. 316-317.
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