The Nuclear Pore Complex 
Laura I. Davis, Ph.D. — Assistant Investigator 
Dr. Davis is also Assistant Professor in the Section of Genetics and the Department of Cell Biology at Duke 
University Medical Center. She received her undergraduate degree from the University of California, San 
Diego, and her Ph.D. degree from the Rockefeller University, where she studied with Giinter Blobel. 
Her postdoctoral work was done with Gerald Fink at the Whitehead Institute. 
ONE of the hallmarks of eukaryotic cells is the 
presence of membranous barriers that di- 
vide the cell into functional compartments called 
organelles. How these compartments are gener- 
ated and how traffic through them is controlled 
are matters of intense interest in cell biology. Our 
focus is on the nuclear envelope, which separates 
the genomic material from the rest of the cell. 
Both RNAs and proteins move across the enve- 
lope, between the nucleus and cytoplasm, and 
the populations found in the two compartments 
are very different. 
For example, newly synthesized mRNA must be 
extensively edited or processed before leaving 
the nucleus. The mechanism that restricts its ex- 
port until processing is complete remains un- 
clear. Similarly, only certain proteins are im- 
ported into the nucleus. These contain specific 
signals that must be recognized by some compo- 
nent of the import apparatus. Recently it has be- 
come clear that the cell can regulate the availabil- 
ity of these signals by changing the context in 
which they are found. For example, transcrip- 
tional activators that control the growth state of 
the cell by turning on gene expression can be 
held in an inactive form in the cytosol if their 
signals are hidden from the import apparatus. 
When the cell is stimulated to begin growth, the 
signals are uncovered, allowing the transcrip- 
tional activators to enter the nucleus and exert 
their function. Thus nucleocytoplasmic transport 
has an important regulatory role in controlling 
gene expression. 
It is thought that all nucleocytoplasmic trans- 
port proceeds through large proteinaceous chan- 
nels that perforate the nuclear envelope. Called 
nuclear pore complexes (NPCs), these structures 
are about 30 times as large as an ion channel and 
are probably composed of over 200 different 
polypeptides. Among the few known compo- 
nents of the NPC are members of a family of re- 
lated proteins called nucleoporins. These pro- 
teins have been localized to an iris-like structure 
in the middle of the NPC, called the central trans- 
porter, and are thought to play an essential role in 
mediating protein (and perhaps RNA) transport. 
They are required for binding of proteins to the 
NPC prior to nuclear import, and antibodies that 
bind to the nucleoporins can block the energy- 
dependent movement of proteins through the 
NPC. While it is possible that the nucleoporins 
themselves recognize the signals on proteins des- 
tined for the nucleus, most of the available evi- 
dence suggests that they are more likely to act as 
docking points for cytosolic receptors that actu- 
ally recognize the signals. 
Our goal has been to understand more about 
the function of the nucleoporins and to identify 
other components of the transport apparatus. We 
use the budding yeast Saccharomyces cerevi- 
siae, since it is amenable to both genetic and bio- 
chemical analysis. Using antibodies that recog- 
nize the mammalian nucleoporins, I previously 
identified homologues of these proteins in yeast 
and cloned the gene encoding one of them 
{NUPl). The genes encoding two others have 
now been isolated (NUP2 and NSPl) . The pro- 
tein encoded by NUPl is essential for viability, 
and we have isolated nupl mutants that confer 
growth at one temperature but not another. These 
conditional mutants have been used to assay the 
phenotype resulting from loss of NUPl function. 
Using immunofluorescence, we have found that 
protein import into the nucleus is severely inhib- 
ited at the nonpermissive temperature, providing 
in vivo evidence that NUPlp is required for pro- 
tein transport. 
Conditional mutants will also be useful for 
identifying proteins that functionally interact 
with NUPlp. To do this, we are using genetic 
screens to find mutations in new genes that either 
enhance or suppress the conditional phenotype 
of the nupl mutants. The assumption is that mu- 
tations in proteins that functionally interact with 
NUPlp will either exacerbate the defect in nupl 
mutants and lead to death at all temperatures, or 
will overcome the defect and allow grouth at the 
restrictive temperature. Using this approach, we 
hope to identify new NPC components, as well as 
cytoplasmic and nuclear proteins that may inter- 
act transiently with the nucleoporins in a func- 
tional manner. 
We have also taken a biochemical approach to 
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