called the nucleoporins, that were first identified in 
mammalian cells. In vitro assays performed by other 
laboratories suggest that the nucleoporins play an 
essential role in mediating protein (and perhaps 
RNA) transport, but their exact function remains 
unknown. 
Dr. Davis's goal is to combine genetics and bio- 
chemistry to elucidate the function of each of the 
nucleoporins and to identify other components of 
the transport apparatus. To accomplish this, she 
uses the budding yeast Saccharomyces cerevisiae as 
an experimental system, because it is amenable to 
both genetic and biochemical analysis. Dr. Davis 
previously identified several yeast homologues of 
the mammalian nucleoporins and cloned the gene 
encoding one of them {NUPl). 
Random mutagenesis has been used to iden- 
tify mutations in NUPl that confer temperature- 
sensitive growth. Immunofluorescence assays were 
employed to examine the efficiency of protein im- 
port as well as mRNA export in one such mutant. 
This mutant failed to import an inducible nuclear 
protein upon shifting to the nonpermissive tempera- 
ture. The mutant also accumulated polyadenylated 
RNA in the nucleus. These results suggest that NUPl 
is required for both protein import and RNA export. 
Thus the elucidation of cellular proteins that func- 
tionally interact with NUPl should provide valuable 
insights into the mechanisms that govern these two 
processes. 
While most yeast strains require NUPl for viabil- 
ity, a strain that carries a naturally occurring sup- 
pressor that bypasses this requirement has recently 
been identified. Deletion of NUPl in this strain 
background causes cells to grow more slowly than 
normal, suggesting that the bypass is not complete. 
By analogy to other systems, mutations in genes en- 
coding proteins that functionally interact with 
NUPl might be expected to exacerbate the eff'ect of 
nupl deletion and prevent growth. These "syn- 
thetic lethals" can be identified by screening for 
mutants that can no longer grow in the absence of 
NUPl. An initial screen has identified several such 
mutants, which remain to be characterized. The 
same question can be addressed directly by crossing 
nupl deletion strains to known mutants suspected 
of functioning in the same pathway. The RNAl 
gene, characterized by Dr. Anita Hopper and her col- 
leagues, encodes a cytosolic protein apparently re- 
quired for export of RNA. Dr. Davis's group has 
found that ma 1-1 exhibits synthetic lethality with 
the suppressed nupl deletion. This observation 
suggests that these two proteins interact function- 
ally and lends credence to the idea that the genetic 
screen will identify functionally relevant proteins. 
NUP 1 p presumably functions through physical as- 
sociation with other proteins. These may be other 
pore complex constituents or nuclear or cytoplas- 
mic proteins that interact transiently with the pore 
complex. Overexpression of NUPl p, which causes 
cytosolic accumulation of the overexpressed pro- 
tein, is lethal to cells. One explanation for this is 
that cytoplasmic NUPlp binds to and conscripts a 
protein or proteins with which it would normally 
interact in the nuclear pore complex. The region of 
NUPlp that mediates this phenotype has been local- 
ized to the amino-tcrminal domain. This domain 
also correlates with the ability of NUPl to function, 
because deletions and point mutations within this 
region render the protein nonfunctional . A biochem- 
ical approach has been used to identify and purify 
two nuclear proteins that bind to this domain. To 
test the relevance of this interaction, nonfunctional 
amino-terminal mutants are now being tested for 
their ability to bind these proteins. If a correlation 
between in vitro binding and in vivo function can 
be established, then efforts to clone the genes en- 
coding these proteins will immediately ensue. A 
complementary genetic approach has also been initi- 
ated. Several clones have been isolated that, when 
expressed in high copy, can rescue the lethality of 
NUPl overexpression. Characterization of these 
genes is currently under way. 
Dr. Davis is also Assistant Professor of Genetics 
and Cell Biology at Duke University Medical 
Center. 
Article 
Davis, L.I. 1992. Control of nucleocytoplasmic 
transport. Curr Opin Cell Biol 4:424-429. 
CELL BIOLOGY AND REGULATION 
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