What Viruses Are Telling Us About Gene Regulation in Mammalian Cells 
the proverbial Trojan horse, VPl6 molecules are 
deposited in the newly infected cell as "ready- 
made" transcription factors. In an elegant series 
of experiments, Bernard Roizman and his col- 
leagues at the University of Chicago showed that 
the activating specificity of VP 16 for herpesvirus 
IE genes is dictated by regulatory DNA sequences 
located upstream of each IE gene. Such IE "en- 
hancers," when linked onto another gene other- 
wise unresponsive to VP 16, conferred a direct 
and specific response. 
On its own, VP 1 6 is incapable of direct associa- 
tion with the IE enhancers. In order to develop an 
understanding of the molecular mechanisms con- 
trolling transcriptional activation by VP 16, ef- 
forts have been undertaken to identify and purify 
cellular proteins that bind to IE enhancers. At 
least four different host cell proteins are required 
for maximal activation of IE gene expression by 
VPl 6. The most recent work in our laboratory has 
entailed the identification and purification of a 
DNA-binding complex that associates avidly and 
specifically with a critical switch point within IE 
enhancers. This particular switch point, termed 
in the jargon of the transcription field a "cis- 
regulatory element," is largely composed of gua- 
nine (G) and adenine (A) residues. The cellular 
DNA-binding protein that interacts specifically 
with the GA-rich cis-regulatory element has been 
termed GA-binding protein (GABP) . 
Kelly LaMarco, a postdoctoral fellow in our lab- 
oratory, purified GABP and found it to be com- 
posed of two polypeptide subunits. After deriv- 
ing the partial amino acid sequence from 
proteolyzed fragments of each subunit. Dr. La- 
Marco, along with postdoctoral associates Cath- 
erine Thompson and Tom Brown, succeeded in 
cloning the genes encoding each GAPB subunit. 
The amino acid sequence of one subunit, termed 
GABPa, exhibits significant similarity to the prod- 
uct of the ETS proto-oncogene. Indeed, the ETS- 
related region of GABPa represents the part of the 
protein that mediates direct contact with DNA. 
This same region of the protein is also neces- 
sary for protein-protein contact with the other 
subunit of the complex, termed GABP/?. Fortu- 
nately, the sequence of GABP^ also exhibits a re- 
gion of amino acid sequence similarity with previ- 
ously studied proteins. GABP/5 contains four 
imperfect repeats, 33 amino acids in length, that 
are related to similarly sized repeats present in 
the products of a number of interesting proteins. 
One such relative is the product of the Notch 
gene of fruit flies studied by Spyridon Artavanis- 
Tsakonas (HHMI, Yale University). The Notch 
gene product is a membrane protein that plays an 
important role in cell-cell communication dur- 
ing fruit fly development. Another protein that 
contains the 33 amino acid repeats is ankyrin, a 
cytoskeletal protein in red blood cells discovered 
by G. Vann Bennett (HHMI, Duke University Med- 
ical Center). Drs. Thompson and Brown found 
that the 33 amino acid repeats of GABP/? consti- 
tute the part of the protein required for direct 
interaction with its matching subunit (GABPa), 
thus providing the first conclusive evidence for 
the mechanist role of this protein structural 
motif. 
Many questions regarding the properties and 
function of this protein complex remain unre- 
solved. For example, how does binding of the 
complex facilitate activation of herpesvirus IE 
genes? What role does GABP play in the control of 
cellular gene expression? Might GABP in some 
way influence the decision of herpesvirus to exe- 
cute its lytic cycle (as it does in epithelial cells) 
or the latent state it enters when the virus infects 
neuronal cells? Given a critical set of molecular 
reagents, including recombinant DNA clones and 
specific antibodies, it should now be possible to 
address these questions directly. 
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