ing the identity elements of the trp operator shows 
the same characteristic pattern of deviations from 
canonical B-DNA as seen in the complex. 
The steroid nuclear receptors. Lipophilic effec- 
tors, such as steroid hormones, thyroid hormone, 
vitamin D, and retinoic acid, regulate the expression 
of genes through the action of their receptors. These 
receptors are transcription factors that are targeted 
as dimers to their respective hormone-responsive 
genes through small, conserved DNA-binding do- 
mains (DBDs). The identity of the DNA target re- 
sides in two features: 1) the sequence of two 6-bp 
half-sites and 2) their arrangement — that is, the 
number of base pairs that separate the half-sites and 
whether the half-sites are arranged as inverted or 
direct repeats. 
The structure of the cocrystalline complex 
formed by the glucocorticoid receptor DNA-binding 
domain (GR-DBD) and its target showed that the 
DBD has two zinc finger-like modules that are 
folded together, one recognizing the half-site and 
the other providing a strong dimer interface that 
places the recognition surface of each subunit in 
register with the opposing half-site. The firm dimer 
interface required to arrange the recognition sur- 
faces is supported by DNA interactions. Thus DNA 
supports (if not induces) conformations that are re- 
sponsible for protein-protein interactions that en- 
able the receptors to recognize their cognate re- 
sponse elements. 
The inferences drawn from the glucocorticoid re- 
ceptor complex have been confirmed by a crystallo- 
graphic study of a mutagenically derived analogue 
of the thyroid receptor's DNA-binding domain (TR'- 
DBD) in a complex where the target's half-sites are 
separated by 0 bp rather than 3 bp. Changes in the 
dimer interface no longer require the recognition 
surfaces of the protein to be separated by 3 bp, and 
the subunits are rearranged to interact with abutting 
half-sites. 
The E2-enhancer complex from bovine papillo- 
mavirus! (BPV-1). The papillomaviruses are a 
family of small DNA viruses that cause hyperplastic 
epithelial lesions. The most notable pathogens are 
the human papillomavirus strains HPV-16 and -18, 
which have been implicated in cervical carcinomas. 
The products of the E2 gene are pivotal, as they regu- 
late viral transcription from all viral promoters and 
are essential for viral replication. All forms of the E2 
protein are dimeric, and their function requires pref- 
erential binding to a highly conserved palindromic 
target sequence, termed the E2-BS, which occurs 17 
times in the viral genome. 
Specific DNA-binding and dimerization is me- 
diated through a carboxyl-terminal, 85-amino acid 
domain that does not include a sequence motif pre- 
viously associated with DNA binding. The crystal 
structure of the E2 DNA-binding/dimerization do- 
main (E2-DBD) of BPV-1, bound to a l6-bp ideal- 
ized E2-binding site, shows a heretofore unde- 
scribed dimeric, eight-stranded antiparallel jS barrel 
made up of four strands from each subunit. A pair of 
a helices symmetrically disposed on the outer cir- 
cumference of the barrel contain all of the amino 
acids directly involved in base-sequence recogni- 
tion. The DNA is severely but smoothly bent around 
the barrel to enable successive major grooves to en- 
gulf these recognition helices. As a result of the very 
high resolution of the study ( 1 .7 A) , the stereochem- 
istry of the protein-DNA interface is seen in unprece- 
dented detail. The highly conserved recognition 
surface of the protein and the identity elements of 
its target form a highly interdependent network of 
specific interactions. 
The arg repressor- operator. Arginine co-represses 
the transcription of arginine biosynthetic genes. The 
regulatory system has an unusual architecture. The 
protein is a robust hexamer that binds a tandem of 
1 8-bp arg boxes separated by 3 bp. The arg repres- 
sor also appears to serve as a binding scaffold for a 
site-specific recombination system. Crystals of the 
arg repressor, both alone and in complexes with 
arg boxes, were grown, and data have been col- 
lected to 2.5 A. 
Transmembrane Signaling 
Receptor-G protein interaction. The retinal 
rod provides a well-characterized example of a 
seven-helical receptor (rhodopsin)-G protein 
(transducin) -target enzyme (phosphodiesterase) 
transmembrane-signaling system. Pure bovine rho- 
dopsin, transducin, and arrestin (and proteolytically 
trimmed fragments thereof) have been prepared in 
tens of milligram amounts in collaboration with Dr. 
Heidi Hamm (University of Illinois, Chicago). 
A structure determination is currently under way 
with crystals of a stably activated transducin com- 
plex (G,„-GTP7S) that diff"ract usefully to 2.0 A. 
The laboratory's objective is to cocrystallize Gt„ 
with photoactivated rhodopsin to see directly the 
molecular mechanism by which the receptor trans- 
duces its photoactivation to G,„ • GTP. The target of 
the activated G,„ • GTP is an oligomeric cGMP phos- 
phodiesterase (PD). The activated transducin acti- 
vates PD by binding the inhibitory subunit, PDy. 
These studies should indicate the structural basis for 
a G protein's capacity to transduce the activation 
signal from the receptor to the target enzyme. 
Dr. Sigler is also Professor of Molecular Biophys- 
ics and Biochemistry at Yale University. 
486 
