Structural Studies of Proteins Involved in Hormonal Signal Transdtwtion 
two receptors, though different in amino acid se- 
quence, are able to recognize the same cytokines. 
We hope also to learn whether receptor-cytokine 
engagement induces significant changes in the 
receptor structure or the interactions among re- 
ceptors that might mediate their internalization 
by cells. 
Fibroblast Growth Factor 
Basic fibroblast growth factor (bPGF) , one of a 
group of seven structurally related proteins, trig- 
gers the induction of mesoderm formation in 
early embryogenesis. In adult animals, the same 
factor initiates processes of wound repair and vas- 
cular regeneration. It is present in the extracellu- 
lar matrix, bound to heparin-containing proteo- 
glycans, where it stimulates the movements of 
cells to sites of tissue generation or repair. The 
intracellular domain of the receptor for bFGF is a 
tyrosine kinase, an enzyme that initiates a cascade 
of intracellular reactions culminating in altered 
patterns of gene expression within the cell. 
The three-dimensional structure of bFGF has 
been determined with high resolution by gradu- 
ate student Jiandong Zhang, using protein sam- 
ples provided by Lawrence Cousens of Chiron, 
Inc. We found that the molecule is structurally 
related to members of the interleukin- 1 family 
that stimulate the growth of white blood cells. 
The structure reveals probable sites for receptor 
and heparin binding at separate but neighboring 
locations on the molecule's surface and also sug- 
gests how phosphorylation of bFGF might alter its 
affinity for the receptor. 
We are collaborating with Phillip Barr of 
Chiron, Inc., and Lewis Williams (HHMI, Univer- 
sity of California, San Francisco) to cocrystallize 
bFGF with its receptor's extracellular domain. At 
least five distinct receptors of bFGF have been 
identified, all of which bear extracellular do- 
mains related to the immunoglobulin family. We 
may find that the complexes between FGF and 
receptor dimers (which form when receptors in- 
teract with the factor) are structurally similar to 
those between antibodies and antigens. 
G Proteins 
The signals generated by the engagement of ex- 
tracellular messenger molecules with their re- 
ceptors are, in many cases, transmitted across the 
plasma membrane to members of the G protein 
family. For example, when epinephrine binds to 
the i8-adrenergic receptor, an intracellular pro- 
tein called Gsa is induced, to bind the nucleotide 
GTP and discard a pair of regulatory subunits. 
The GTP-bound Gsa is then an activator of mem- 
brane-bound enzyme that catalyzes the synthesis 
of the intracellular second messenger cyclic 
AMP. Gsa eventually hydrolyzes the GTP, allow- 
ing it to rebind its inhibitory subunits and return 
to an inactive state in which it can no longer stim- 
ulate cyclase. 
We are undertaking crystallographic studies of 
Gsa to learn how this protein may interact with 
other components of the signal transduction sys- 
tem in which it operates. The initial steps toward 
a structural determination of Gsa have now been 
taken in our laboratory by research associate 
David Coleman, who has crystallized Gsa and a 
nonhydrolyzable GTP analogue in complex with 
a protein expressed in the laboratory of Alfred 
Gilman (University of Texas Southwestern Medi- 
cal Center at Dallas) . Our goal is to understand 
the series of GTP-dependent structural changes 
within Gsa that account for its regulatory 
properties. 
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