Dr. Hurley is also using molecular, immunological, 
and genetic methods to study G proteins in Dro- 
sophila. Several unique G proteins have been iden- 
tified, and transformation experiments will be used 
to determine how their properties determine devel- 
opment and physiological responses of a whole or- 
ganism. 
The laboratory of Associate Investigator David P. 
Corey, Ph.D. (Massachusetts General Hospital) has 
made further progress in understanding the molec- 
ular mechanism of sensory perception by the 
receptor cells of the inner ear. The generally ac- 
cepted hypothesis for the transduction mechanism 
is that there are ion channels in the tips of me- 
chanically sensitive cilia that are directly activated 
by tension in fine filamentous links between the 
cilia. An adjustment or adaptation process is 
thought to involve some molecular motor that con- 
tinually adjusts the tension in the filaments. They 
have found that the adjustment rate is constant 
when the filaments are thought to be slack, con- 
sistent with the hypothesis. They have also found 
that the voltage changes that are thought to alter 
the tension cause submicroscopic movements of 
the cilia with the appropriate time course. Bio- 
chemical experiments are under way to separate 
the protein constituents of the cilia, to identify 
those that might be involved in the transduction 
and motility processes. 
Recent studies by Investigator Ronald M. Evans, 
Ph.D. (The Salk Institute for Biological Studies) and 
his colleagues have led to significant advances in 
our understanding of how steroid, retinoid, and 
thyroid hormones act to exert their regulatory ef- 
fects on development and physiologic homeostasis. 
Although these hormones are structurally and bio- 
synthetically distinct, cloning studies have shown 
that their receptors are members of a superfamily 
of regulatory genes that act as transcription factors. 
As such these receptors are of great interest for 
studying the mechanisms of transcriptional control. 
The various receptors display considerable specific- 
ity and selectivity in the genetic programs that they 
ultimately influence. This modulation of gene ex- 
pression leads to profound changes in protein syn- 
thesis within cells and consequent changes in cell 
function. It is the combination of these final 
changes that ultimately determines the physiologic 
effect of the relevant hormones. The study of these 
molecules can be used to provide a rather direct 
link between molecular events in the genome and 
physiological events in the organism. 
468 
