Divergent Members of the SKY Family of Transcriptional Regulators Bind an 
Insulin-Responsive Element, IRE-A 
is contained in several T cell-specific genes that 
are bound with high affinity by TCF-la (T cell 
factor la). Thus diverse members of the HMG 
family of proteins may modulate transcription 
through a similar spectrum of sequences that 
contain a core motif. Identification of such a mo- 
tif may provide a clue to the identity of important 
physiological targets of the IRE-ABP and SRY-like 
family of transcriptional regulators. 
The laboratories of Peter Goodfellow and Ro- 
bin Lovell-Badge have identified 5/?Fas the testis- 
determining region on the basis of genetic evi- 
dence. Several patients with an XY genotype who 
failed to differentiate to the male phenotype have 
mutations in the HMG box domain of SRY. The 
SRY locus is widely presumed to encode a se- 
quence-specific DNA-binding protein because it 
contains an HMG domain within its open reading 
frame. The DNA-binding properties and mecha- 
nism by which this protein regulates transcrip- 
tion have not been defined. Certain of the muta- 
tions that were found in the sex-reversed patients 
were not de novo mutations, and thus it was not 
possible to deduce whether these mutations were 
completely unrelated to the sex-reversed pheno- 
type or contributed to it. Because IRE-ABP was 
isolated on the basis of its ability to bind the IRE-A 
motif, it was possible to examine the effect of 
these mutations on the binding affinity of IRE- 
ABP and SRY derivatives. Derivatives that con- 
tained the mutations associated with sex reversal 
at positions 3 and 7 in the HMG box domain 
showed marked impairment in their ability to 
protect the IRE-A motif from DNase I digestion. 
Furthermore, derivatives of IRE-ABP and 5.^y that 
contain a switch between IRE-ABP and SRYat po- 
sition 3 in the HMG box domain show altered 
binding affinity for the IRE-A motif. Thus posi- 
tions 3 and 7 appear to be important determi- 
nants of binding affinity for this family. 
Future Directions 
Identification of the IRE-A binding protein as a 
member of the 5J?Ffamily suggests many avenues 
of investigation. Studies are under way to define 
the binding specificity of IRE-ABP- and SRY-like 
family members. Definition of the preferred bind- 
ing site for these related proteins will facilitate 
the identification of other insulin-sensitive genes 
that are regulated by IRE-ABP and potential tar- 
gets of SRY. For example, insulin simultaneously 
activates and inhibits diverse metabolic pro- 
cesses to alter the flux of metabolites into glyco- 
gen and fat. We have located the proposed con- 
sensus sequence in the upstream region of genes 
that are regulated in a positive and negative direc- 
tion by insulin, and we can now establish 
whether IRE-ABP plays a role in regulating these 
diverse metabolic processes. 
The observation that the SRY protein and 
IRE-ABP share binding specificity for a se- 
quence located in a glycolytic gene implies that 
these proteins carry out similar functions in the 
specific tissues in which they are expressed. It 
is clear, for instance, that spermatogenesis in 
the adult testis requires high lactate produc- 
tion; thus both gene products may be involved 
in regulating glycolysis/gluconeogenesis in 
their respective target tissues. Conversely, IRE- 
ABP-like genes may play a role in regulating 
processes that show sexual dimorphism in 
adult tissues or promoting differentiation of its 
target tissues during embryogenesis. 
Studies on the regulation of IRP-A gene expres- 
sion have led to an understanding of the mecha- 
nism by which insulin modulates the expression 
of specific genes in specific tissues involved in 
the maintenance of normal glucose and lipid me- 
tabolism. Ultimately we expect these studies will 
lead to an understanding of the signal transduc- 
tion process by which insulin modulates the ex- 
pression of these genes. Understanding the hor- 
monal control of lipid metabolism at a molecular 
level will provide insights into two disease states 
of major importance, obesity and diabetes. 
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