Basal expression of the glycoprotein hormone alpha-subunit gene in pituitar
y gonadotrophs is partially dependent on a gonadotroph specific element (GS
E) which binds the nuclear receptor, steroidogenic factor-1 (SF-1). We have
used surface plasmon resonance (SPR) to determine the association (k(ass))
, dissociation (k(diss)) and affinity (K-A) constants of SF-1 binding to im
mobilized oligonucleotides containing either the GSE consensus motif or a G
SE mutant with a 2 bp substitution in the GSE site (GSE(MUT)).
In vitro translated SF-1 protein bound the consensus GSE with a threefold i
ncrease in affinity constant (P<0.01) compared with the GSE(MUT). This was
due primarily to a significant increase (P<0.05) in the k(ass) for SF-1 to
the GSE and a slower k(diss) (P<0 05). The binding interaction was specific
and could be significantly inhibited (P<0.001) by eitheranti-SF-1 antibody
or excess non-biotinylated GSE. The addition of 14 bp wild-type flanking s
equences significantly reduced the affinity of SF-1 to both the GSE (P<0.05
) and the GSE(MUT) (P<0.01). This was due to a significant (P<0.01) decreas
e in k(ass) for the wild-type and mutant long oligonucleotides compared wit
h the short GSE. Nuclear extracts from alpha T3-1 gonadotroph cells also bo
und the GSE and GSE(MUT), giving k(diss) values which were two- to threefol
d slower than those obtained with in vitro translated SF-1.
Thus, SPR is a powerful technique for examining kinetic interaction between
SF-1 and its binding site, and is able to demonstrate the effects of mutat
ions and flanking sequences on that interaction.