Donald Johnston wrote:
>> My reading of EGF-R scatchard analysis vis a vis mutant receptors is that
> dimerization is not a factor in defining high and low affinity binding
> sites. On one level it would seem to be simple allostery - binding of one
> egf would alter the conformation of the second receptor in the dimer,
> increasing the affinity of the second for egf binding.
I did not mean to imply that receptor dimerization is the only
alternative explanation of the non-linear Scatchard plots, and I agree
that there are other views of what is going on in this particular
system. Allostery is a good (and even likely) explanation of this
phenomenon, but again an allosteric mechanism gives what appears to be
two "classes" of binding sites even though there is no physical/chemical
difference between different receptor molecules.
The main thing I was trying to point out is that a Scatchard analysis is
really only valid for a simple 1:1 binding mechanism without allostery,
receptor dimerization, etc. One of the confusing things about
interpreting the data for the EGF receptor system is that different
groups have fundamentally different views about the stoichiometry. Some
groups believe that a single EGF binds to two receptors, while others
argue for 2:2 complexes. I believe a recent excellent paper by M. A.
Lemmon et al. (EMBO J. 16, 281, 1997) firmly establishes the 2:2
stoichiometry, but these results still allow for two fundamentally
different mechanisms for receptor dimerization.
>> So, does receptor dimerization occur under the conditions of Scatchard
> analysis?
>
Yes, I believe it nearly always does.
> As I read your interpretation, a one to one correspondance would be
> expected if the receptors did not dimerize. Is this correct?
I assume you mean a one-to-one correspondence between the affinity as
determined from the slope of the Scatchard plot and a true thermodynamic
binding constant for EGF binding to a single receptor. If so, the
answer is YES if, and only if, it is truly a simple 1:1 binding event
(no allostery, no simultaneous contact of EGF with 2 receptors, etc.),
otherwise the answer is generally NO.
I hope this clarifies the points I was trying to make. This discussion
is, I suspect, getting too specific and narrow to be of general interest
to the group, so if you want to continue it I suggest we correspond
directly.
John Philo, Amgen
*** Disclaimer: These are the opinions of the poster not Amgen Inc.***
