Protein Interactions

stanyon at stanyon at
Wed Sep 11 01:47:02 EST 1996

A question for Protein Chemists.

With what sort of frequency might a protein oscillate or vibrate between two or
more shapes, when one of these shapes usually requires some post-translational
modification such as phosphorylation to have occurred before it becomes the
dominant form?

As background to this question, I should add that I am studying protein-protein
interactions in the Yeast Two-hybrid system. A recurrent observation is that
interactions - as assessed by transcription of a gene which confers prototrophy
upon the yeast - do not give an all-or-nothing readout. Although the DNA
required to produce the proteins may be present in the yeast, the frequncy at
which this causes the observed phenotype ranges from 50-60% in the case of
strongly interacting proteins such as fos & jun, down to less than 1% in the
case of "weakly" interacting proteins. This correlates with the activity of
b-galactosidase in some reports. In effect, the degree of penetrance of the
observable phenotype appears to vary considerably.
The other interesting observation is that even though the frequency may differ
considerably, the size of colonies does not appear to be affected. Thus, fos-jun
pairings will give colonies of similar size to "weaker" pairings, though the
numbers of colonies may be 10-fold apart when compared to the number carrying
the necessary DNA.

I am attempting to find out whether there are any recommendable methods for
studying the stability of protein complexes with a view to establishing whether
or not complexes between non-post-translationally modified proteins are stable,
even though the complex formation may be quite infrequent. Essentially, a
quantitative measure would be necessary, in terms of both frequency and

        Feel free to mail me directly, and thank-you in advance.

Clem Stanyon

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