From: Jo Butler <pjgb@mrc-lmb.cam.ac.uk>
  To  : RASMB Noticeboard <rasmb@alpha.bbri.org>
  Date: Tue, 05 Jan 1999 16:29:49 +0000

RNA-protein complex

I agree with both John Philo and John Correia, and also think that I can
add something from my own experience with such complexes.

I had been asked to help with the stoichiometry of an RNA-protein complex.
>From SDS gels one knew the ratio of the (multiple) protein species, but
there was no good measure of the RNA/protein ratio in the complex(es), only
a range of possible estimates.
Since we did not know the molecular shape, we dismissed velocity as a
useful method and went straight for equilibrium centrifugation (where the
diffusion coefficient is not relevant).  Plots of Mw,app had the expected
increase against c, showing that complex was forming.  We therefore worked
out the possible vbars for all the reasonable complexes and replotted the
results for each.  Only one approached asymptotically towards the mass of
the complex whose vbar had been used and so we settled on this as the most
probable complex for the RNA-protein mix.  (Incidentally this was also the
most reasonable complex, increasing my confidence in the method!)

I therefore urge Art to go ahead, but to switch to equilibrium rather than
velocity.  You may not get a definitive answer, but even the indication can
be useful.

Jo

Dr P.J.G. Butler,
MRC Laboratory of Molecular Biology,
Hills Road,
Cambridge, CB2 2QH,
UK.
Tel. 01223 248011 (or 01223 402296 DDI)
FAX. 01223 213556