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Integral Membrane Proteins?

Cornelius Krasel krasel at alf.biochem.mpg.de
Sat Feb 11 07:42:37 EST 1995

[This has been crossposted to bionet.molbio.proteins.7tms_r]

T. J. Murphy (medtjm at bimcore.emory.edu) wrote:
> My interests include "7TM" receptors coupled to GTP-binding proteins.
> Since "7TM" receptors are integral membrane proteins, they have proven
> refractory to known approaches for crystallization and structural solution
> (as I understand it).

1) It is possible to crystallize integral membrane proteins (see for
   example the photosynthetic reaction center or the porins). It is,
   however, much more difficult than crystallizing soluble proteins.

2) It is very difficult (yet) to express G-protein coupled receptors in
   quantities needed for expression. E. coli are rather reluctant to
   produce these proteins (you get about five to ten molecules per cell).
   Baculovirus expression is better, but still not comparable to
   soluble proteins in baculovirus. In mammalian cells, expression can
   be forced to rather high levels but here arise the problems of
   culturing large amounts of cells.

3) There is a projection structure of bovine rhodopsin available by
   G. Schertler. Bovine rhodopsin can be purified rather easily in
   large amounts from eyes (thereby circumventing the problems pointed
   out in #2 above).

> if one produces segments of a 7TM receptor protein, each containing a single 
> predicted membrane spanning region, what is the likelihood that these
> segments could be crystallized, and solved individually?

I am not sure if this approach has been tried with membrane segments of
G-protein coupled receptors, but the PDB contains some NMR structures
of membrane-spanning peptides derived from bacteriorhodopsin whose
structure has been solved by NMR.

It is very likely that indeed the transmembrane structures of G-protein
coupled receptors are more or less amphipathic helices, with the
more hydrophilic part pointing towards the center of the receptor.
Based on this assumption and on mutational data, a model has been
constructed by Joyce Baldwin (Baldwin, Curr. Op. Struct. Biol. 6,
180-190, 1994 and references therein) which attempts to fit all the
experimental data.

Now let the modellers stand up :-)


/* Cornelius Krasel, Abt. Lohse, Genzentrum, D-82152 Martinsried, Germany  */
/* email: krasel at alf.biochem.mpg.de                 fax: +49 89 8578 3795  */
/* "Science is the game you play with God to find out what His rules are." */

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