I have seen that there was a small disscusion on
the crystallization of membrane proteins:
1. It is true that more membrane proteins that crystal structure
is obtain is by cleavage of the extracellular part. But....
2. A nobel price was given for the solution of the crystal structure
of the PS complex from bacteria - by all means a membrane protein.
(Michel H. J. Mol Biol. 158, 567, 1982 is one ref) and many porteins
were crystallized, some were even solved (See Mitchel H. Crystallization
of membrane proteins, CRC 1991).
3. More then this, Some membrane proteins have their natural state as
two dimentional crystals, and the structure of Bacteriorodopsin from
purple membranes was solved using EM crystallography (diffractions of
electrons in EM used for the same as X-ray in X-ray crystallography)
Therefore it is to be expected to see more of these in the future.
Once a structural Immunologist
Ofer Markman