What's important to know when switching from crystallizing soluble proteins to crystallizing membrane proteins? I've compiled a list of points that I've made in the past when attempting to answer this question.
1. Go nano volume: Sample preparation involves the use of solubilizing detergent, and membrane proteins are notoriously unstable - unless you or your biochemist friend has worked out "conditions" (buffer, lipid, additives, temperature...) that keep the membrane protein from aggregating. This is all about getting the biochemistry right and often requires a lot more effort since standard conditions that are typically applied for soluble proteins may not be sufficient to keep the protein sample alive for a period of time that's compatible with crystal formation. Due to sample loss and cost in most cases you'll start with substantially less protein sample volume than what you're used to. Don't even think about uL-sized crystallization experiments.
2. Set up more crystallization experiments: Get used to screening more extensively, preparing more crystallization experiments and geting fewer crystal hits. Compared to soluble proteins, there are more parameters to screen for. This is due to the presence of an additional component, amphiphiles (detergent type, concentration, lipids...) and their complex behavior in solution. This dramatically increases the dimensionality of the already multidimensional protein crystallization phase space.
3. Spend more time at the microscope. The phenomenology of drop content is, how shall I say it without discouraging you, 'richer'. There are separate detergent rich phases that can look like oiled out protein, some phases are turbid and there are detergent crystals devoid of any membrane protein that may get you on the wrong track. Some membrane protein crystals may not even have proper facets.
You see, this is a funner game.
Of the Practical Membrane Protein Crystallization Tips listed here I think these 3 are the most useful:
4. Membrane proteins often require harsh detergents for their extraction out of the native membrane. Often crystals grow better with milder, shorter chain detergents.
5. Try to control detergent concentration (measuring it and reducing it). Often the detergent concentrates with the membrane protein and when low MWCO filters are used for sample concentration.
6. Start with crystallization screens that are rich in PEG as opposed to salts. For example the Ozma series of Emerald BioSystems' crystallization screens.
And finally:
7. Read this "Pedestrian guide to membrane protein crystallization" by Michael Wiener (Methods 34, 364-372, 2004).
8. By all means, explore non-traditional crystallization experiments that have worked for a number of membrane proteins. For example, utilizing bicelles or lipidic cubic phases (see primer here, and the Cubic LCP Kit).
Enjoy,
Peter