Promoting crystallization of intrinsic membrane proteins with conjugated micelles

A new technique for promoting nucleation and growth of membrane protein (MP) crystals from micellar environments is reported. It relies on the conjugation of micelles that sequester MPs in protein detergent complexes (PDCs). Conjugation via amphiphilic [metal:chelator] complexes presumably takes place at the micelle/water interface, thereby bringing the PDCs into proximity, promoting crystal nucleation and growth. We have successfully applied this approach to two light-driven proton pumps: bacteriorhodopsin (bR) and the recently discovered King Sejong 1–2 (KS1–2), using the amphiphilic 4,4′-dinonyl-2,2′-dipyridyl (Dinonyl) (0.7 mM) chelator in combination with Zn(2+), Fe(2+), or Ni(2+) (0.1 mM). Crystal growth in the presence of the [metal–chelator] complexes leads to purple, hexagonal crystals (50–75 µm in size) of bR or pink, rectangular/square crystals (5–15 µm) of KS1–2. The effects of divalent cation identity and concentration, chelator structure and concentration, ionic strength and pH on crystal size, morphology and process kinetics, are described.