Structure of Oligomeric β B2-crystallin: an application of the T2 translation function to an asymmetric unit containing two dimers

The molecular structure of the main subunit of the fl-crystallins, components of the vertebrate eye lens, has recently been solved by molecular replacement at 2.1 A, resolution [Bax, Lapatto, Nalini, Driessen, Lindley, Mahadevan, Blundell & Slingsby (1990). Nature (London), 347, 776-780]. The protein, fiB2, is a dimer in solution, but a tetramer in the crystal with one subunit in the asymmetric unit of space group 1222. Using the crystallographic dimer from this /-centred form the structure of a C222 crystal form of the fiB2 protein with four subunits in the asymmetric unit has now been solved by molecular replacement at 3.3 A. The solution involved the use of a new translation function for non-crystallographic symmetry, based on the T2 function of Crowther & Blow [Acta Cryst. (1967), 23, 544-548]