STRUCTURE OF THE R121D MUTANT N-TERMINAL LOBE OF HUMAN LACTOFERRIN. B.F. Anderson,W. Breyer, R.L. Kingston, H.R. Faber, C.L. Day and E.N. Baker, Department of Biochemistry, Massey University, Palmerston North, New Zealand.

The three dimensional structure of the iron free, R121D mutant N-terminal half molecule of human lactoferrin is currently being investigated by molecular replacement methods. Small colourless crystals were obtained by microdialysis from low-salt/Tris/Isopropanol solutions and a data set complete to 3Å was collected on image plates with CuK[[alpha]] radiation. Although the diffraction pattern appears hexagonal the crystals are trigonal P3₁ with a=b=151.3Å, c=48.6Å,[[gamma]]=120deg.,. The program AMORE, using the two domains of the recombinant N-terminal lobe of human lactoferrin, Lf_N, (Day et al., J. Mol. Biol., (1993). 232, 1084-1100) independently, located two independent molecules in the assumed P6₄ asymmetric unit (correlation coefficients > 0.5). One molecule however seriously overlaps with its symmetry mates about the screw axis. A possible expanation of this is that there is merohedral twinning about a two fold axis parallel to the unique direction of a P3₁ crystal. Statistical tests on the observed data confirm that twinning is present with an approximate twin fraction of 0.4. Further rotation/translation searches with AMORE in space group P3₁ and using as search model one of the full molecules found previously gives four strong sites which can be divided into pairs related by a diad coincident with the 3₁ axis. Since this model gives a V_m of 4.4, our present efforts are directed toward locating the rest of the cell contents (probably one more molecule in the untwinned asymmetric unit) while continuing the search for less twinned crystals and higher resolution data.

The structure shows an `open' conformation and as such appears very similar to that shown by the N lobe of apolactoferrin. Comparison with Lf_N shows that there has been a rotational movement of the N2 domain with respect to N1 of approximately 52deg..