3-DIMENSIONAL RECONSTRUCTION OF LIMULUS-POLYPHEMUS HEMOCYANIN FROM CRYOELECTRON MICROSCOPY

Hemocyanin (Hc) the respiratory pigment of the horseshoe crab Limulus polyphemus (Lp) is composed of 48 similar to 75 kDa copper-containing subunits arranged in eight hexameric groups. In this study, we used th e random conical tilt series method to do a three-dimensional (3D) rec onstruction of Lp Hc observed in vitreous ice. This approach allowed t he unambiguous determination of the handedness of the molecule. Lp Hc contains two superimposed 4 x 6mer structures possessing the same stru ctural features as the other 4 x 6meric Hcs, namely flip and flop view s and a rocking effect. Moreover, 3D fitting of the X-ray structure of subunit LpII with the reconstruction volume shows that the intra4 x 6 meric contacts described in arthropod Hcs also occur within Limulus Hc . The two half-molecules composing the 8 x 6mer have their flop faces in contact (flop/flop association), the main links being formed by sub units LpIV. Model building shows that the flop/flop association is the only possible arrangement which allows the assembly of the whole part icle. The two alternate constructions (flip/flop and flip/flip) are fo rbidden because of steric hindrance. (C) 1997 Academic Press Limited.