Three-dimensional reconstruction of Limulus polyphemus hemocyanin from cryoelectron microscopy.

Abstract Hemocyanin (Hc) the respiratory pigment of the horseshoe crab Limulus polyphemus ( Lp ) is composed of 48 ∼75 kDa copper-containing subunits arranged in eight hexameric groups. In this study, we used the random conical tilt series method to do a three-dimensional (3D) reconstruction of Lp Hc observed in vitreous ice. This approach allowed the unambiguous determination of the handedness of the molecule. Lp Hc contains two superimposed 4 × 6mer structures possessing the same structural features as the other 4 × 6meric Hcs, namely flip and flop views and a rocking effect. Moreover, 3D fitting of the X-ray structure of subunit Lp II with the reconstruction volume shows that the intra 4 × 6meric contacts described in arthropod Hcs also occur within Limulus Hc. The two half-molecules composing the 8 × 6mer have their flop faces in contact (flop/flop association), the main links being formed by subunits Lp IV. Model building shows that the flop/flop association is the only possible arrangement which allows the assembly of the whole particle. The two alternate constructions (flip/flop and flip/flip) are forbidden because of steric hindrance.