Structural similarity between lung surfactant protein D and conglutinin.

Preparations of bovine lung surfactant D (SP-D) and conglutinin were examined by electron microscopy, gel-filtration and SDS/PAGE. SP-D is composed of non-covalently linked subunits, of 160 kDa, which each contain three, disulphide-linked, 44-kDa polypeptide chains. In the electron microscope a single 160-kDa subunit of SP-D appears as a 45.8 ± 3-nm-long rod connected to a small globular ‘head’. Particles were also seen which correspond to non-covalently linked dimmers, trimers and tetramers of the 160-kDa monomer subunit of SP-D. The tetramer structure contains 12 polypeptide chains and is very similar to the electron microscopy images and model reported by Strang et al. [Strang, C. J., Slayter, US., Lachmann, P. J. and Davis, A. E. (1986) Biochem. J. 236, 3811–389] for bovine conglutinin in which four 160-kDa subunits are disulphide-linked to give a molecule of expected molecular mass of 528 kDa. This study confirmed the findings by Strang et al. in the above paper for intact conglutinin and also emphasized that the rod-like structures, of length 37.6 ± 3.7 nm. Seen in the conglutinin subunits were significantly shorter than those in SP-D despite the close similarity in amino acid sequence (79% identity) and chain length between the two proteins. In addition, a truncated form of conglutinin was found in the conglutinin preparations, due to limited proteolysis of the Arg-Ala bond at position 54 in the 44-kDa chains. These truncated conglutinin chains yield a subunit composed of three shortened, non-disulophide-linked, chains and this subunit appears as a monomer with a rod length of 34.2 ± 2.8 nm in the electron microscope. On gel-filtration, a proportion of the SP-D preparation behaved, as expected, as a molecule with an apparent molecular mass of 600 kDa. The remainder of the SP-D preparation behaved as aggregated material with a molecular mass greater than 900 kDa which yielded no distinct structures in the electron microscope. Intact conglutinin was wluted at a position greater than 900 kDa but yet provided clear electron microscopy images of the tetramer structure described above. Although it is difficult to explain fully the anomalous behaviour of SP-D and conglutinin on gel-filtration, it is proposed that the natural form of both Sp-D and conglutinin is the highest distinct oligomer form seen in the electron microscope, i.e. a tetramer of the 160-kDa subunits in which the four rod-like elements are linked in a tail-to-tail fashion to a central core. [ABSTRACT FROM AUTHOR]