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 scen which correspond to non‐covalently linked dimers, 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 emphasised 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‐disulphide‐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 eluted 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 anomolous 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.