(A) A model of SP-D structure. (Upper panel) The SP-D monomer (43 kDa) consists of a carbohydrate recognition domain which forms the globular head structure. This domain is connected to the collagen-like helical tail domain by a short, 30–amino acid, neck domain. At the end of the tail domain is the amino terminus in which cysteines 15 and 20 are positioned (shown as yellow projections). (Lower panel) Stylized representation of SP-D multimer assembly (note tail domains are shown shortened for ease of visualization). The head and neck domains drive the aggregation of the SP-D monomer to form a trimer of ∼130 kDa. These trimers associate to form a dodecamer (∼520 kDa). The forces holding this dodecamer together are unclear, although there is a dependency upon the amino-terminal cysteines as mutant lacking these cysteines do not form dodecamers. These dodecamers can assemble to a multimer of greater than 1 MDa. It is unclear whether the dodecamer is an essential intermediate in multimer formation. It should be noted that neither the trimer nor the dodecamer are globular proteins, due to the presence of the long collagen tail and thus under native conditions will behave as molecules with greater molecular radius.
(B) Hydropathy plot of SP-D. A Kyle Doolittle hydropathy plot for the SP-D sequence was constructed using a window size of nine residues. A positive value indicates a region of hydrophobicity. The position of cysteines 15 and 20 are marked within the tail domain. As one can see, this is the most hydrophobic portion of the molecule.