Pulmonary surfactant protein D (SP-D), a lung host defense protein, is assembled as multimers of trimeric subunits. Trimerization of SP-D monomers is required for high affinity saccharide binding, and the oligomerization of trimers is required for many of its functions. A peptide containing the α-helical neck region can spontaneously trimerize in vitro. However, it is not known whether this sequence is necessary for the complete cellular assembly of disulfide-cross-linked, trimeric subunits and dodecamers. For the present studies, we synthesized mutant cDNAs with deletions or site-directed substitutions in the neck domain of rat SP-D, and examined the assembly of the newly synthesized proteins after transfection of CHO-K1 cells. The neck domain contains three "classical" heptad repeat motifs with leucine residues at the "d position," and a distinctive C-terminal repeat previously suggested to drive trimeric chain association. Deletion of the highly conserved core of the latter repeat (FSRYLKK) did not interfere with the secretion of dodecamers with lectin activity. By contrast, deletion of the entire neck domain or deletion of one or two amino-terminal repeats resulted in defective molecular assembly. The secreted proteins eluted in the position of monomers by gel filtration under nondenaturing conditions. In addition, the neck + carbohydrate recognition domain of SP-D was necessary and sufficient for the trimerization of a heterologous collagen sequence located amino-terminal to the trimeric coiled-coil. These studies provide strong evidence that the amino-terminal heptad repeats of the neck domain are necessary for the intracellular, trimeric association of SP-D monomers and for the assembly and secretion of functional dodecamers.