CD154 variant lacking tumor necrosis factor homologous domain inhibits cell surface expression of wild-type protein

X-linked hyper-IgM (XHIM) syndrome is an immunological disorder resulting f rom mutations in the CD154 gene. Some mutations occur in splicing sites and result in transcripts encoding wild-type and mutant proteins. These mutant s lack the tumor necrosis factor homologous (TNFH) domain and consequently fail to trimerize. Given that the TNFH domain is responsible for trimerizat ion, one may predict that the TNFH mutant can not participate in the assemb ly of wild-type CD154. Thus, it was puzzling why these patients exhibit XHI M phenotype, presumably resulting from a lack of functional CD154. One poss ibility is that the TNFH mutant exhibits a dominant negative effect over th e wild-type protein. To investigate this, we coexpressed the wild-type prot ein and a TNFH mutant and examined the biochemical and functional propertie s of the resulting CD154 products. We demonstrate that despite the lack of the TNFH domain, the TNFH mutant can associate with the wild-type protein. Furthermore, such an association compromises the ability of the wild-type p rotein to mature onto the cell surface. These results provide a mechanism f or the defect of CD154 in XHIM patients producing both wild-type and TNFH v ariants and suggest that besides the TNFH domain, the stalk region particip ates in the assembly of CD154 trimers.