The lymphotoxin-α (LTα) subunit is essential for the assembly, but not for the receptor specificity, of the membrane-anchored LTα1β2 heterotrimeric ligand

Abstract

The lymphotoxins (LT) α and β, members of the tumor necrosis factor (TNF) cytokine superfamily, are implicated as important regulators and developmental factors for the immune system. LTα is secreted as a homotrimer and signals through two TNF receptors of 55-60 kDa (TNFR60) or 75-80 kDa (TNFR80). LTα also assembles with LTβ into a membrane-anchored, heterotrimeric LTα1β2 complex that engages a distinct cognate receptor, the LTβ receptor (LTβR). To investigate the role of the LTα subunit in the function of the membrane LTα1β2 complex, gene transfer via baculovirus was used to assemble LTα and -β complexes in insect cells. LTα containing mutations at D50N or Y108F are secreted as homotrimers that fail to bind either TNF receptor and are functionally inactive in triggering cell death of the HT29 adenocarcinoma cell line. In contrast, these mutant LTα proteins retain the ability to co-assemble with LTβ into membrane-anchored LTα1β2 complexes that engage the LTβR and trigger the death of HT29 cells. Membrane-anchored LTβ expressed on the cell surface in absence of the LTα subunit binds the LTβR but is functionally inactive in the cell death assay. These results indicate that the TNF receptor-binding regions of the LTα subunit are not necessary for engagement of the LTβR, but the LTα subunit is required for the assembly of LTβ into a functional heteromeric ligand.