THE LYMPHOTOXIN-ALPHA (LT-ALPHA) SUBUNIT IS ESSENTIAL FOR THE ASSEMBLY, BUT NOT FOR THE RECEPTOR SPECIFICITY, OF THE MEMBRANE-ANCHORED LT-ALPHA-1-BETA-2 HETEROTRIMERIC LIGAND

The lymphotoxins (LT) alpha and beta, members of the tumor necrosis fa ctor (TNF) cytokine superfamily, are implicated as important regulator s and developmental factors for the immune system. LT alpha is secrete d as a homotrimer and signals through two TNF receptors of 55-60 kDa ( TNFR60) or 75-80 kDa (TNFR80), LT alpha also assembles with LT beta in to a membrane-anchored, heterotrimeric LT alpha 1 beta 2 complex that engages a distinct cognate receptor, the LT beta receptor (LT beta R), To investigate the role of the LT alpha subunit in the function of th e membrane LT alpha 1 beta 2 complex, gene transfer via baculovirus wa s used to assemble LT alpha and -beta complexes in insect cells, LT al pha 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 contra st, these mutant LT alpha proteins retain the ability to co-assemble w ith LT beta into membrane-anchored LT alpha 1 beta 2 complexes that en gage the LT beta R and trigger the death of HT29 cells, Membrane-ancho red LT beta expressed on the cell surface in absence of the LT alpha s ubunit binds the LT beta R but is functionally inactive in the cell de ath assay, These results indicate that the TNF receptor-binding region s of the LT alpha subunit are not necessary for engagement of the LT b eta R, but the LT alpha subunit is required for the assembly of LT bet a into a functional heteromeric ligand.