β1 integrins show specific association with CD98 protein in low density membranes

Background: The CD98 (4F2, FRP-1) is a widely expressed cell surface protein heterodimer composed of a glycosylated heavy chain and a non-glycosylated light chain. Originally described as a T cell activation antigen, it was later shown to function in amino acid transport, cell fusion and homotypic cell aggregation. Several lines of evidence suggest its functional interaction with integrins but the biochemical basis for this interaction has been unclear. Results: We demonstrate that CD98 constitutively and specifically associates with beta 1 integrins (alpha 2 beta 1, alpha 3 beta 1, alpha 5 beta 1 and alpha 6 beta 1), but minimally with alpha 4 beta 1. Integrin-CD98 association was established by reciprocal immunoprecipitation experiments, and confirmed by CD98-induced clustering of alpha 3 beta 1 but not alpha 4 beta 1 on the surface of rhabdomyosarcoma cells. Integrin-CD98 association is independent of the a subunit cytoplasmic tail, is maintained in alpha 3 beta 1 ligand-interaction deficient mutants, and is not inhibited by EDTA. Within the CD98 heavy chain, a C109S mutation (but not a C330S mutation) caused a loss of beta 1 integrin association. The same C109S mutation also caused a loss of CD98 light chain association. Importantly, CD98 associated selectively with beta 1 integrins present in low density "light membrane" fractions on a sucrose gradient. CD98 was not present in dense fractions that contained the majority of beta 1 integrins. Notably, the C109S mutant of CD98, that did not associate with beta 1 integrins, showed also a reduced localization into light membrane fractions. Conclusions: We demonstrate that CD98 association with beta 1 integrins is specific, occurs in the context of low density membranes, and may require the CD98 light chain.