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

We demonstrate that CD98 constitutively and specifically associates with β1 integrins (α2β1,α3β1, α5β1 and α6β1), but minimally with α4β1. Integrin-CD98 association was established by reciprocal immunoprecipitation experiments, and confirmed by CD98-induced clustering of α3β1 but not α4β1 on the surface of rhabdomyosarcoma cells. Integrin-CD98 association is independent of the α subunit cytoplasmic tail, is maintained in α3β1 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 β1 integrin association. The same C109S mutation also caused a loss of CD98 light chain association. Importantly, CD98 associated selectively with β1 integrins present in low density "light membrane" fractions on a sucrose gradient. CD98 was not present in dense fractions that contained the majority of β1 integrins. Notably, the C109S mutant of CD98, that did not associate with β1 integrins, showed also a reduced localization into light membrane fractions.We demonstrate that CD98 association with β1 integrins is specific, occurs in the context of low density membranes, and may require the CD98 light chain.The CD98 (4F2, FRP-1) molecule, a cell surface disulfide-linked heterodimer, was originally described as a T cell activation antigen [1], and later was shown to provide a co-stimulatory signal for CD3-mediated T-cell activation [2], independent of CD28/CD80/CD86 interaction [3]. In other studies, triggering of human monocyte CD98 could suppress T cell proliferation [4], or promote homotypic cell aggregation of monocytes [5]. Also, CD98 may be a target antigen for natural killer cells [6], may mediate fusion of blood monocytes leading to osteoclast formation [7,8], and may modulate hematopoietic cell survival and differentiation [9].The CD98 molecule is also widely expressed on rapidly growing non-hematopoietic cells, where it may modulate oncogenic transformation [10,11], metal ion transp