Abstract
We have studied the role of the cytoplasmic domain of alpha 6 in the assembly and function of the alpha 6 beta 4 integrin, and compared it with the role of alpha 6 in the assembly and function of alpha 6 beta 1, by transfection of cDNAs encoding cytoplasmic mutants of alpha 6 into K562 cells with or without full-length beta 4 cDNA. Des-(1022-1050)-alpha 6, which contains a deletion C-terminal to the GFFKR motif, was expressed in association with beta 1, but associated preferentially with beta 4, whereas the wild-type alpha 6 subunit associated efficiently with beta 1 and beta 4. Des-(1016-1050)-alpha 6, which lacked also the GFFKR sequence, was only expressed at the cell surface when beta 4 was available. Transient expression in COS-7 cells showed that des-(1016-1050)-alpha 6 was retained in the endoplasmic reticulum as a monomer, which suggests that truncation of the cytoplasmic domain reduces the affinity of alpha 6 for beta 1, particularly when the GFFKR sequence is absent. Although the GFFKR motif is not essential for association of alpha 6 with beta 4, it increases the stability of the alpha 6 beta 4 integrin. The cytoplasmic domain of alpha 6 is essential for inside-out and outside-in signaling via the alpha 6 beta 1 receptor, but not for adhesion via alpha 6 beta 4. We show that alpha 6 beta 4 is a constitutively active receptor. Thus, unlike adhesion by most other integrins, adhesion by alpha 6 beta 4 does not seem to depend on any active cellular process. Binding of alpha 6 beta 4 to ligand was only slightly affected by truncation of the alpha 6 cytoplasmic domain N-terminal to the GFFKR sequence and became partially dependent on metabolic energy. These data indicate that truncations of the cytoplasmic domain of the alpha 6 subunit affect the assembly and function of alpha 6 beta 1 more strongly than those of alpha 6 beta 4. This difference may be due to the greater affinity of alpha 6 for beta 4 than for beta 1, which makes alpha 6 beta 4 less susceptible to the effect of truncations.