The role of the cytoplasmic domain of alpha 6 integrin in the assembly and function of alpha 6 beta 1 and alpha 6 beta 4.
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.
De Melker
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The Netherlands Cancer Institute, Division of Cell Biology, Amsterdam, The Netherlands.
Sonnenberg
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D000595:Amino Acid Sequence; D000818:Animals; D000954:Antigens, Surface / Q000378:metabolism*; D001386:Azides / Q000494:pharmacology; D019556:COS Cells; D002465:Cell Movement / Q000502:physiology; D003001:Cloning, Molecular; D003847:Deoxyglucose / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D005353:Fibronectins / Q000378:metabolism; D005434:Flow Cytometry; D015870:Gene Expression / Q000235:genetics; D039121:Integrin alpha6beta1; D039161:Integrin alpha6beta4; D016023:Integrins / Q000378:metabolism*; D007797:Laminin / Q000378:metabolism; D008969:Molecular Sequence Data; D011233:Precipitin Tests; D017384:Sequence Deletion / Q000235:genetics; D019810:Sodium Azide; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014162:Transfection / Q000235:genetics; D014407:Tumor Cells, Cultured
D000954:Antigens, Surface; D001386:Azides; D005353:Fibronectins; D039121:Integrin alpha6beta1; D039161:Integrin alpha6beta4; D016023:Integrins; D007797:Laminin; D019810:Sodium Azide; D003847:Deoxyglucose; D013755:Tetradecanoylphorbol Acetate