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| - Homodimer (PubMed:20375010). Part of a complex composed of CD19, CR2/CD21, CD81 and IFITM1/CD225 in the membrane of mature B cells. Interacts (via the second extracellular domain) with CD19; this interaction is initiated early during biosynthesis in the ER and enables trafficking of only properly folded CD19 (PubMed:1383329, PubMed:16449649). Part of a complex that includes MHC class II/HLA-DR molecules and IFITM1 (PubMed:8409388). Interacts with IFITM1 (PubMed:2398277, PubMed:26354436). Interacts with IFITM2 and IFITM3 (By similarity). Part of integrin-tetraspanin complex composed of CD9, CD81, beta-1 and beta-2 integrins in the membrane of monocyte/macrophages (PubMed:12796480). Interacts (via the second extracellular domain) with integrin ITGAV:ITGB3 (PubMed:27993971). Interacts with CD247/CD3 zeta, ICAM1 and CD9 at the immune synapse on T cell membrane (PubMed:23858057). Part of a GPCR-tetraspanin complex consisting at least of ADGRG1, CD81, possibly CD9, and GNA11 in which CD81 enhances the association of ADGRG1 with GNA11 (PubMed:15004227). Part of a complex composed of CD9, CD81, PTGFRN and IGSF8 (By similarity). Interacts directly with IGSF8 (PubMed:11504738). Interacts with CD53 and SCIMP (PubMed:21930792). Interacts with SAMHD1 (via its C-terminus) (PubMed:28871089). Interacts with glypican GPC3 and with the transcriptional repressor HHEX; binding to GPC3 decreases the availability of free CD81 for binding to HHEX, resulting in nuclear translocation of HHEX and transcriptional repression (By similarity). Interacts with CLDN1 (PubMed:20375010, PubMed:21516087). Interacts with CLDN6 and CLDN9 (PubMed:20375010).
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