title: A Transmembrane Domain GGxxG Motif in CD4 Contributes to Its Lck-Independent Function but Does Not Mediate CD4 Dimerization.
abstract: CD4 interactions with class II major histocompatibility complex (MHC) molecules are essential for CD4+ T cell development, activation, and effector functions. While its association with p56lck (Lck), a Src kinase, is important for these functions CD4 also has an Lck-independent role in TCR signaling that is incompletely understood. Here, we identify a conserved GGxxG motif in the CD4 transmembrane domain that is related to the previously described GxxxG motifs of other proteins and predicted to form a flat glycine patch in a transmembrane helix. In other proteins, these patches have been reported to mediate dimerization of transmembrane domains. Here we show that introducing bulky side-chains into this patch (GGxxG to GVxxL) impairs the Lck-independent role of CD4 in T cell activation upon TCR engagement of agonist and weak agonist stimulation. However, using Forster's Resonance Energy Transfer (FRET), we saw no evidence that these mutations decreased CD4 dimerization either in the unliganded state or upon engagement of pMHC concomitantly with the TCR. This suggests that the CD4 transmembrane domain is either mediating interactions with an unidentified partner, or mediating some other function such as membrane domain localization that is important for its role in T cell activation.
authors.affiliation: Department of Immunobiology, The University of Arizona College of Medicine, Tucson, Arizona, United States of America.
authors.forename: Heather L
authors.initials: HL
authors.lastname: Parrish
authors.affiliation: Department of Immunobiology, The University of Arizona College of Medicine, Tucson, Arizona, United States of America.
authors.forename: Caleb R
authors.initials: CR
authors.lastname: Glassman
authors.affiliation: Department of Immunobiology, The University of Arizona College of Medicine, Tucson, Arizona, United States of America.
authors.forename: Madeline M
authors.initials: MM
authors.lastname: Keenen
authors.affiliation: Department of Immunobiology, The University of Arizona College of Medicine, Tucson, Arizona, United States of America; The Arizona Center on Aging, The University of Arizona College of Medicine, Tucson, Arizona, United States of America.
authors.forename: Neha R
authors.initials: NR
authors.lastname: Deshpande
authors.affiliation: Department of Immunobiology, The University of Arizona College of Medicine, Tucson, Arizona, United States of America.
authors.forename: Matthew P
authors.initials: MP
authors.lastname: Bronnimann
authors.affiliation: Department of Immunobiology, The University of Arizona College of Medicine, Tucson, Arizona, United States of America; The Arizona Center on Aging, The University of Arizona College of Medicine, Tucson, Arizona, United States of America; The BIO-5 Institute, The University of Arizona College of Medicine, Tucson, Arizona, United States of America.
authors.forename: Michael S
authors.initials: MS
authors.lastname: Kuhns
mesh_terms: D020816:Amino Acid Motifs; D000818:Animals; D015704:CD4 Antigens; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D031541:Fluorescence Resonance Energy Transfer; D006801:Humans; D008213:Lymphocyte Activation / Q000502:physiology*; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck) / Q000235:genetics / Q000378:metabolism*; D055503:Protein Multimerization / Q000502:physiology*; D017434:Protein Structure, Tertiary; D011948:Receptors, Antigen, T-Cell / Q000235:genetics / Q000378:metabolism*; D013601:T-Lymphocytes / Q000166:cytology / Q000378:metabolism*
chemical_list: D015704:CD4 Antigens; D011948:Receptors, Antigen, T-Cell; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck)