title: Identification of CMTM7 as a transmembrane linker of BLNK and the B-cell receptor.
abstract: BLNK is a pivotal adaptor protein in the signal transduction pathway from the IgM class B-cell receptor. BLNK is phosphorylated by Syk and binds various signaling intermediates, leading to cellular events including MAP-kinase activation, culminating in cellular activation. It remains unclear how BLNK is initially recruited to the surface IgM (sIgM) complex to which Syk is also recruited. Here we show that CMTM7, a tetra-spanning membrane protein of unknown function, co-localized with clathrin and sIgM at the plasma membrane. RNA-interference-mediated knockdown of CMTM7 expression in B cells resulted in an impairment of sIgM-ligation-induced tyrosine phosphorylation of BLNK, which was due to an impaired interaction of BLNK and Syk, and in a failure to activate JNK and ERK, but not upstream kinases such as Src-family kinases and Syk. CMTM7 was bound to BLNK in a membrane fraction, and their association was augmented after sIgM ligation. Exogenous CMTM7 or a mutant with an N-terminal deletion (ΔN), but not one with a C-terminal deletion (ΔC) that is defective in membrane localization, were able to restore BLNK-Syk binding, BLNK phosphorylation and ERK activation in the CMTM7-knockdown B cells. In addition, CMTM7 and the ΔN, but not the ΔC, were constitutively associated with sIgM, and this binding was required for BLNK recruitment to sIgM. From these data, we conclude that CMTM7 functions to link sIgM and BLNK in the plasma membrane, to recruit BLNK to the vicinity of Syk, and to initiate the BLNK-mediated signal transduction.
authors.affiliation: Division of Molecular Biology Laboratory, Research Institute for Biological Sciences (RIBS), Tokyo University of Science, Noda, Chiba, Japan.
authors.forename: Atsuko
authors.initials: A
authors.lastname: Miyazaki
authors.forename: Satomi
authors.initials: S
authors.lastname: Yogosawa
authors.forename: Akikazu
authors.initials: A
authors.lastname: Murakami
authors.forename: Daisuke
authors.initials: D
authors.lastname: Kitamura
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism*; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism*; D018925:Chemokines / Q000737:chemistry / Q000378:metabolism*; D002966:Clathrin / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D007075:Immunoglobulin M / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D062792:MARVEL Domain-Containing Proteins; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism*; D051379:Mice; D008954:Models, Biological; D010766:Phosphorylation; D011485:Protein Binding; D021381:Protein Transport; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011947:Receptors, Antigen, B-Cell / Q000378:metabolism*; D015398:Signal Transduction; D013329:Structure-Activity Relationship; D000072377:Syk Kinase
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C113939:B cell linker protein; C552173:CMTM7 protein, human; C573093:CMTM7 protein, mouse; D018925:Chemokines; D002966:Clathrin; D007075:Immunoglobulin M; D047908:Intracellular Signaling Peptides and Proteins; D062792:MARVEL Domain-Containing Proteins; D008565:Membrane Proteins; D011947:Receptors, Antigen, B-Cell; D011505:Protein-Tyrosine Kinases; C000606896:SYK protein, human; D000072377:Syk Kinase; C000606897:Syk protein, mouse