B-cell linker (BLNK)protein is expressed inB cellsandmacrophagesand plays a large role inB cell receptorsignaling.[5]Like alladaptor proteins,BLNK has no known intrinsic enzymatic activity.[6]Its function is to temporally and spatially coordinate and regulatedownstreamsignaling effectors inB cell receptor(BCR) signaling, which is important in B cell development.[7]Binding of these downstream effectors is dependent on BLNK phosphorylation.[8][9]BLNK is encoded by theBLNKgene[8][10]and is also known asSLP-65,[11]BASH,[12]andBCA.[13]
BLNK consists of a N-terminalleucine zippermotif followed by an acidic region, a proline-rich region, and a C-terminalSH2 domain.[14][5]The leucine zipper motif allows BLNK to localize to the plasma membrane, presumably by coiled-coil interactions with a membrane protein.[5]This leucine zipper motif distinguishes BLNK fromlymphoctye cytosolic protein 2,also known as LCP-2 or SLP-76, which plays a similar role inT cell receptorsignaling.[15]Although LCP-2 has an N-terminal heptad-like organization of leucine and isoleucine residues like BLNK, it has not been experimentally shown to have the leucine zipper motif.[16]Recruitment of BLNK to the plasma membrane is also achieved by binding of the SH2 domain of BLNK to a non-ITAMphospho-tyrosine on the cytoplasmic domain ofCD79A,which is a part ofIgαand the B cell receptor complex.[17][18][19]
BLNK’s function and interaction shown in a schematic of BCR signaling pathways. BCR antigen recognition activatesSrc family kinases,including theSYKandBTKtyrosine kinases.Syk then phosphorylates BLNK, which can recruit downstream signaling molecules such asGrb2,PLCG2,VavandNck.
BLNK's function and importance in B cell development were first illustrated in BLNK deficient DT40 cells, achickenB cellline.[7]DT40 cells had interrupted B cell development: there was no calcium mobilization response in the B cell, impaired activation of themitogen-activated protein (MAP) kinasesp38,JNK,and somewhat inhibitedERKactivation upon (BCR) activation as compared to wild type DT40 cells.[7]Inknockout mice,BLNK deficiency results in a partial block in B cell development,[20][21]and in humans BLNK deficiency results in a much more profound block in B cell development.[22][5]
Linker or adaptor proteins provide mechanisms by which receptors can amplify and regulate downstream effector proteins.[6]BLNK is essential for normal B-cell development as part of the B cell receptor signaling pathway. [supplied by OMIM][10][23][24]
Evidence also suggests that BLNK may have tumor suppressive activity through its interaction withBruton's tyrosine kinase(Btk)[25][26]and regulation of the pre-B cell checkpoint.[14][27]
The acidic region of BLNK contains several inducibly phosphorylatedtyrosineresidues, at least five of which are found in humans.[28]Evidence suggests that BLNK is phosphorylated by thetyrosine-protein kinase Sykafter B cell receptor activation.[8][9][24][29]Phosphorylation of these residues provides docking sites necessary for downstreamprotein-protein interactionsbetween BLNK and the SH2 domain-containing proteinsGrb2,[8][11][17][30]PLCG2,Btk,theVavprotein family, andNck.[31][9][8]BLNK has also been shown to interact withSH3KBP1[32]andMAP4K1.[33]A more recentmass spectrometrystudy of BLNK in DT40 cells found that at least 41 unique serine, threonine, and tyrosine residues are phosphorylated on BLNK.[34]
^"Human PubMed Reference:".National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:".National Center for Biotechnology Information, U.S. National Library of Medicine.
^abcdKöhler F, Storch B, Kulathu Y, Herzog S, Kuppig S, Reth M, Jumaa H (February 2005). "A leucine zipper in the N terminus confers membrane association to SLP-65".Nature Immunology.6(2): 204–210.doi:10.1038/ni1163.PMID15654340.S2CID10708737.
^abBorowicz P, Chan H, Hauge A, Spurkland A (November 2020). "Adaptor proteins: Flexible and dynamic modulators of immune cell signalling".Scandinavian Journal of Immunology.92(5): e12951.doi:10.1111/sji.12951.hdl:10852/82328.PMID32734639.S2CID220892370.
^Gangi-Peterson L, Peterson SN, Shapiro LH, Golding A, Caricchio R, Cohen DI, et al. (January 1998). "bca: an activation-related B-cell gene".Molecular Immunology.35(1): 55–63.doi:10.1016/s0161-5890(98)00008-x.PMID9683264.
^abHerzog S, Storch B, Jumaa H (2006). "Dual role of the adaptor protein SLP-65: organizer of signal transduction and tumor suppressor of pre-B cell leukemia".Immunologic Research.34(2): 143–155.doi:10.1385/ir:34:2:143.PMID16760574.S2CID11515343.
^Koretzky GA, Abtahian F, Silverman MA (January 2006). "SLP76 and SLP65: complex regulation of signalling in lymphocytes and beyond".Nature Reviews. Immunology.6(1): 67–78.doi:10.1038/nri1750.PMID16493428.S2CID22368341.
^Pappu R, Cheng AM, Li B, Gong Q, Chiu C, Griffin N, et al. (December 1999). "Requirement for B cell linker protein (BLNK) in B cell development".Science.286(5446): 1949–1954.doi:10.1126/science.286.5446.1949.PMID10583957.
^Minegishi Y, Rohrer J, Coustan-Smith E, Lederman HM, Pappu R, Campana D, et al. (December 1999). "An essential role for BLNK in human B cell development".Science.286(5446): 1954–1957.doi:10.1126/science.286.5446.1954.PMID10583958.
^Hashimoto S, Iwamatsu A, Ishiai M, Okawa K, Yamadori T, Matsushita M, et al. (October 1999). "Identification of the SH2 domain binding protein of Bruton's tyrosine kinase as BLNK--functional significance of Btk-SH2 domain in B-cell antigen receptor-coupled calcium signaling".Blood.94(7): 2357–2364.doi:10.1182/blood.V94.7.2357.419k40_2357_2364.PMID10498607.S2CID21014231.
^Hendriks RW, Kersseboom R (February 2006). "Involvement of SLP-65 and Btk in tumor suppression and malignant transformation of pre-B cells".Seminars in Immunology.18(1): 67–76.doi:10.1016/j.smim.2005.10.002.PMID16300960.
Maruyama K, Sugano S (January 1994). "Oligo-capping: a simple method to replace the cap structure of eukaryotic mRNAs with oligoribonucleotides".Gene.138(1–2): 171–174.doi:10.1016/0378-1119(94)90802-8.PMID8125298.
Suzuki Y, Yoshitomo-Nakagawa K, Maruyama K, Suyama A, Sugano S (October 1997). "Construction and characterization of a full length-enriched and a 5'-end-enriched cDNA library".Gene.200(1–2): 149–156.doi:10.1016/S0378-1119(97)00411-3.PMID9373149.
Hashimoto S, Iwamatsu A, Ishiai M, Okawa K, Yamadori T, Matsushita M, et al. (October 1999). "Identification of the SH2 domain binding protein of Bruton's tyrosine kinase as BLNK--functional significance of Btk-SH2 domain in B-cell antigen receptor-coupled calcium signaling".Blood.94(7): 2357–2364.doi:10.1182/blood.V94.7.2357.419k40_2357_2364.PMID10498607.S2CID21014231.