B-cell linker (BLNK) protein is expressed in B cells and macrophages and plays a large role in B cell receptor signaling.[5] Like all adaptor proteins, BLNK has no known intrinsic enzymatic activity.[6] Its function is to temporally and spatially coordinate and regulate downstream signaling effectors in B 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 the BLNKgene[8][10] and is also known as SLP-65,[11]BASH,[12] and BCA.[13]
Structure and localization
BLNK consists of a N-terminal leucine zipper motif followed by an acidic region, a proline-rich region, and a C-terminal SH2 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 from lymphoctye cytosolic protein 2, also known as LCP-2 or SLP-76, which plays a similar role in T cell receptor signaling.[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-ITAM phospho-tyrosine on the cytoplasmic domain of CD79A, which is a part of Igα and the B cell receptor complex.[17][18][19]
Function
BLNK’s function and interaction shown in a schematic of BCR signaling pathways. BCR antigen recognition activates Src family kinases, including the SYK and BTKtyrosine kinases. Syk then phosphorylates BLNK, which can recruit downstream signaling molecules such as Grb2, PLCG2, Vav and Nck.
BLNK's function and importance in B cell development were first illustrated in BLNK deficient DT40 cells, a chickenB cell line.[7] DT40 cells had interrupted B cell development: there was no calcium mobilization response in the B cell, impaired activation of the mitogen-activated protein (MAP) kinasesp38, JNK, and somewhat inhibited ERK activation upon (BCR) activation as compared to wild type DT40 cells.[7] In knockout 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 with Bruton's tyrosine kinase (Btk) [25][26] and regulation of the pre-B cell checkpoint.[14][27]
Phosphorylation and interactions
The acidic region of BLNK contains several inducibly phosphorylated tyrosine residues, at least five of which are found in humans.[28] Evidence suggests that BLNK is phosphorylated by the tyrosine-protein kinase Syk after B cell receptor activation.[8][9][24][29] Phosphorylation of these residues provides docking sites necessary for downstream protein-protein interactions between BLNK and the SH2 domain-containing proteins Grb2,[8][11][17][30]PLCG2, Btk, the Vav protein family, and Nck.[31][9][8] BLNK has also been shown to interact with SH3KBP1[32] and MAP4K1.[33] A more recent mass spectrometry study 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.
