{"id":600,"date":"2017-01-20T03:24:00","date_gmt":"2017-01-20T08:24:00","guid":{"rendered":"http:\/\/www.bocsci.com\/blog\/?p=600"},"modified":"2017-01-20T03:24:00","modified_gmt":"2017-01-20T08:24:00","slug":"regulation-of-syk","status":"publish","type":"post","link":"https:\/\/www.bocsci.com\/blog\/regulation-of-syk\/","title":{"rendered":"Regulation of Syk"},"content":{"rendered":"

In cells, it has been shown that Syk<\/a> becomes active after binding to the\u00a0phosphorylated ITAMs located on the BCR. Syk binds the two ITAM\u00a0phosphotyrosines through its tandem SH2 domains. Hydrogen\/deuterium\u00a0exchange and mass spectrometric analyses show that the binding of Syk to a\u00a0diphosphorylated ITAM peptide induces a structural change that stabilizes the linker A helix. The binding of the SH2 domains of Syk to the\u00a0phosphotyrosine residues of the ITAM most likely occurs in a two step process. There is an initial binding of one of the SH2 domains to one\u00a0phosphotyrosine in the ITAM. A conformational change occurs that is followed\u00a0by the binding of the second SH2 domain to the second ITAM phosphotyrosine. Linker A is crucial for Syk\u2019s binding to the receptor and for the subsequent\u00a0relay of signals. The crystal structure of Syk\u2019s tandem SH2 domains bound\u00a0to an ITAM and the solution structure of the C-terminal SH2 domain indicate that\u00a0the SH2 domains of Syk also can function independently from each other. Studies using a gain of function S2 insect cell system that reconstitutes\u00a0BCR signaling show that the binding of the tandem SH2 domains of Syk to the\u00a0BCR is absolutely crucial for Syk\u2019s activation in cells. In fact in vitro <\/i><\/em>experiments\u00a0using a diphosphorylated ITAM demonstrate that binding to the ITAM peptide\u00a0alone is sufficient for the full activation of Syk.<\/p>\n

Syk undergoes autophosphorylation in vitro. <\/i><\/em>The tyrosine residues that\u00a0are phosphorylated have been identified through in vitro <\/i><\/em>kinase assays,\u00a0phosphopeptide mapping, and mass spectrometric analyses as Y130, Y290,\u00a0Y317, Y342, Y346, Y519, Y520, Y624, and Y625.\u00a0Syk\u2019s\u00a0autophosphorylation leads to its activation. Syk can autophosphorylate in<\/i><\/em>\u00a0<\/i><\/em>vitro <\/i><\/em>in the presence of ATP and presumably this also is one mechanism of its\u00a0activation in cells.\u00a0In fact it has been found that phosphorylation alone\u00a0also can fully activate Syk.<\/p>\n

The phosphorylated tyrosine residues on Syk<\/a> have different roles\u00a0including changes in conformation and changes in protein binding. Tyrosine 130 of murine Syk is phosphorylated in cells following BCR ligation. Phosphorylation of Y130 within linker A leads to a conformational\u00a0change within the protein. This change increases the activity of Syk in cells,\u00a0but decreases the affinity of Syk for the ITAMs. Tyrosine 290 on Syk is\u00a0phosphorylated in vitro during an autophosphorylation reaction, but has not been\u00a0found to be phosphorylated in intact cells of the immune system. Using\u00a0mutants of Syk with Y290 mutated to a phenylalanine, no significant contribution\u00a0of this residue to the ability of Syk to rescue Fc\u03b5RI-mediated degranulation in\u00a0Syk-deficient mast cells or antigen-induced tyrosine phosphorylation in T cells\u00a0can be found. Tyrosine 317 on murine Syk is important for the regulation of\u00a0Syk in B cells by acting as a site for the binding of the E3 ubiquitin ligase, cellular\u00a0homolog of Casitas B lineage lymphoma (c-Cbl). When Y317 of Syk is\u00a0mutated to a phenylalanine, Syk-dependent, anti-IgM-induced B cell signaling\u00a0events such as NFAT- and ELK-1-mediated gene transcription are enhanced,\u00a0suggesting a negative regulatory role for this site. Phosphorylated Y317 on\u00a0Syk also is important for the binding of p85, the regulatory subunit of\u00a0phosphoinositide-3-kinase (PI3K). Using gene knockout studies and\u00a0dominant negative mutants, a Syk-dependent increase in phosphatidylinositol-3,4,5-trisphosphate following BCR-ligation is observed. These studies\u00a0indicate that Syk is important for the regulation of the PI3K pathway. However\u00a0the exact role that the phosphorylation of Y317 plays in the activation of PI3K in\u00a0B cells has not been examined fully.<\/p>\n

Tyrosine residues 342 and 346 of Syk are phosphorylated both in vitro <\/i><\/em>by\u00a0autophosphorylation as demonstrated using kinase assays in conjunction with phosphopeptide mapping and in intact, anti-IgM stimulated cells as shown using\u00a0mass spectrometry, site specific antibodies, and phosphopeptide mapping. The phosphorylation of these sites in cells occurs by\u00a0autophosphorylation or through phosphorylation by Lyn kinase following either BCR-ligation or treatment with H2O2 or pervanadate. Phosphorylation of\u00a0Y342 and Y346 are crucial for the efficient activation of B cells. DT40 B\u00a0cells expressing a mutant version of Syk in which tyrosines 342 and\/or 346 are\u00a0mutated to phenylalanines have defects in calcium mobilization and NFAT\u00a0mediated transcription following BCR ligation. Tyrosines 342 and 346 of\u00a0Syk can bind to different proteins depending on the site that is phosphorylated\u00a0and the stoichiometry of phosphorylation. The phosphorylation of tyrosines\u00a0342 and 346 of Syk also enhances the catalytic activity of Syk. The zeta chain\u00a0associated protein kinase 70, Zap-70, which is a second member of the\u00a0Syk\u00a0family of tyrosine kinases, contains corresponding tyrosine residues that\u00a0contribute structurally to the stability of the autoinhibited state of the kinase.\u00a0If these tyrosines are phosphorylated, structural studies suggest that they would
\nno longer stabilize this autoinhibited state resulting in the activation of the kinase.\u00a0It is likely that Syk is regulated in an analogous manner.<\/p>\n

 <\/p>\n

Reference:<\/p>\n

Leela Louise Paris. THE PHOSPHORYLATION OF S291 ON SYK AND ITS ROLE IN B CELL\u00a0SIGNAL TRANSDUCTION<\/p>\n","protected":false},"excerpt":{"rendered":"

In cells, it has been shown that Syk becomes active after binding to the\u00a0phosphorylated ITAMs located on the BCR. Syk binds the two ITAM\u00a0phosphotyrosines through its tandem SH2 domains. Hydrogen\/deuterium\u00a0exchange […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[181],"tags":[],"_links":{"self":[{"href":"https:\/\/www.bocsci.com\/blog\/wp-json\/wp\/v2\/posts\/600"}],"collection":[{"href":"https:\/\/www.bocsci.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.bocsci.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.bocsci.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.bocsci.com\/blog\/wp-json\/wp\/v2\/comments?post=600"}],"version-history":[{"count":1,"href":"https:\/\/www.bocsci.com\/blog\/wp-json\/wp\/v2\/posts\/600\/revisions"}],"predecessor-version":[{"id":601,"href":"https:\/\/www.bocsci.com\/blog\/wp-json\/wp\/v2\/posts\/600\/revisions\/601"}],"wp:attachment":[{"href":"https:\/\/www.bocsci.com\/blog\/wp-json\/wp\/v2\/media?parent=600"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bocsci.com\/blog\/wp-json\/wp\/v2\/categories?post=600"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bocsci.com\/blog\/wp-json\/wp\/v2\/tags?post=600"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}