Subcellular localization and signalling of Brutons tyrosine kinase (Btk)

<p>Brutons tyrosine kinase (Btk) is a non-receptor tyrosine kinase related to the Src family of kinases. Mutations in various parts of the gene have been shown to cause X-linked agammaglobulinemia (XLA), a primary immunodeficiency in humans, characterized by a defect in B-cell development. XLA patients lack B-cells and consequentially have very low levels of immunoglobulins in their serum. Thus, these patients suffer from an increased susceptibility mainly to extracellular bacterial infections.</p><p>The molecular mechanism(s) underlying Btk localization, activation and signaling are not fully understood. We analyzed the subcellular localization of Btk employing a recombinant chimeric Btk fused with the Green Fluorescent Protein (GFP) with subsequent analysis of images using digital confocal microscopy. Different biochemical protein analyses were also performed.</p><p>During this study we have found that Btk can translocate to the plasma membrane of living cells and play an important role as a potent inducer of cytoskeletal reorganization resulting in membrane ruffle formation. Moreover, we found that Btk can translocate to the nucleus and that Btk utilizes functional CRM-1 dependent nuclear export signal(s) to shuffle between the nucleus and the cytoplasm. We also found that Tec family kinases bind to caveolin-1, a major structural component of caveolae (rafts or microdomains) located in the plasma membrane. Finally, we demonstrate that Cbl acts as an E3-ubiquitin ligase for Btk and that ubiquitinated Btk is targeted for proteasomal degradation when Btk is expressed at high levels. Furthermore, upregulation of the small-ubiquitin-related-modifier (SUMO-1) downregulates Btk.</p><p>In conclusion, the subcellular localization of Btk has implications regarding cytoskeletal regulation and /or potential targets inside the nucleus, which may be of relevance for B-cell development and differentiation. Also, Cbl-dependent ubiquitination as well as sumoylation are likely to provide a deeper insight into the negative regulation of Btk- mediated cell signaling.</p><h3>List of scientific papers</h3><p>I. Nore BF, Vargas L, Mohamed AJ, Branden LJ, Backesjo CM, Islam TC, Mattsson PT, Hultenby K, Christensson B, Smith CI (2000). "Redistribution of Brutons tyrosine kinase by activation of phosphatidylinositol 3-kinase and Rho-family GTPases. " Eur J Immunol 30(1): 145-54 <br><a href="https://pubmed.ncbi.nlm.nih.gov/10602036">https://pubmed.ncbi.nlm.nih.gov/10602036</a><br><br></p><p>II. Mohamed AJ, Vargas L, Nore BF, Backesjo CM, Christensson B, Smith CI (2000). "Nucleocytoplasmic shuttling of Brutons tyrosine kinase. " J Biol Chem 275(51): 40614-9 <br><a href="https://pubmed.ncbi.nlm.nih.gov/11016936">https://pubmed.ncbi.nlm.nih.gov/11016936</a><br><br></p><p>III. Vargas L, Nore BF, Berglof A, Heinonen JE, Mattsson PT, Smith CI, Mohamed AJ (2002). "Functional interaction of caveolin-1 with Brutons tyrosine kinase and Bmx. " J Biol Chem 277(11): 9351-7 <br><a href="https://pubmed.ncbi.nlm.nih.gov/11751885">https://pubmed.ncbi.nlm.nih.gov/11751885</a><br><br></p><p>IV. Vargas L, Mohamed AJ, Nore BF, Heinonen JE, Smith E (2002). "Proteasome inhibition and sumoylation upregulation enhance degradation of Brutons tyrosine kinase(Btk)." (Submitted)</p>