Requirement of the SH4 and tyrosine-kinase domains but not the kinase activity of Lyn for its biosynthetic targeting to caveolin-positive Golgi membranes

Enhancing extracellular vesicle cargo loading and functional delivery by engineering protein-lipid interactions

Naturally generated lipid nanoparticles termed extracellular vesicles (EVs) hold significant promise as engineerable therapeutic delivery vehicles. However, active loading of protein cargo into EVs in a manner that is useful for delivery remains a challenge. Here, we demonstrate that by rationally designing proteins to traffic to the plasma membrane and associate with lipid rafts, we can enhance loading of protein cargo into EVs for a set of structurally diverse transmembrane and peripheral membrane proteins. We then demonstrate the capacity of select lipid tags to mediate increased EV loading and functional delivery of an engineered transcription factor to modulate gene expression in target cells. We envision that this technology could be leveraged to develop new EV-based therapeutics that deliver a wide array of macromolecular cargo.

[Novel Tyrosine Phosphorylation Signals in the Nucleus and on Mitotic Spindle Fibers and Lysosomes Revealed by Strong Inhibition of Tyrosine Dephosphorylation]

Protein-tyrosine phosphorylation is one of the posttranslational modifications and plays critical roles in regulating a wide variety of cellular processes, such as cell proliferation, differentiation, adhesion, migration, survival, and apoptosis. Protein-tyrosine phosphorylation is reversibly regulated by protein-tyrosine kinases and protein-tyrosine phosphatases. Strong inhibition of protein-tyrosine phosphatase activities is required to undoubtedly detect tyrosine phosphorylation. Our extremely careful usage of Na₃VO₄, a potent protein-tyrosine phosphatase inhibitor, has revealed not only the different intracellular trafficking pathways of Src-family tyrosine kinase members but also novel tyrosine phosphorylation signals in the nucleus and on mitotic spindle fibers and lysosomes. Furthermore, despite that the first identified oncogene product v-Src is generally believed to induce transformation through continuous stimulation of proliferation signaling by its strong tyrosine kinase activity, v-Src-driven transformation was found to be caused not by continuous proliferation signaling but by v-Src tyrosine kinase activity-dependent stochastic genome alterations. Here, I summarize our findings regarding novel tyrosine phosphorylation signaling in a spatiotemporal sense and highlight the significance of the roles of tyrosine phosphorylation in transcriptional regulation inside the nucleus and chromosome dynamics.

The role of the tyrosine kinase Lyn in allergy and cancer

With worsening air pollution brought by global social development, the prevalence of allergic diseases has increased dramatically in the past few decades. The novel Lck/yes-related protein tyrosine kinase (Lyn) belongs to the Src kinase family (SFK) and plays a pivotal role in the pathogenesis of inflammation, tumor, and allergy. This signaling molecule is vital in the IgE/FcεRI signaling pathway that regulates allergy. The Lyn-FcεRIβ interaction is essential for mast cell activation. The signaling pathway of Lyn has become the focus of immune, inflammatory, tumor, and allergy research. This molecule has positive and negative regulatory effects, which have attracted researchers' attention. This paper reviews the basic characteristics of Lyn and its regulatory mechanism and role in tumor and other diseases, specifically in allergies.

Recruitment of Lyn from endomembranes to the plasma membrane through calcium-dependent cell-cell interactions upon polarization of inducible Lyn-expressing MDCK cells

Src-family kinases, expressed in a wide variety of cell types, are anchored to cellular membranes through posttranslational lipid modifications and involved in diverse cellular signalling. In epithelial cells, Src-family kinases are localized at the plasma membrane and participate in epithelial functions. Epithelial cell polarity is achieved through dynamic reorganization of protein trafficking. To examine the trafficking of Src-family kinases between polarized and non-polarized epithelial cells, we generated an MDCK cell line that can inducibly express a protein of interest in a polarized state at any time. We show here that Lyn, a member of Src-family kinases, mainly localizes to the plasma membrane in polarized MDCK cells and to endomembranes in non-polarized MDCK cells. Cell-cell interactions between adjacent MDCK cells recruit Lyn from endomembranes to the plasma membrane even without cell attachment to extracellular matrix scaffolds, and loss of cell-cell interactions by calcium deprivation relocates Lyn from the plasma membrane to endomembranes through Rab11-mediated recycling. Therefore, using our MDCK cells expressing inducible Lyn, we reveal that calcium-dependent cell-cell interactions play a critical role in plasma membrane localization of Lyn in polarized MDCK cells.

Association of Lyn kinase with membrane rafts determines its negative influence on LPS-induced signaling

Bacterial lipopolysaccharide activates Toll-like receptor 4 (TLR4) and triggers proinflammatory reactions of macrophages. TLR4 signaling is negatively regulated by Lyn tyrosine kinase, provided the kinase accumulates in membrane rafts as a result of palmitoylation, the catalytic activity, and SH2- and SH3-mediated intermolecular interactions.

Lipopolysaccharide (LPS) is the component of Gram-negative bacteria that activates Toll-like receptor 4 (TLR4) to trigger proinflammatory responses. We examined the involvement of Lyn tyrosine kinase in TLR4 signaling of macrophages, distinguishing its catalytic activity and intermolecular interactions. For this, a series of Lyn-GFP constructs bearing point mutations in particular domains of Lyn were overexpressed in RAW264 macrophage-like cells or murine peritoneal macrophages, and their influence on LPS-induced responses was analyzed. Overproduction of wild-type or constitutively active Lyn inhibited production of TNF-α and CCL5/RANTES cytokines and down-regulated the activity of NFκB and IRF3 transcription factors in RAW264 cells. The negative influence of Lyn was nullified by point mutations of Lyn catalytic domain or Src homology 2 (SH2) or SH3 domains or of the cysteine residue that undergoes LPS-induced palmitoylation. Depending on the cell type, overproduction of those mutant forms of Lyn could even up-regulate LPS-induced responses, and this effect was reproduced by silencing of endogenous Lyn expression. Simultaneously, the Lyn mutations blocked its LPS-induced accumulation in the raft fraction of RAW264 cells. These data indicate that palmitoylation, SH2- and SH3-mediated intermolecular interactions, and the catalytic activity of Lyn are required for its accumulation in rafts, thereby determining the negative regulation of TLR4 signaling.

Protective role for lipid modifications of Src-family kinases against chromosome missegregation

Src-family tyrosine kinases, which are expressed in various cell types, play critical roles in cell signalling at the cytoplasmic side of the plasma membrane through their lipid modifications. Src-family kinases are cotranslationally myristoylated and posttranslationally palmitoylated in the amino-terminal region. The Src-family member Lyn contains a myristoylation site at glycine-2 and a palmitoylation site at cysteine-3, whereas c-Src has a myristoylation site at glycine-2 but not any palmitoylation sites. However, little is known about the role for lipid modifications of Src-family kinases in cell division. Here, we show that non-lipid-modified Lyn and c-Src, Lyn(G2A/C3A) and c-Src(G2A), are delocalized from membranes to the cytoplasm and the nucleus, which gives rise to a significant increase in the rate of chromosome missegregation, such as chromosome lagging and anaphase chromosome bridging, in a tyrosine kinase activity-dependent manner. Treatment with the Src inhibitor PP2 shows that the kinase activity of non-lipid-modified, non-membrane-bound Src during M phase is critical for giving rise to chromosome missegregation. Given that only a fraction of Src-family kinases fails in lipid modifications during biosynthesis, these results suggest that Src’s membrane anchorage through their lipid modifications from prophase to anaphase plays a protective role against induction of chromosome missegregation.

Two-Dimensional Crowding Uncovers a Hidden Conformation of α-Synuclein

The intrinsically disordered protein (IDP), α-synuclein (αS), is well-known for phospholipid membrane binding-coupled folding into tunable helical conformers. Here, using single-molecule experiments in conjunction with ensemble assays and a theoretical model, we present a unique case demonstrating that the interaction-folding landscape of αS can be tuned by two-dimensional (2D) crowding through simultaneous binding of a second protein on the bilayer surface. Unexpectedly, the experimental data show a clear deviation from a simple competitive inhibition model, but are consistent with a bimodal inhibition mechanism wherein membrane binding of a second protein (a membrane interacting chaperone, Hsp27, in this case) differentially inhibits two distinct modules of αS-membrane interaction. As a consequence, αS molecules are forced to access a hidden conformational state on the phospholipid bilayer in which only the higher-affinity module remains membrane-bound. Our results demonstrate that macromolecular crowding in two dimensions can play a significant role in shaping the conformational landscape of membrane-binding IDPs with multiple binding modes.

The SLE variant Ala71Thr of BLK severely decreases protein abundance and binding to BANK1 through impairment of the SH3 domain function

The B-lymphocyte kinase (BLK) gene is associated genetically with several human autoimmune diseases including systemic lupus erythematosus. We recently described that the genetic risk is given by two haplotypes: one covering several strongly linked single-nucleotide polymorphisms within the promoter of the gene that correlated with low transcript levels, and a second haplotype that includes a rare nonsynonymous variant (Ala71Thr). Here we show that this variant, located within the BLK SH3 domain, is a major determinant of protein levels. In vitro analyses show that the 71Thr isoform is hyperphosphorylated and promotes kinase activation. As a consequence, BLK is ubiquitinated, its proteasomal degradation enhanced and the average life of the protein is reduced by half. Altogether, these findings suggest that an intrinsic autoregulatory mechanism previously unappreciated in BLK is disrupted by the 71Thr substitution. Because the SH3 domain is also involved in protein interactions, we sought for differences between the two isoforms in trafficking and binding to protein partners. We found that binding of the 71Thr variant to the adaptor protein BANK1 is severely reduced. Our study provides new insights on the intrinsic regulation of BLK activation and highlights the dominant role of its SH3 domain in BANK1 binding.

Impact of the mitogen-activated protein kinase pathway on the subproteome of detergent-resistant microdomains of colon carcinoma cells

Lipid rafts play a key role in the regulation of fundamentally important cellular processes, including cell proliferation, differentiation, and survival. The composition of such detergent-resistant microdomains (DRMs) is altered under pathologic conditions, including cancer. Although DRMs have been analyzed in colorectal carcinoma little information exists about their composition upon treatment with targeted drugs. Hence, a quantitative proteomic profiling approach was performed to define alterations within the DRM fraction of colorectal carcinoma cells upon treatment with the drug U0126, an inhibitor of the mitogen-activated protein kinase pathway. Comparative expression profilings resulted in the identification of 300 proteins, which could partially be linked to key oncogenic signaling pathways and tumor-related cellular features, such as cell proliferation, adhesion, motility, invasion, and apoptosis resistance. Most of these proteins were downregulated upon inhibitor treatment. In addition, quantitative proteomic profilings of cholesterol-depleted versus intact lipid rafts were performed to define, which U0126-regulated target structures represent bona fide raft proteins. Selected differentially abundant raft proteins were validated at the mRNA and/or protein level using U0126- or Trametinib-treated cells. The presented data provide insights into the molecular mechanisms associated with the response to the treatment with MEK inhibitors and might also lead to novel candidates for therapeutic interventions.

Activation of Lyn tyrosine kinase through decreased membrane cholesterol levels during a change in its membrane distribution upon cell detachment

Cellular membranes, which can serve as scaffolds for signal transduction, dynamically change their characteristics upon cell detachment. Src family kinases undergo post-translational lipid modification and are involved in a wide range of signaling events at the plasma membrane, such as cell proliferation, cell adhesion, and survival. Previously, we showed the differential membrane distributions among the members of Src family kinases by sucrose density gradient fractionation. However, little is known about the regulation of the membrane distribution of Src family kinases upon cell detachment. Here, we show that cell detachment shifts the main peak of the membrane distribution of Lyn, a member of Src family kinase, from the low density to the high density membrane fractions and enhances the kinase activity of Lyn. The change in Lyn distribution upon cell detachment involves both dynamin activity and a decrease in membrane cholesterol. Cell detachment activates Lyn through decreased membrane cholesterol levels during a change in its membrane distribution. Furthermore, cholesterol incorporation decreases Lyn activity and reduces the viability of suspension cells. These results suggest that cell detachment-induced Lyn activation through the change in the membrane distribution of Lyn plays an important role in survival of suspension cells.

The Lyn kinase C-lobe mediates Golgi export of Lyn through conformation-dependent ACSL3 association

The Src-family tyrosine kinase Lyn has a role in signal transduction at the cytoplasmic face of the plasma membrane upon extracellular ligand stimulation. After synthesis in the cytoplasm, Lyn accumulates on the Golgi and is subsequently transported to the plasma membrane. However, the mechanism of Lyn trafficking remains elusive. We show here that the C-lobe of the Lyn kinase domain is associated with long-chain acyl-CoA synthetase 3 (ACSL3) on the Golgi in a manner that is dependent on Lyn conformation but is independent of its kinase activity. Formation of a closed conformation by CSK prevents Lyn from associating with ACSL3, resulting in blockade of Lyn export from the Golgi. Overexpression and knockdown of ACSL3 accelerates and blocks Golgi export of Lyn, respectively. The post-Golgi route of Lyn, triggered by ACSL3, is distinct from that of vesicular stomatitis virus glycoprotein (VSV-G) and of caveolin. Moreover, an ACSL3 mutant lacking the LR2 domain, which is required for the catalytic activity, retains the ability to associate with Lyn and accelerate Golgi export of Lyn. These results suggest that initiation of Golgi export of Lyn involves association of ACSL3 with the Lyn C-lobe, which is exposed to the molecular surface in an open conformation.