Inter- and intramolecular interactions of highly purified Rous sarcoma virus-transforming protein, pp60v-src

Lck-dependent Fyn activation requires C terminus-dependent targeting of kinase-active Lck to lipid rafts

Mechanisms regulating the activation and delivery of function of Lck and Fyn are central to the generation of the most proximal signaling events emanating from the T cell antigen receptor (TcR) complex. Recent results demonstrate that lipid rafts (LR) segregate Lck and Fyn and play a fundamental role in the temporal and spatial coordination of their activation. Specifically, TcR-CD4 co-aggregation-induced Lck activation outside LR results in Lck translocation to LR where the activation of LR-resident Fyn ensues. Here we report a structure-function analysis toward characterizing the mechanism supporting Lck partitioning to LR and its capacity to activate co-localized Fyn. Using NIH 3T3 cells ectopically expressing FynT, we demonstrate that only LR-associated, kinase-active (Y505F)Lck reciprocally co-immunoprecipitates with and activates Fyn. Mutational analyses revealed a profound reduction in the formation of Lck-Fyn complexes and Fyn activation, using kinase domain mutants K273R and Y394F of (Y505F)Lck, both of which have profoundly compromised kinase activity. The only kinase-active Lck mutants tested that revealed impaired physical and enzymatic engagement with Fyn were those involving truncation of the C-terminal sequence YQPQP. Remarkably, sequential truncation of YQPQP resulted in an increasing reduction of kinase-active Lck partitioning to LR, in both fibroblasts and T cells. This in turn correlated with an ablation of the capacity of these truncates to enhance TcR-mediated interleukin-2 production. Thus, Lck-dependent Fyn activation is predicated by proximity-mediated transphosphorylation of the Fyn kinase domain, and targeting kinase-active Lck to LR is dependent on the C-terminal sequence QPQP.

Regulation of c-Src activity by the expression of wild-type v-Src and its kinase-dead double Y416F-K295N mutant

Active, wild-type v-Src and its kinase-dead double Y416F-K295N mutant were expressed in hamster fibroblasts. Expression of the active v-Src induced activation of endogenous c-Src and increased general protein-tyrosine phosphorylation in the infected cells. Expression of the kinase-dead mutant induced hypophosphorylation of Tyr416 of the endogenous c-Src. The inactivation of c-Src was reversible, as confirmed by in vitro kinase activity of c-Src immunoprecipitated from the kinase-dead v-Src-expressing cells. Both activation and inactivation of c-Src may be explained by direct interaction of the v-Src and c-Src that may either facilitate transphosphorylation of the regulatory Tyr416 in the activation loop, or prevent it by formation of transient dead-end complexes of the Y416F-K295N mutant with c-Src. The interaction was also indicated by co-localization of v- and c-Src proteins in immunofluorescent images of the infected cells. These results suggest that dimerization of Src plays an important role in the regulation of Src tyrosine kinase activity.

Role of the PH domain in regulating in vitro autophosphorylation events required for reconstitution of PDK1 catalytic activity

In addition to its catalytic domain, phosphoinsositide-dependent protein kinase-1 (PDK1) contains a C-terminal pleckstrin homology (PH) domain, which binds the membrane-bound phosphatidylinositol (3,4,5)-triphosphate [PI(3,4,5)P3] second messenger. Here, we report in vitro kinetic, phosphopeptide mapping, and oligomerization studies that address the role of the PH domain in regulating specific autophosphorylation events, which are required for PDK1 catalytic activation. First, 'inactive' unphosphorylated forms of N-terminal His6 tagged full length (His6-PDK1) and catalytic domain constructs [His6-PDK1(Delta PH)] were generated by treatment with Lambda protein phosphatase (lambda PP). Reconstitution of lambda PP-treated His6-PDK1(Delta PH) catalytic activity required activation loop Ser-241 phosphorylation, which occurred only upon trans-addition of 'active' PDK1 with an apparent bimolecular rate constant of (app)k1(S241) = 374+/-29 M(-1) s(-1). In contrast, full length lambda PP-treated His6-PDK1 catalyzed Ser-241 cis-autophosphorylation with an apparent first-order rate constant of (app)k1(S241) = (5.0+/-1.5) x 10(-4) s(-1) but remained 'inactive'. Reconstitution of lambda PP-treated His(6)-PDK1 catalytic activity occurred only when autophosphorylated in the presence of PI(3,4,5)P3 containing vesicles. PI(3,4,5)P3 binding to the PH domain activated apparent first-order Ser-241 autophosphorylation by 20-fold [(app)k1(S241) = (1.1+/-0.1) x 10(-2) s(-1)] and also promoted biphasic Thr-513 trans-autophosphorylation [(app)k2(T513) = (4.9+/-1.1) x 10(2) M(-1) s(-1) and(app)k3(T513) = (1.5+/-0.2) x 10(3) M(-1) s(-1)]. The results of mutagenesis studies suggest that Thr-513 phosphorylation may cause dissociation of autoinhibitory contacts formed between the contiguous regulatory PH and catalytic kinase domains.

In silico activation of Src tyrosine kinase reveals the molecular basis for intramolecular autophosphorylation

Structural data suggest that important hinge-bending motions of the two lobes that shape the catalytic domain of Src tyrosine kinase, together with reorganization of an alpha helix (helix C), are needed for the activation loop to adopt the catalytically competent conformation. The phosphorylation of a Tyr residue (Tyr-416) in this loop also seems to be essential for enzyme activation. However, no information is available about the dynamics of this activation process. By comparing the inactive and active forms of the catalytic domains of Src and Lck, another member of the Src family, we first identified a short stretch that can act as a hinge for the interlobe motion. The opening of the lobes was then simulated using a targeted molecular dynamics approach. The results obtained suggested that pulling the two lobes apart is not enough to induce the required conformational change in the activation loop. Rather unexpectedly, however, swinging of the lobes situated Tyr-416 in a suitable position for intramolecular autophosphorylation, and further simulation of Tyr-416-phosphorylated Src in the presence of ADP did then result in a conformational change that placed the activation loop in a position similar to that found in the active open conformation of Lck. Taken together, our results establish a physical link between intramolecular autophosphorylation and loop activation.

Spontaneous autophosphorylation of Lyn tyrosine kinase at both its activation segment and C-terminal tail confers altered substrate specificity

Two tyrosyl residues have been reported to play a crucial role in the regulation of protein tyrosine kinases of the Src family: autophosphorylation of Tyr416 (c-Src numbering) located in the catalytic domain correlates with enzyme activation, while Csk-mediated phosphorylation of the C-terminal tyrosine Tyr527 (c-Src numbering) gives rise to inactive forms of Src kinases. Here we show that the Src-related Lyn kinase undergoes spontaneous and stoichiometric autophosphorylation at both Tyr396 (homologous to c-Src Tyr416) and Tyr507 (homologous to c-Src Tyr527). Such a doubly autophosphorylated form of Lyn is hyperactive toward peptide substrates and insensitive to Csk-induced downregulation. In contrast, doubly autophosphorylated Lyn exhibits reduced activity toward protein substrates such as phospho-p50/HS1 (hematopoietic-lineage cell-specific protein) and p57/PDI (protein disulfide isomerase related protein), whose multiple sequential/processive phosphorylation relies on the accessibility of the SH2 domain of the kinase. These data disclose a novel conformation of Lyn that is catalytically active despite the presence of an intramolecular interaction between the phosphorylated tail and the SH2 domain. This enzyme conformation is expected to display a reduced oncogenic potential resulting from its defective recognition of a subset of protein substrates whose targeting is mediated by the Lyn SH2 domain.

Domain-specific stabilization of photoreceptor membrane guanylyl cyclase by adenine nucleotides and guanylyl cyclase activating proteins (GCAPs)

In photoreceptor outer segments, particulate guanylyl cyclase (RetGC) is stimulated at low intracellular Ca2+ concentrations by guanylyl cyclase activating protein (GCAP), a Ca2+-sensitive activator, to resynthesize light-depleted cGMP. In washed outer segment membranes, we find that GCAP-stimulable RetGC is rapidly inactivated at physiological temperatures (30-37 degrees C). Under the same conditions, RetGC remains competent for stimulation by S-100 protein preparations or Mn2+/Triton X-100, indicating that the cyclase catalytic domain remains functional. GCAPs and adenine nucleotides protect against inactivation. Protection by GCAPs is independent of Ca2+ concentration, suggesting that there is a Ca2+-independent interaction between GCAP and RetGC. Protection by ATP (EC50 = 150 microM) is not due to phosphorylation, since the nonhydrolyzable analogue adenylyl imidodiphosphate (AMP-PNP) protects equally well. In addition to their roles in protection, ATP and AMP-PNP also slowly stimulate cyclase activity. In parallel with the functional change in RetGC at physiological temperatures, we also observe a structural change. A 62-kDa intracellular fragment of RetGC-1 becomes more sensitive to cleavage by trypsin after preincubation at 30 degrees C unless ATP, AMP-PNP, or GCAP is present. Adenine nucleotides and GCAPs thus protect RetGC structurally, as well as functionally.

Regulation, substrates and functions of src

Src is the best understood member of a family of 9 tyrosine kinases that regulates cellular responses to extracellular stimuli. Activated mutants of Src are oncogenic. Using Src as an example, and referring to other Src family members where appropriate, this review describes the structure of Src, the functions of the individual domains, the regulation of Src kinase activity in the cell, the selection of substrates, and the biological functions of Src. The review concentrates on developments in the last 6-7 years, and cites data resulting from the isolation and characterization of Src mutants, crystallographic studies of the structures of SH2, SH3 and tyrosine kinase domains, biochemical studies of Src kinase activity and binding properties, and the biology of transgenic and knockout mouse strains.

Autophosphorylation of purified c-Src at its primary negative regulation site

The phosphorylating and transforming activities of c-Src are negatively regulated by phosphorylation at Tyr-527 near its carboxyl terminus. Previous studies have indicated that c-Src preferentially autophosphorylates Tyr-416, a residue in the middle of the catalytic domain, in vitro, and that Tyr-527 is phosphorylated by the carboxyl-terminal Src kinase, Csk. However, indirect evidence suggests that c-Src may also autophosphorylate Tyr-527 as part of a negative feedback loop. While some in vivo evidence suggests that Tyr-527 can be autophosphorylated in an intermolecular interaction, it has not previously been possible to directly demonstrate significant autophosphorylation in vitro. Here we show that c-Src purified from recombinant bacteria can autophosphorylate Tyr-527 to high levels in vitro when incubated with sufficiently high concentrations of ATP (KM(Mg2+/ATP) approximately equal to 20 microM) that are well above those that have been used previously. In vitro Tyr-527 autophosphorylation can occur both as an intra- and intermolecular interaction; higher enzyme concentrations are required for intermolecular Tyr-527 phosphorylation than for Tyr-416 autophosphorylation. These results support the possibility that, like G-proteins, c-Src can switch itself off in vivo by its own enzymatic activity.

Autophosphorylation: a salient feature of protein kinases

Most protein kinases catalyze autophosphorylation, a process which is generally intramolecular and is modulated by regulatory ligands. Either serine/threonine or tyrosine serves as the phosphoacceptor, and several sites on the same kinase subunit are usually autophosphorylated. Autophosphorylation affects the functional properties of most protein kinases. Members of the protein kinase family exhibit diversity in the characteristics and functions of autophosphorylation, but certain common themes are emerging.

Characterization of a GDP-sensitive phosphorylation in plasma membranes of D. discoideum

In a previous study, we reported the GDP-dependent phosphorylation of a 36 kD membrane protein, p36, in D. discoideum membranes prepared from starved (aggregation competent) cells (Anschutz et al., 1989). Here we show that p36 can be phosphorylated when membranes are supplied either ATP or GTP as the phosphate donor, but that a greater level of p36 phosphorylation is achieved with GTP. The rate of phosphorylation of p36, using either nucleotide triphosphate, is enhanced by GDP. This reflects a decrease in the apparent Km of the enzyme for the particular nucleotide triphosphate. p36 can also be phosphorylated in membranes prepared from vegetative cells. However, the ability of GDP to stimulate p36 phosphorylation is not observed in vegetative cell membranes. Competition experiments indicate that there are also developmental differences in the nucleotide triphosphate site(s) available to phosphorylate p36.

Isolation of a catalytically competent phosphorylated tyrosine kinase from Rous sarcoma virus-induced rat tumor by immunoadsorption to and hapten elution from phosphotyrosine binding antibodies

A procedure has been developed for the isolation of a catalytically competent phosphorylated tyrosine kinase (RSV Y-kinase) from avian sarcoma virus-induced rat tumors. The procedure involves reaction of partially purified RSV Y-kinase with ATP to effect tyrosyl phosphorylation of catalytically competent RSV Y-kinase. Tyrosyl phosphorylated RSV Y-kinase was isolated from the heterogeneous reaction mixture by immunoadsorption on immobilized phosphotyrosyl binding antibodies and elution with the hapten p-nitrophenyl phosphate. Estimation of the phosphate content of the purified phosphorylated RSV Y-kinase indicated that 1-3 tyrosyl groups had been phosphorylated upon reaction with ATP. The specific activity toward histone 2B of the purified phosphorylated RSV Y-kinase was at least 30-fold greater than that estimated for the RSV Y-kinase prepared previously by immunoadsorption on immobilized antiserum from tumor bearing rabbits.

Enzymatically inactive p60c-src mutant with altered ATP-binding site is fully phosphorylated in its carboxy-terminal regulatory region

Cellular src protein, p60c-src, is phosphorylated on tyrosine 527 in chicken embryo fibroblasts, and this phosphorylation is implicated in suppressing the protein-tyrosine kinase activity and transforming potential of p60c-src. To determine whether tyrosine 527 phosphorylation is dependent on p60c-src kinase activity, the ATP-binding site of chicken p60c-src was destroyed by substitution of lysine 295 with methionine. The resultant protein, p60c-src(M295), expressed either in chicken cells or in yeast, lacked detectable kinase activity. Nevertheless, tyrosine and serine phosphorylation of p60c-src(M295) overproduced in chicken cells were indistinguishable from that of authentic p60c-src. By contrast, p60c-src(M295) was not phosphorylated on tyrosine in yeast. These results suggest that a protein kinase present in chicken cells but not in yeast phosphorylates tyrosine 527 in trans, and are consistent with the possibility that this kinase is distinct from p60c-src.