Strong increase in the tyrosine phosphorylation of actin upon inhibition of oxidative phosphorylation: correlation with reversible rearrangements in the actin skeleton of Dictyostelium cells

Formation of Cytoplasmic Actin-Cofilin Rods is Triggered by Metabolic Stress and Changes in Cellular pH

Actin dynamics plays a crucial role in regulating essential cell functions and thereby is largely responsible to a considerable extent for cellular energy consumption. Certain pathological conditions in humans, like neurological disorders such as Alzheimer’s disease or amyotrophic lateral sclerosis (ALS) as well as variants of nemaline myopathy are associated with cytoskeletal abnormalities, so-called actin-cofilin rods. Actin-cofilin rods are aggregates consisting mainly of actin and cofilin, which are formed as a result of cellular stress and thereby help to ensure the survival of cells under unfavorable conditions. We have used Dictyostelium discoideum, an established model system for cytoskeletal research to study formation and principles of cytoplasmic actin rod assembly in response to energy depletion. Experimentally, depletion of ATP was provoked by addition of either sodium azide, dinitrophenol, or 2-deoxy-glucose, and the formation of rod assembly was recorded by live-cell imaging. Furthermore, we show that hyperosmotic shock induces actin-cofilin rods, and that a drop in the intracellular pH accompanies this condition. Our data reveal that acidification of the cytoplasm can induce the formation of actin-cofilin rods to varying degrees and suggest that a local reduction in cellular pH may be a cause for the formation of cytoplasmic rods. We hypothesize that local phase separation mechanistically triggers the assembly of actin-cofilin rods and thereby influences the material properties of actin structures.

Calcium-dependent, non-apoptotic, large plasma membrane bleb formation in physiologically stimulated mast cells and basophils

Large membrane derangements in the form of non-detaching blebs or membrane protrusions occur in a variety of cell stress and physiological situations and do not always reflect apoptotic processes. They have been studied in model mast cells under conditions of cell stress, but their potential physiological relevance to mast cell function and formation in primary mast cells or basophils have not been addressed. In the current study, we examine the large, non-detaching, non-apoptotic, membrane structures that form in model and primary mast cells under conditions of stimulation that are relevant to allergy, atopy and Type IV delayed hypersensitivity reactions. We characterized the inflation kinetics, dependency of formation upon external free calcium and striking geometric consistency of formation for large plasma membrane blebs (LPMBs). We describe that immunologically stimulated LPMBs in mast cells are constrained to form in locations where dissociation of the membrane-associated cytoskeleton occurs. Mast cell LPMBs decorate with wheat germ agglutinin, suggesting that they contain plasma membrane (PM) lectins. Electrophysiological capacitance measurements support a model where LPMBs are not being formed from internal membranes newly fused into the PM, but rather arise from stretching of the existing membrane, or inflation and smoothing of a micro-ruffled PM. This study provides new insights into the physiological manifestations of LPMB in response to immunologically relevant stimuli and in the absence of cell stress, death or apoptotic pathways.

Reversible membrane pearling in live cells upon destruction of the actin cortex

Membrane pearling in live cells is observed when the plasma membrane is depleted of its support, the cortical actin network. Upon efficient depolymerization of actin, pearls of variable size are formed, which are connected by nanotubes of ~40 nm diameter. We show that formation of the membrane tubes and their transition into chains of pearls do not require external tension, and that they neither depend on microtubule-based molecular motors nor pressure generated by myosin-II. Pearling thus differs from blebbing. The pearling state is stable as long as actin is prevented from polymerizing. When polymerization is restored, the pearls are retracted into the cell, indicating continuity of the membrane. Our data suggest that the alternation of pearls and strings is an energetically favored state of the unsupported plasma membrane, and that one of the functions of the actin cortex is to prevent the membrane from spontaneously assuming this configuration.

Tyrosine phosphorylation of actin during microcyst formation and germination in Polysphondylium pallidum

High osmolarity causes amoebae of the cellular slime mould Polysphondylium pallidum to individually encyst, forming microcysts. During microcyst differentiation, actin is tyrosine phosphorylated. Tyrosine phosphorylation of actin is independent of encystment conditions and occurs during the final stages of microcyst formation. During microcyst germination, actin undergoes dephosphorylation prior to amoebal emergence. Renewed phosphorylation of actin in germinating microcysts can be triggered by increasing the osmolarity of the medium which inhibits emergence. Immunofluorescence reveals that actin is dispersed throughout the cytoplasm in dormant microcysts. Following the onset of germination, actin is observed around vesicles where it co-localizes with phosphotyrosine. Prior to emergence, actin localizes to patches near the cell surface. Increasing osmolarity disrupts this localization and causes actin to redistribute throughout the cytoplasm, a situation similar to that observed in dormant microcysts. The tyrosine phosphorylation state of actin does not appear to influence the long-term viability of dormant microcysts. Together, these results indicate an association between actin tyrosine phosphorylation, organization of the actin cytoskeleton, and microcyst dormancy.

The exocytic gene secA is required for Dictyostelium cell motility and osmoregulation

We investigated the link between cell movement and plasma membrane recycling using a fast-acting, temperature-sensitive mutant of the Dictyostelium SecA exocytic protein. Strikingly, most mutant cells become almost paralysed within minutes at the restrictive temperature. However, they can still sense cyclic-AMP (cAMP) gradients and polymerise actin up-gradient, but form only abortive pseudopodia, which cannot expand. They also relay a cAMP signal normally, suggesting that cAMP is released by a non-exocytic mechanism. To investigate why SecA is required for motility, we examined membrane trafficking in the mutant. Plasma membrane circulation is rapidly inhibited at the restrictive temperature and the cells acquire a prominent vesicle. Organelle-specific markers show that this is an undischarged contractile vacuole, and we found the cells are correspondingly osmo-sensitive. Electron microscopy shows that many smaller vesicles, probably originating from the plasma membrane, also accumulate at the restrictive temperature. Consistent with this, the surface area of mutant cells shrinks. We suggest that SecA mutant cells cannot move at the restrictive temperature because their block in exocytosis results in a net uptake of plasma membrane, reducing its area, and so restricting pseudopodial expansion. This demonstrates the importance of proper surface area regulation in cell movement.

Anaplasma phagocytophilum induces actin phosphorylation to selectively regulate gene transcription in Ixodes scapularis ticks

Anaplasma phagocytophilum, the agent of human anaplasmosis, persists in ticks and mammals. We show that A. phagocytophilum induces the phosphorylation of actin in an Ixodes ricinus tick cell line and Ixodes scapularis ticks, to alter the ratio of monomeric/filamentous (G/F) actin. A. phagocytophilum–induced actin phosphorylation was dependent on Ixodes p21-activated kinase (IPAK1)–mediated signaling. A. phagocytophilum stimulated IPAK1 activity via the G protein–coupled receptor Gβγ subunits, which mediated phosphoinositide 3-kinase (PI3K) activation. Disruption of Ixodes gβγ, pi3k, and pak1 reduced actin phosphorylation and bacterial acquisition by ticks. A. phagocytophilum–induced actin phosphorylation resulted in increased nuclear G actin and phosphorylated actin. The latter, in association with RNA polymerase II (RNAPII), enhanced binding of TATA box–binding protein to RNAPII and selectively promoted expression of salp16, a gene crucial for A. phagocytophilum survival. These data define a mechanism that A. phagocytophilum uses to selectively alter arthropod gene expression for its benefit and suggest new strategies to interfere with the life cycle of this intracellular pathogen, and perhaps other Rickettsia-related microbes of medical importance.

Expression of Y53A-actin in Dictyostelium disrupts the cytoskeleton and inhibits intracellular and intercellular chemotactic signaling

We showed previously that phosphorylation of Tyr(53), or its mutation to Ala, inhibits actin polymerization in vitro with formation of aggregates of short filaments, and that expression of Y53A-actin in Dictyostelium blocks differentiation and development at the mound stage (Liu, X., Shu, S., Hong, M. S., Levine, R. L., and Korn, E. D. (2006) Proc. Natl. Acad. Sci. U.S.A. 103, 13694-13699; Liu, X., Shu, S., Hong, M. S., Yu, B., and Korn, E. D. (2010) J. Biol. Chem. 285, 9729-9739). We now show that expression of Y53A-actin, which does not affect cell growth, phagocytosis, or pinocytosis, inhibits the formation of head-to-tail cell streams during cAMP-induced aggregation, although individual amoebae chemotax normally. We show that expression of Y53A-actin causes a 50% reduction of cell surface cAMP receptors, and inhibits cAMP-induced increases in adenylyl cyclase A activity, phosphorylation of ERK2, and actin polymerization. Trafficking of vesicles containing adenylyl cyclase A to the rear of the cell and secretion of the ACA vesicles are also inhibited. The actin cytoskeleton of cells expressing Y53A-actin is characterized by numerous short filaments, and bundled and aggregated filaments similar to the structures formed by copolymerization of purified Y53A-actin and wild-type actin in vitro. This disorganized actin cytoskeleton may be responsible for the inhibition of intracellular and intercellular cAMP signaling in cells expressing F-Y53A-actin.

Mutation of actin Tyr-53 alters the conformations of the DNase I-binding loop and the nucleotide-binding cleft

All but 11 of the 323 known actin sequences have Tyr at position 53, and the 11 exceptions have the conservative substitution Phe, which raises the following questions. What is the critical role(s) of Tyr-53, and, if it can be replaced by Phe, why has this happened so infrequently? We compared the properties of purified endogenous Dictyostelium actin and mutant constructs with Tyr-53 replaced by Phe, Ala, Glu, Trp, and Leu. The Y53F mutant did not differ significantly from endogenous actin in any of the properties assayed, but the Y53A and Y53E mutants differed substantially; affinity for DNase I was reduced, the rate of nucleotide exchange was increased, the critical concentration for polymerization was increased, filament elongation was inhibited, and polymerized actin was in the form of small oligomers and imperfect filaments. Growth and/or development of cells expressing these actin mutants were also inhibited. The Trp and Leu mutations had lesser but still significant effects on cell phenotype and the biochemical properties of the purified actins. We conclude that either Tyr or Phe is required to maintain the functional conformations of the DNase I-binding loop (D-loop) in both G- and F-actin, and that the conformation of the D-loop affects not only the properties that directly involve the D-loop (binding to DNase I and polymerization) but also allosterically modifies the conformation of the nucleotide-binding cleft, thus increasing the rate of nucleotide exchange. The apparent evolutionary "preference" for Tyr at position 53 may be the result of Tyr allowing dynamic modification of the D-loop conformation by phosphorylation (Baek, K., Liu, X., Ferron, F., Shu, S., Korn, E. D., and Dominguez, R. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 11748-11753) with effects similar, but not identical, to those of the Ala and Glu mutations.

Pulsed-laser creation and characterization of giant plasma membrane vesicles from cells

Femtosecond-pulsed laser irradiation was found to initiate giant plasma membrane vesicle (GPMV) formation on individual cells. Laser-induced GPMV formation resulted from intracellular cavitation and did not require the addition of chemical stressors to the cellular environment. The viscosity, structure, and contents of laser-induced GPMVs were measured with fluorescence microscopy and single-particle tracking. These GPMVs exhibit the following properties: (1) GPMVs grow fastest immediately after laser irradiation; (2) GPMVs contain barriers to free diffusion of incorporated fluorescent beads; (3) materials from both the cytoplasm and surrounding media flow into the growing GPMVs; (4) the GPMVs are surrounded by phospholipids, including phosphatidylserine; (5) F-actin is incorporated into the vesicles; and (6) caspase activity is not essential for GPMV formation. The effective viscosity of 65 nm polystyrene nanoparticles within GPMVs ranged from 32 to 434 cP. The nanoparticle diffusion was commonly affected by relatively large, macromolecular structures within the bleb.

Visualizing myosin-actin interaction with a genetically-encoded fluorescent strain sensor

Many proteins have been shown to undergo conformational changes in response to externally applied force in vitro, but whether the force-induced protein conformational changes occur in vivo remains unclear. To reveal the force-induced conformational changes, or strains, within proteins in living cells, we have developed a genetically encoded fluorescent "strain sensor," by combining the proximity imaging (PRIM) technique, which uses spectral changes of 2 GFP molecules that are in direct contact, and myosin-actin as a model system. The developed PRIM-based strain sensor module (PriSSM) consists of the tandem fusion of a normal and circularly permuted GFP. To apply strain to PriSSM, it was inserted between 2 motor domains of Dictyostelium myosin II. In the absence of strain, the 2 GFP moieties in PriSSM are in contact, whereas when the motor domains are bound to F-actin, PriSSM has a strained conformation, leading to the loss of contact and a concomitant spectral change. Using the sensor system, we found that the position of the lever arm in the rigor state was affected by mutations within the motor domain. Moreover, the sensor was used to visualize the interaction between myosin II and F-actin in Dictyostelium cells. In normal cells, myosin was largely detached from F-actin, whereas ATP depletion or hyperosmotic stress increased the fraction of myosin bound to F-actin. The PRIM-based strain sensor may provide a general approach for studying force-induced protein conformational changes in cells.

Modulation of actin structure and function by phosphorylation of Tyr-53 and profilin binding

On starvation, Dictyostelium cells aggregate to form multicellular fruiting bodies containing spores that germinate when transferred to nutrient-rich medium. This developmental cycle correlates with the extent of actin phosphorylation at Tyr-53 (pY53-actin), which is low in vegetative cells but high in viable mature spores. Here we describe high-resolution crystal structures of pY53-actin and unphosphorylated actin in complexes with gelsolin segment 1 and profilin. In the structure of pY53-actin, the phosphate group on Tyr-53 makes hydrogen-bonding interactions with residues of the DNase I-binding loop (D-loop) of actin, resulting in a more stable conformation of the D-loop than in the unphosphorylated structures. A more rigidly folded D-loop may explain some of the previously described properties of pY53-actin, including its increased critical concentration for polymerization, reduced rates of nucleation and pointed end elongation, and weak affinity for DNase I. We show here that phosphorylation of Tyr-53 inhibits subtilisin cleavage of the D-loop and reduces the rate of nucleotide exchange on actin. The structure of profilin-Dictyostelium-actin is strikingly similar to previously determined structures of profilin-beta-actin and profilin-alpha-actin. By comparing this representative set of profilin-actin structures with other structures of actin, we highlight the effects of profilin on the actin conformation. In the profilin-actin complexes, subdomains 1 and 3 of actin close around profilin, producing a 4.7 degrees rotation of the two major domains of actin relative to each other. As a result, the nucleotide cleft becomes moderately more open in the profilin-actin complex, probably explaining the stimulation of nucleotide exchange on actin by profilin.

The actinome of Dictyostelium discoideum in comparison to actins and actin-related proteins from other organisms

Actin belongs to the most abundant proteins in eukaryotic cells which harbor usually many conventional actin isoforms as well as actin-related proteins (Arps). To get an overview over the sometimes confusing multitude of actins and Arps, we analyzed the Dictyostelium discoideum actinome in detail and compared it with the genomes from other model organisms. The D. discoideum actinome comprises 41 actins and actin-related proteins. The genome contains 17 actin genes which most likely arose from consecutive gene duplications, are all active, in some cases developmentally regulated and coding for identical proteins (Act8-group). According to published data, the actin fraction in a D. discoideum cell consists of more than 95% of these Act8-type proteins. The other 16 actin isoforms contain a conventional actin motif profile as well but differ in their protein sequences. Seven actin genes are potential pseudogenes. A homology search of the human genome using the most typical D. discoideum actin (Act8) as query sequence finds the major actin isoforms such as cytoplasmic beta-actin as best hit. This suggests that the Act8-group represents a nearly perfect actin throughout evolution. Interestingly, limited data from D. fasciculatum, a more ancient member among the social amoebae, show different relationships between conventional actins. The Act8-type isoform is most conserved throughout evolution. Modeling of the putative structures suggests that the majority of the actin-related proteins is functionally unrelated to canonical actin. The data suggest that the other actin variants are not necessary for the cytoskeleton itself but rather regulators of its dynamical features or subunits in larger protein complexes.

An active Src kinase-β-actin association is linked to actin dynamics at the periphery of colon cancer cells

Src controls the dynamic actin cytoskeleton in fibroblasts and in cancer cells, although it is not known how direct its effects are. Using FRET/FLIM imaging, we found that wild type Src associates directly, or indirectly, with peripheral beta-actin at integrin adhesions after serum stimulation, and that an active Src kinase domain is essential. Beta-actin can be directly tyrosine-phosphorylated by Src in vitro, and in a Src-dependent manner in cells. Moreover, beta-actin dynamics are suppressed when Src is rendered kinase-inactive. Surprisingly, debilitating mutations in the Src SH2 or SH3 domains do not suppress association of Src with beta-actin. This may therefore be an example of a spatially regulated Src kinase/substrate interaction that is controlling peripheral actin dynamics. Interestingly, there is no FRET between Src and beta-actin at cadherin-mediated cell-cell contacts, despite apparent co-localization there, demonstrating precise spatial specificity of Src/beta-actin complexes.

Cracking up: symmetry breaking in cellular systems

The shape of animal cells is, to a large extent, determined by the cortical actin network that underlies the cell membrane. Because of the presence of myosin motors, the actin cortex is under tension, and local relaxation of this tension can result in cortical flows that lead to deformation and polarization of the cell. Cortex relaxation is often regulated by polarizing signals, but the cortex can also rupture and relax spontaneously. A similar tension-induced polarization is observed in actin gels growing around beads, and we propose that a common mechanism governs actin gel rupture in both systems.

Can electrons travel through actin microfilaments and generate oxidative stress in retinol treated Sertoli cell?

In early reports our research group has demonstrated that 7 microM retinol (vitamin A) treatment leads to many changes in Sertoli cell metabolism, such as up-regulation of antioxidant enzyme activities, increase in damage to biomolecules, abnormal cellular division, pre-neoplasic transformation, and cytoskeleton conformational changes. These effects were observed to be dependent on the production of reactive oxygen species (ROS), suggesting extra-nuclear (non-genomic) effects of retinol metabolism. Besides 7 microM retinol treatment causing oxidative stress, we have demonstrated that changes observed in cytoskeleton of Sertoli cells under these conditions were protective, and seem to be an adaptive phenomenon against a pro-oxidant environment resulting from retinol treatment. We have hypothesized that the cytoskeleton can conduct electrons through actin microfilaments, which would be a natural process necessary for cell homeostasis. In the present study we demonstrate results correlating retinol metabolism, actin architecture, mitochondria physiology and ROS, in order to demonstrate that the electron conduction through actin microfilaments might explain our results. We believe that electrons produced by retinol metabolism are dislocated through actin microfilaments to mitochondria, and are transferred to electron transport chain to produce water. When mitochondria capacity to receive electrons is overloaded, superoxide radical production is increased and the oxidative stress process starts. Our results suggested that actin cytoskeleton is essential to oxidative stress production induced by retinol treatment, and electrons conduction through actin microfilaments can be the key of this correlation.

Phosphorylation of actin Tyr-53 inhibits filament nucleation and elongation and destabilizes filaments

Dictyostelium actin was shown to become phosphorylated on Tyr-53 late in the developmental cycle and when cells in the amoeboid stage are subjected to stress but the phosphorylated actin had not been purified and characterized. We have separated phosphorylated and unphosphorylated actin and shown that Tyr-53 phosphorylation substantially reduces actin's ability to inactivate DNase I, increases actin's critical concentration, and greatly reduces its rate of polymerization. Tyr-53 phosphorylation substantially, if not completely, inhibits nucleation and elongation from the pointed end of actin filaments and reduces the rate of elongation from the barbed end. Negatively stained electron microscopic images of polymerized Tyr-53-phosphorylated actin show a variable mixture of small oligomers and filaments, which are converted to more typical, long filaments upon addition of myosin subfragment 1. Tyr-53-phosphorylated and unphosphorylated actin copolymerize in vitro, and phosphorylated and unphosphorylated actin colocalize in amoebae. Tyr-53 phosphorylation does not affect the ability of filamentous actin to activate myosin ATPase.

Staurosporine induces tyrosine phosphorylation in Dictyostelium discoideum proteins

The treatment of cells with staurosporine results in inhibition and less frequently activation of protein kinases, in a cell-type specific manner. In the social amoeba Dictyostelium discoideum, staurosporine induces marked changes in cell morphology affecting growth and development. Here we describe that incubation of D. discoideum growing or starved cells with staurosporine results in a rapid and unexpected tyrosine phosphorylation on two polypeptides of approximately 64 and approximately 62 kDa. These proteins emerge as novel substrates for tyrosine phosphorylation opening up new perspectives for the study of cell signalling in D. discoideum.

Differential display proteomic analysis of Picea meyeri pollen germination and pollen-tube growth after inhibition of actin polymerization by latrunculin B

To investigate roles of the actin cytoskeleton in growth of the pollen tube of Picea meyeri, we used the actin polymerization inhibitor latrunculin B (LATB) under quantitatively controlled conditions. At low concentrations, LATB inhibited polymerization of the actin cytoskeleton in the growing pollen tube, which rapidly inhibited tip growth. The proteomic approach was used to analyse protein expression-profile changes during pollen germination and subsequent pollen-tube development with disturbed organization of the actin cytoskeleton. Two-dimensional electrophoresis and staining with Coomassie Brilliant Blue revealed nearly 600 protein spots. A total of 84 of these were differentially displayed at different hours with varying doses of LATB, and 53 upregulated or downregulated proteins were identified by mass spectrometry. These proteins were grouped into distinct functional categories including signalling, actin cytoskeleton organization, cell expansion and carbohydrate metabolism. Moreover, actin disruption affected the morphology of Golgi stacks, mitochondria and amyloplasts, along with a differential expression of proteins involved in their functions. These findings provide new insights into the multifaceted mechanism of actin cytoskeleton functions and its interaction with signalling, cell-expansion machinery and energy-providing pathways.

Cortical actomyosin breakage triggers shape oscillations in cells and cell fragments

Cell shape and movements rely on complex biochemical pathways that regulate actin, microtubules, and substrate adhesions. Some of these pathways act through altering the cortex contractility. Here we examined cellular systems where contractility is enhanced by disassembly of the microtubules. We found that adherent cells, when detached from their substrate, developed a membrane bulge devoid of detectable actin and myosin. A constriction ring at the base of the bulge oscillated from one side of the cell to the other. The movement was accompanied by sequential redistribution of actin and myosin to the membrane. We observed this oscillatory behavior also in cell fragments of various sizes, providing a simplified, nucleus-free system for biophysical studies. Our observations suggest a mechanism based on active gel dynamics and inspired by symmetry breaking of actin gels growing around beads. The proposed mechanism for breakage of the actomyosin cortex may be used for cell polarization.

A novel partner for Dictyostelium filamin is an α-helical developmentally regulated protein

The filamins are a family of highly homologous actincrosslinking proteins that stabilize three-dimensional actin networks, link them to membrane proteins and direct intracellular signaling reactions to the actin scaffold through interaction with various binding partners. Here, we describe the first Dictyostelium filamin-interacting protein to be isolated - FIP, a 229.8 kDa protein with two α-helical coiled coil domains. FIP was identified in a yeast two-hybrid screen using the rod domain of filamin as bait. FIP can also be coimmunoprecipitated with filamin from cellular extracts. Deletion analysis located the interaction domain of FIP to a C-terminal region; by contrast, in filamin rods, repeats 2-4 interacted with the recombinant FIP protein. The 7 kb transcript of FIP is upregulated during early development. Monoclonal antibodies raised against a bacterially expressed FIP polypeptide recognize a 230 kDa developmentally regulated protein in western blots. Immunofluorescence analysis shows a punctate staining pattern in the cytosol and, in cell fractionation experiments, FIP is mainly found in the cytosolic fraction. A fusion protein composed of GFP and the C-terminal part localizes to the plasma membrane and is associated with the cytoskeleton. Expression of the fusion protein affects development and influences the size of the multicellular aggregates and the phototactic behavior of slugs. Thus, FIP might provide a candidate link between the dynamic actin cytoskeleton and signal transduction events during the multicellular stages of Dictyostelium amoebae.

Dictyostelium stress-activated protein kinase alpha, a novel stress-activated mitogen-activated protein kinase kinase kinase-like kinase, is important for the proper regulation of the cytoskeleton

Mitogen-activated protein kinase cascades regulate various cellular functions, including growth, cell differentiation, development, and stress responses. We have identified a new Dictyostelium kinase (stress-activated protein kinase [SAPK]alpha), which is related to members of the mixed lineage kinase class of mitogen-activated protein kinase kinases. SAPKalpha is activated by osmotic stress, heat shock, and detachment from the substratum and by a membrane-permeable cGMP analog, a known regulator of stress responses in Dictyostelium. SAPKalpha is important for cellular resistance to stresses, because SAPKalpha null cells exhibit reduced viability in response to osmotic stress. We found that SAPKalpha mutants affect cellular processes requiring proper regulation of the actin cytoskeleton, including cell motility, morphogenesis, cytokinesis, and cell adhesion. Overexpression of SAPKalpha results in highly elevated basal and chemoattractant-stimulated F-actin levels and strong aggregation and developmental defects, including a failure to polarize and chemotax, and abnormal morphogenesis. These phenotypes require a kinase-active SAPKalpha. SAPKalpha null cells exhibit reduced chemoattractant-stimulated F-actin levels, cytokinesis, developmental and adhesion defects, and a motility defect that is less severe than that exhibited by SAPKalpha-overexpressing cells. SAPKalpha colocalizes with F-actin in F-actin-enriched structures, including membrane ruffles and pseudopodia during chemotaxis. Although SAPKalpha is required for these F-actin-mediated processes, it is not detectably activated in response to chemoattractant stimulation.

Rapid patterning and zonal differentiation in a two-dimensionalDictyosteliumcell mass: the role of pH and ammonia

Recently it was demonstrated that a rapidly forming, self-organizing pattern that emerges within two-dimensional Dictyostelium discoideumcell cultures could later give rise to stripes of distinct zones, each comprising different cell types. Here we report physiological aspects of the initial rapid patterning and its relationship to cell differentiation. We found that as the temperature is lowered the characteristic length of the pattern increases. From this we estimated the activation energy of the patterning kinetics. Fluorescence of fluorescein-conjugated dextran revealed that the cytosolic pH of cells in the inside zone becomes lower than that in the outer zone facing the air. The patterning could be inhibited by addition of the plasma-membrane proton pump inhibitors diethystilbestrol (DES) or miconazole. Preincubation of cells with weak acid delayed the timing of the patterning, whereas weak base hastened it. A pH-indicating dye revealed localized accumulation of ammonia in the extracellular space. These results suggest that gradients of secreted metabolites may be directly responsible for the rapid patterning and its consequence on cell differentiation in a confined geometrical situation. Possible diffusible candidate molecules and a reaction scheme coupled to the imposed oxygen gradient are discussed.

Kinetics of binding, uptake and degradation of live fluorescent (DsRed) bacteria by Dictyostelium discoideum

The kinetics of binding, uptake and degradation of bacteria by vegetative Dictyostelium amoeba using Escherichia coli expressing the recombinant fluorescent protein DsRed have been characterized. There are significant advantages to using DsRed-expressing bacteria for phagocytosis assays. Stable expression of the fluorescent protein, DsRed, provides living bacteria with a bright internal fluorescent signal that is degradable in the phagolysosomal pathway. Unlike assays with chemically labelled bacteria or latex beads, the bacteria are alive and possess a natural, unaltered external surface for receptor interaction. Dictyostelium cells rapidly bind and phagocytose DsRed bacteria. Pulse-chase experiments show that the signal derived from DsRed is degraded with a half-life of approximately 45 min. To distinguish internalized bacteria from those bound to the surface, an assay was developed in which sodium azide was used to release surface-bound particles. Surprisingly, surface particle release appears to be independent of myosin II function. Using this assay it was shown that the uptake of bacteria into cells is extremely rapid. After 1 min incubation, 20% of the signal is derived from internalized bacteria. The proportion of the signal from internalized bacteria increases gradually and reaches 50% at steady state. This assay will be useful in investigations of the molecular machinery of phagocytosis and post-internalization vesicle trafficking.

S-Adenosyl-L-homocysteine hydrolase is sequestered into actin rods in Dictyostelium discoideum spores

Here we show evidence that S-adenosyl-L-homocysteine hydrolase (SAHH) is linked to the actin cytoskeleton. Actin rods formed in Dictyostelium discoideum spores during the final stage of development are structurally composed of novel bundles of actin filaments. SAHH only accumulates with actin at this stage of development in the life cycle of D. discoideum. Recently SAHH is believed to be a target for antiviral chemotherapy and the suppression of T cells. Our finding may contribute to designing novel antiviral and immunosuppressive drugs.

Rapid patterning in 2-D cultures of Dictyostelium cells and its relationship to zonal differentiation

Rapid patterning has been observed in confined 2-D cultures of Dictyostelium discoideum Ax-2 cells as an outer dark zone and a inner light zone. The width of outer zone was usually approximately100 microm, irrespective of the size of cell masses under atmospheric conditions. The width of the outer zone, however, changed depending on external O2 concentrations and reached up to 250 microm at 100% O2. A clear regional difference in tetramethyl rhodamine methyl ester (TMRM) staining was noticed between the outer zone and the inner zone: the inner zone was more strongly stained with TMRM than the outer zone, which faced the air. Using inhibitors of oxidative phosphorylation (dinitrophenol (DNP) or NaN3) and a specific inhibitor of CN-resistant respiration (benzohydroxamic acid (BHAM)), it has been demonstrated that the outer zone is basically formed by the O2 threshold for oxidative phosphorylation, while the inner cells mainly perform cyanide-resistant respiration. When cells around the early mound stage (just before prestalk and prespore differentiation) were cultured as 2-D cell masses, ecmA-expressing cells (pstA cells), ecmB-expressing cells (pstB cells) and D19-expressing cells (prespore; psp cells), arose in a position-dependent manner in the outer zone. In the inner zone, cell motility seemed to be markedly impaired and neither prestalk nor prespore differentiation occurred. In addition, once-differentiated prespore cells were found to dedifferentiate rapidly in the inner zone. The reason for dedifferentiation as well as for failure of cells to differentiate in the inner zone is discussed with reference to O2 radicals.

Glucose-induced pathways for actin tyrosine dephosphorylation during Dictyostelium spore germination

In the presence of germination signals, dormant spores of Dictyostelium discoideum rapidly germinate to start a new life cycle. Previously we have shown that half of the actin molecules in spores are maintained in a tyrosine-phosphorylated state, and a decline of the actin phosphorylation levels is a prerequisite for spore swelling. In this study, we have established d-glucose as a trigger molecule for the actin dephosphorylation. Present in a nutrient germination medium, d-glucose both may act as a trigger molecule and/or may serve as a substrate within a pathway for actin dephosphorylation depending upon spore age. However, the glucose-induced actin dephosphorylation was insufficient for spores to swell. Other factors in the nutrient medium were required for complete germination of young spores aged 1 to 5 days. In contrast, dispersion in nonnutrient buffer was necessary and sufficient for a decline of actin phosphorylation levels and even the emergence of amoebae in older spores (6 days and beyond). Moreover, the dephosphorylation pathway in the older spores was independent of energy production. We propose that the diversification of the actin dephosphorylation pathway may enable spores to increase their probability of germination upon spore aging.

DAip1, a Dictyostelium homologue of the yeast actin-interacting protein 1, is involved in endocytosis, cytokinesis, and motility

The 64-kD protein DAip1 from Dictyostelium contains nine WD40-repeats and is homologous to the actin-interacting protein 1, Aip1p, from Saccharomyces cerevisiae, and to related proteins from Caenorhabditis, Physarum, and higher eukaryotes. We show that DAip1 is localized to dynamic regions of the cell cortex that are enriched in filamentous actin: phagocytic cups, macropinosomes, lamellipodia, and other pseudopodia. In cells expressing green fluorescent protein (GFP)-tagged DAip1, the protein rapidly redistributes into newly formed cortical protrusions. Functions of DAip1 in vivo were assessed using null mutants generated by gene replacement, and by overexpressing DAip1. DAip1-null cells are impaired in growth and their rates of fluid-phase uptake, phagocytosis, and movement are reduced in comparison to wild-type rates. Cytokinesis is prolonged in DAip1-null cells and they tend to become multinucleate. On the basis of similar results obtained by DAip1 overexpression and effects of latrunculin-A treatment, we propose a function for DAip1 in the control of actin depolymerization in vivo, probably through interaction with cofilin. Our data suggest that DAip1 plays an important regulatory role in the rapid remodeling of the cortical actin meshwork.

Regulation of Dictyostelium protein-tyrosine phosphatase-3 (PTP3) through osmotic shock and stress stimulation and identification of pp130 as a PTP3 substrate

Osmotic shock and growth-medium stimulation of Dictyostelium cells results in rapid cell rounding, a reduction in cell volume, and a rearrangement of the cytoskeleton that leads to resistance to osmotic shock. Osmotic shock induces the activation of guanylyl cyclase, a rise in cGMP mediating the phosphorylation of myosin II, and the tyrosine phosphorylation of actin and the approximately 130-kDa protein (p130). We present data suggesting that signaling pathways leading to these different responses are, at least in part, independent. We show that a variety of stresses induce the Ser/Thr phosphorylation of the protein-tyrosine phosphatase-3 (PTP3). This modification does not alter PTP3 catalytic activity but correlates with its translocation from the cytosol to subcellular structures that co-localize to endosomal vesicles. This translocation is independent of PTP3 activity. Mutation of the catalytically essential Cys to a Ser results in inactive PTP3 that forms a stable complex with tyrosine-phosphorylated p130 (pp130) in vivo and in vitro, suggesting that PTP3 has a substrate specificity for pp130. The data suggest that stresses activate several interacting signaling pathways controlled by Ser/Thr and Tyr phosphorylation, which, along with the activation of guanylyl cyclase, mediate the ability of this organism to respond to adverse changes in the external environment.

Regulation of integrin-mediated adhesion during cell migration

Migrating cells form dynamic and highly regulated adhesive interactions with their environment. In particular, integrin-mediated adhesions to the extracellular matrix (ECM) play a central role in cell migration. This review focuses on recent advances in understanding the adhesive mechanisms that regulate cell detachment at the rear of migrating fibroblasts and neutrophils. The contribution of several key adhesive regulators is discussed, including myosin mediated cell contractility, tyrosine phosphorylation, rho, calcium fluxes, and calpain. A challenge for future investigation will be to determine how adhesive events are spatially and temporally coordinated to promote productive directional cell movements.

High levels of actin tyrosine phosphorylation: correlation with the dormant state of Dictyostelium spores

Upon removal of nutrients, the amoebae of the cellular slime mold Dictyostelium discoideum differentiate into dormant spores which survive starvation stress. In this study, we demonstrate that half of the actin molecules in the spores are tyrosine-phosphorylated. The phosphorylated actin is distributed around immobile crenate mitochondria and vesicles, as well as in the cytoplasm of the spores. The actin isolated from spore lysates contains phosphorylated and unphosphorylated forms at the same molar ratio as that of the original whole spore lysate. Under actin polymerizing conditions they form actin filaments and then they are completely depolymerized under actin depolymerizing conditions, indicating that tyrosine phosphorylation of actin may not prohibit actin polymerization nor stimulate depolymerization. The phosphorylation levels increase at the end of the culmination stage when spores have matured morphologically and physiologically, and reach maximum levels after an additional 12 hours of development. The levels are stable for 20 days following spore maturation, and decline to undetectable levels within the next 10 days. Spores having high levels of phosphorylation show high viability, and vice versa. Following activation of spores with nutrient medium containing spore germination promoters, the phosphorylation levels quickly decrease with a half-life of about 5 minutes. After 20 minutes spores begin to swell. At this later time, most of the phosphorylated actin already has been dephosphorylated. Also, in heat-activated spores actin dephosphorylation occurs prior to spore swelling. However, addition of phosphatase inhibitors following heat-activation, prevented spore swelling and dephosphorylation of actin. Our data indicate that the high levels of actin tyrosine phosphorylation, specific to the spore stage, may be required for maintaining dormancy to withstand starvation stress. The rapid dephosphorylation of actin leads to a reactivated dynamic actin system which participates in spore swelling, vesicle movement, and mitochondrial shape changes during the spore germination process.

Three-dimensional patterns and redistribution of myosin II and actin in mitotic Dictyostelium cells

Myosin II is not essential for cytokinesis in cells of Dictyostelium discoideum that are anchored on a substrate (Neujahr, R., C. Heizer, and G. Gerisch. 1997. J. Cell Sci. 110:123-137), in contrast to its importance for cell division in suspension (DeLozanne, A., and J.A. Spudich. 1987. Science. 236:1086-1091; Knecht, D.A., and W.F. Loomis. 1987. Science. 236: 1081-1085.). These differences have prompted us to investigate the three-dimensional distribution of myosin II in cells dividing under one of three conditions: (a) in shaken suspension, (b) in a fluid layer on a solid substrate surface, and (c) under mechanical stress applied by compressing the cells. Under the first and second conditions outlined above, myosin II does not form patterns that suggest a contractile ring is established in the furrow. Most of the myosin II is concentrated in the regions that flank the furrow on both sides towards the poles of the dividing cell. It is only when cells are compressed that myosin II extensively accumulates in the cleavage furrow, as has been previously described (Fukui, Y., T.J. Lynch, H. Brzeska, and E.D. Korn. 1989. Nature. 341:328-331), i.e., this massive accumulation is a response to the mechanical stress. Evidence is provided that the stress-associated translocation of myosin II to the cell cortex is a result of the dephosphorylation of its heavy chains. F-actin is localized in the dividing cells in a distinctly different pattern from that of myosin II. The F-actin is shown to accumulate primarily in protrusions at the two poles that ultimately form the leading edges of the daughter cells. This distribution changes dynamically as visualized in living cells with a green fluorescent protein-actin fusion.

Dictyostelium discoideum protein disulfide isomerase, an endoplasmic reticulum resident enzyme lacking a KDEL-type retrieval signal

The primary activity of protein disulfide isomerase (PDI), a multifunctional resident of the endoplasmic reticulum (ER), is the isomerization of disulfide bridges during protein folding. We isolated a cDNA encoding Dictyostelium discoideum PDI (Dd-PDI). Phylogenetic analyses and basic biochemical properties indicate that it belongs to a subfamily called P5, many members of which differ from the classical PDIs in many respects. They lack an intervening inactive thioredoxin module, a C-terminal acidic domain involved in Ca2+ binding and a KDEL-type retrieval signal. Despite the absence of this motif, the ER is the steady-state location of Dd-PDI, suggesting the existence of an alternative retention mechanism for P5-related enzymes.

Myosin II-independent processes in mitotic cells of Dictyostelium discoideum: redistribution of the nuclei, re-arrangement of the actin system and formation of the cleavage furrow

Mitosis was studied in multinucleated and mononucleated mutant cells of Dictyostelium discoideum that lack myosin II (Manstein et al. (1989) EMBO J. 8, 923–932). Multinucleated cells were produced by culture in suspension, mononucleated cells were enriched by growth on a solid surface (DeLozanne and Spudich (1987) Science 236, 1086–1091). The multinucleated cells disclosed interactions of mitotic complexes with the cell cortex that were not apparent in normal, mononucleated cells. During the anaphase stage, entire mitotic complexes consisting of spindle, microtubule asters, and separated sets of chromosomes were translocated to the periphery of the cells. These complexes were appended at a distance of about 3 microns from the cell surface, in a way that the spindle became orientated in parallel to the surface. Subsequently, lobes of the cell surface were formed around the asters of microtubules. These lobes were covered with tapered protrusions rich in coronin, an actin associated protein that typically accumulates in dynamic cell-surface projections (DeHostos et al. (1991) EMBO J. 10, 4097–4104). During their growth on a solid surface, mononucleated myosin II-null cells passed through the mitotic cleavage stages with a speed comparable to wild-type cells. Cytokinesis as linked to mitosis is distinguishable by several parameters from traction mediated cytofission, which results in the pinching off of pieces of a multinucleated cell in the interphase. The possibility is discussed that cells can cleave during mitosis without forming a contractile ring at the site of the cleavage furrow.

Classification of tyrosine kinases from Dictyostelium discoideum with two distinct, complete or incomplete catalytic domains

Two new kinases of Dictyostelium discoideum were identified by screening of a (lambda)gt11 expression library with a phosphotyrosine specific antibody. Amino-acid sequences derived from cDNA and genomic clones indicate that DPYK3 is a protein of 150 kDa and DPYK4, a protein of 75 kDa. The C-terminal fragments of each protein were produced in Escherichia coli and shown to be autocatalytically phosphorylated at tyrosine residues. A common feature of these kinases is the presence of two different sequence stretches in tandem that are related to kinase catalytic domains. The sequence relationships of DPYK3 and 4 to other protein kinases, and the positions of their catalytic domain sequences within the phylogenetic tree of protein kinases were analysed. Domains I of both kinases and domain II of DPYK3 constitute, together with the catalytic domains of two previously described tyrosine kinases of D. discoideum, a branch of their own, separate from the tyrosine kinase domains in sensu strictu. Domain II in DPYK4 is found on a different branch close to serine/threonine kinases.

Structure/function studies on the pH-dependent actin-binding protein hisactophilin in Dictyostelium mutants

Our previous studies have shown that the actin-binding protein hisactophilin from Dictyostelium discoideum is a candidate for organizing the actin cytoskeleton at the plasma membrane in a pH-dependent manner. To further characterize this interaction we isolated hisactophilin overexpression (hisII+) and hisactophilin minus (his-) mutants. D. discoideum contains two hisactophilin isoforms; both genes are independently transcribed and carry a short intron at the same position of the coding region. The deduced amino acid sequence of hisactophilin II showed a characteristic high content of 35 histidine residues out of a total 118 amino acids. After transformation of Dictyostelium AX2 wild-type cells with a genomic fragment designed to inactivate the hisactophilin I gene we obtained hisactophilin II overexpressing mutants (hisII+). Multiple integration of the vector led to strong overexpression of hisactophilin II which even outnumbered the actin concentration by a factor of two. Hisactophilin II protein showed the same biochemical properties as hisactophilin I during purification and in its pH-dependent binding to F-actin; as shown by mass spectrometry the hisactophilin II fraction was almost completely myristoylated despite of this high overexpression. The inactivation of both hisactophilin genes was achieved by gene replacement with a vector construct encompassing parts of gene I and gene II connected by a geneticin cassette. The properties of the hisII+ and his- cells with regard to growth in shaking culture and on Klebsiella plates, development, chemotaxis and morphology were not affected under normal conditions. However, the hisII+ transformants revealed a significant difference to wild-type cells and his- cells when the cytoplasmic pH was lowered by diethylstilbestrol (DES), a proton pump inhibitor. HisII+ cells were more resistant to the acidification; in contrast to AX2 wild-type cells and his- cells they did not form plasma membrane protrusions, showed an increase in F-actin content, and contained large clusters of F-actin. Lowering the internal pH caused an accumulation of hisactophilin below the plasma membrane. The fact that cells deficient in hisactophilin again lose resistance to acidification is in good agreement with the hypothesis that hisactophilin functions as a pH sensor at the plasma membrane by reversibly connecting the membrane with the actin cortical network upon local changes of the proton concentration.

Protection against osmotic stress by cGMP-mediated myosin phosphorylation

Conventional myosin functions universally as a generator of motive force in eukaryotic cells. Analysis of mutants of the microorganism Dictyostelium discoideum revealed that myosin also provides resistance against high external osmolarities. An osmo-induced increase of intracellular guanosine 3',5'-monophosphate was shown to mediate phosphorylation of three threonine residues on the myosin tail, which caused a relocalization of myosin required to resist osmotic stress. This redistribution of myosin allowed cells to adopt a spherical shape and may provide physical strength to withstand extensive cell shrinkage in high osmolarities.

NDP kinase can modulate contraction of Dictyostelium cytoskeletons

Extraction of Dictyostelium amoebae with Triton X-100 produces robust cytoskeletons composed mainly of actin and myosin II. These cytoskeletons rapidly contract when mixed with Mg-ATP in simple buffers. The Triton-soluble fraction was found to contain a GTP-dependent activity that prevented contraction by Mg-ATP. This activity was purified, and identified, as nucleoside diphosphate kinase (NDP kinase). The apparent inhibition resulted from pre-contraction of the cytoskeletons. Tightly bound cytoskeletal ADP was presumably phosphorylated, and the resulting ATP powered contraction. NDP kinase appeared to be unique in this capacity, since other regenerating systems did not cause pre-contraction. Reconstitution experiments demonstrated that the kinase must be in physical contact with the cytoskeleton. These results suggest that Dictyostelium NDP kinase is able to channel ATP to the myosin molecule, and this could play a role in directly regulating cytoskeletal contraction or in facilitating contraction under conditions where intracellular ATP concentrations are low. This ability to modulate cytoskeletal contraction could help to explain observations in other systems whereby defects in NDP kinase result in abnormal development or changes in the metastatic potential of cancer cells.

The Dictyostelium cytoskeleton

New avenues of cytoskeleton research in Dictyostelium discoideum have opened up with the cloning of the alpha- and beta-tubulin genes and the characterization of kinesins and cytoplasmic dynein. Much research, however, continues to focus on the actin cytoskeleton and its dynamics during chemotaxis, morphogenesis, and other motile processes. New actin-associated proteins are being identified and characterized by biochemical means and through isolation of mutants lacking individual components. This work is shedding light on the roles of specific actin assemblies in various biological processes.

Stress-induced tyrosine phosphorylation of actin in Dictyostelium cells and localization of the phosphorylation site to tyrosine-53 adjacent to the DNase I binding loop

Actin is known to be phosphorylated at tyrosine, serine, or threonine residues in various cells. In cells of Dictyostelium discoideum, a rise in the tyrosine phosphorylation of actin is observed in response to ATP depletion. An actin fraction rich in phosphotyrosine was obtained by chromatography on the weak anion exchanger Mono-P. Mass spectrometry and amino acid sequencing of protease cleavage products indicated that a single tyrosine residue was phosphorylated. Localization of this residue to position 53 of the actin sequence attributed the modification to a site that is critical for the capability of actin to polymerize. Induction of the tyrosine phosphorylation by heat shock and Cd2+ ions indicates that this modification of actin is implicated in the response of Dictyostelium cells to stress.

Tyrosine-phosphorylated T cell receptor zeta chain associates with the actin cytoskeleton upon activation of mature T lymphocytes

The multichain T cell antigen receptor (TCR) is composed of an antigen binding (alpha/beta) domain and associated signal-transducing complexes, the CD3 (gamma, delta, and epsilon) and the zeta chains. The zeta chain (TCR zeta) plays a key role in signal transduction. We show here that TCR ligation induces association of tyrosine-phosphorylated TCR zeta with the detergent-insoluble cell fraction. The microfilament poison, cytochalasin D, disrupts this association and enhances the coprecipitation of actin with TCR zeta after receptor ligation. This microfilament association is specific to TCR-associated polypeptides containing at least one intact immunoreceptor tyrosine-based activation motif (ITAM). Mapping studies using transfectants and chimeric TCR zeta chain constructs suggest that the third ITAM is necessary and sufficient for association, if the distal tyrosine is intact. This cytoskeletal association is directly correlated with IL-2 production, and ligation of TCR on immature thymocytes does not induce TCR zeta-cytoskeleton association. These data thus provide direct evidence of a developmentally regulated activation-dependent interaction between a lymphocyte antigen receptor and the actin cytoskeleton.

Chemoattractant-controlled accumulation of coronin at the leading edge of Dictyostelium cells monitored using a green fluorescent protein-coronin fusion protein

BACKGROUND

The highly motile cells of Dictyostelium discoideum rapidly remodel their actin filament system when they change their direction of locomotion either spontaneously or in response to chemoattractant. Coronin is a cytoplasmic actin-associated protein that accumulates at the coritcal sites of moving cells and contributes to the dynamics of the actin system. It is a member of the WD-repeat family of proteins and is known to interact with actin-myosin complexes. In coronin null mutants, cell locomotion is slowed down and cytokinesis is impaired.

RESULTS

We have visualized the redistribution of coronin by fluorescence imaging of motile cells that have been transfected with an expression plasmid containing the coding sequence of coronin fused to the sequence encoding the green fluorescent protein (GFP). This coronin-GFP fusion protein (GFP). This coronin-GFP fusion protein transiently accumulates in the front regions of growth-phase cells, reflecting the changing positions of leading edges and the competition between them. During the aggregation stage, local accumulation of coronin-GFP is biased by chemotactic orientation of the cells in gradients of cAMP. The impairment of cell motility in coronin null mutants shows that coronin has an important function at the front region of the cells. The mutant cells are distinguished by the formation of extended particle-free zones at their front regions, from where pseudopods often break out as blebs. Cytochalasin A reduces the size of these zones, indicating that actin filaments prevent entry of the particles.

CONCLUSIONS

These data demonstrate that coronin is reversibly recruited from the cytoplasm and is incorporated into the actin network of a nascent leading edge, where it participates in the reorganization of the cytoskeleton. Monitoring the dynamics of protein assembly using GFP fusion proteins and fluorescence microscopy promises to be a generally applicable method for studying the dynamics of cytoskeletal proteins in moving and dividing cells.

Epidermal growth factor induces serine phosphorylation of actin

Stimulation of cells by epidermal growth factor induces a rapid polymerisation of actin in the cortical skeleton. Activation of the EGF-receptor leads to autophosphorylation and to phosphorylation of specific intracellular substrates. Here we show that actin is phosphorylated in vitro and in vivo upon EGF stimulation. Two-dimensional phospho-amino acid analysis shows that phosphorylation occurs on serine, not on tyrosine residues.