Induction of apoptosis by SLK, a Ste20-related kinase

Maternal Nanos represses hid/skl-dependent apoptosis to maintain the germ line in Drosophila embryos

Nanos (Nos) is an evolutionarily conserved protein essential for the survival of primordial germ cells. In Drosophila, maternal Nos partitions into pole cells and suppresses apoptosis to permit proper germ-line development. However, how this critical event is regulated by Nos has remained elusive. Here, we report that Nos represses apoptosis of pole cells by suppressing translation of head involution defective (hid), a member of the RHG gene family that is required for Caspase activation. In addition, we demonstrate that hid acts in concert with another RHG gene, sickle (skl), to induce apoptosis. Expression of skl is induced in pole cells by maternal tao-1, a ste20-like serine/threonine kinase. Tao-1-dependent skl expression is required to potentiate hid activity. However, skl expression is largely suppressed in normal pole cells. Once the pole cells lack maternal Nos, Tao-1-dependent skl expression is fully activated, suggesting that skl expression is also restricted by Nos. These findings provide the first evidence that the germ line is maintained through the regulated expression of RHG genes.

Prostate-derived sterile 20-like kinase 1-alpha induces apoptosis. JNK- and caspase-dependent nuclear localization is a requirement for membrane blebbing

We have demonstrated previously that full-length prostate-derived sterile 20-like kinase 1-alpha (PSK1-alpha) binds to microtubules via its C terminus and regulates their organization and stability independently of its catalytic activity. Here we have shown that apoptotic and microtubule-disrupting agents promote catalytic activation, C-terminal cleavage, and nuclear translocation of endogenous phosphoserine 181 PSK1-alpha and activated N-terminal PSK1-alpha-induced apoptosis. PSK1-alpha, unlike its novel isoform PSK1-beta, stimulated the c-Jun N-terminal kinase (JNK) pathway, and the nuclear localization of PSK1-alpha and its induction of cell contraction, membrane blebbing, and apoptotic body formation were dependent on JNK activity. PSK1-alpha was also a caspase substrate, and the broad spectrum caspase inhibitor benzyloxycarbonyl-VAD-fluoromethyl ketone or mutation of a putative caspase recognition motif ((916)DPGD(919)) blocked nuclear localization of PSK1-alpha and its induction of membrane blebs. Additional inhibition of caspase 9 was needed to prevent cell contraction. PSK1-alpha is therefore a bifunctional kinase that associates with microtubules, and JNK- and caspase-mediated removal of its C-terminal microtubule-binding domain permits nuclear translocation of the N-terminal region of PSK1-alpha and its induction of apoptosis.

The 3′-untranslated region of the Ste20-like kinase SLK regulates SLK expression

Ste20-like kinase, SLK, a germinal center kinase found in kidney epithelial cells, signals to promote apoptosis. Expression of SLK mRNA and protein and kinase activity are increased during kidney development and recovery from ischemic acute renal failure. The 3′-untranslated region (3′-UTR) of SLK mRNA contains multiple adenine and uridine-rich elements, suggesting that 3′-UTR may regulate mRNA stability. This was confirmed in COS cell transient transfection studies, which showed that expression of the SLK open-reading frame plus 3′-UTR mRNA was reduced by 35% relative to the open-reading frame alone. To further characterize the SLK-3′-UTR, this nucleotide sequence was subcloned downstream of enhanced green fluorescent protein (EGFP) cDNA. In COS, 293T, and glomerular epithelial cells, expression of EGFP mRNA and protein was markedly reduced in the presence of the SLK-3′-UTR. After transfection and subsequent addition of actinomycin D, EGFP mRNA remained stable in cells for at least 6 h, whereas EGFP-SLK-3′-UTR mRNA decayed with a half-life of ∼4 h. A region containing five AUUUA motifs within the SLK-3′-UTR destabilized EGFP mRNA. Deletion of this region from the SLK-3′-UTR, in part, restored mRNA stability. By UV cross-linking and SDS-PAGE, the SLK-3′-UTR bound to protein(s) of ∼30 kDa in extracts of COS cells, glomerular epithelial cells, and kidney. Cotransfection of HuR (a RNA binding protein of ∼30 kDa) increased the steady-state mRNA level of EGFP-SLK-3′-UTR but not EGFP. Thus the SLK-3′-UTR may interact with kidney RNA-binding proteins to regulate expression of SLK mRNA during kidney development and after ischemic injury.

v-Src-dependent Down-regulation of the Ste20-like Kinase SLK by Casein Kinase II

We have previously shown that the Ste20-like kinase SLK is a microtubule-associated protein inducing actin stress fiber disassembly. Here, we show that v-Src expression can down-regulate SLK activity. This down-regulation is independent of focal adhesion kinase but requires v-Src kinase activity and membrane translocation. SLK down-regulation by v-Src is indirect and is accompanied by SLK hyperphosphorylation on serine residues. Deletion analysis revealed that casein kinase II (CK2) sites at position 347/348 are critical for v-Src-dependent modulation of SLK activity. Further studies show that CK2 can directly phosphorylate SLK at these positions and that inhibition of CK2 in v-Src-transformed cells results in normal kinase activity. Finally, CK2 and SLK can be co-localized in fibroblasts spreading on fibronectin-coated substrates, suggesting a mechanism whereby SLK may be regulated at sites of actin remodeling, such as membrane lamellipodia and ruffles, through CK2.

The ERK cascade: a prototype of MAPK signaling

Sequential activation of protein kinases within the mitogen-activated protein kinase (MAPK) cascades is a common mechanism of signal transduction in many cellular processes. Four such cascades have been elucidated thus far, and named according to their MAPK tier component as the ERK1/2, JNK, p38MAPK, and ERK5 cascades. These cascades cooperate in transmitting various extracellular signals, and thus control cellular processes such as proliferation, differentiation, development, stress response, and apoptosis. Here we describe the classic ERK1/2 cascade, and concentrate mainly on the properties of MEK1/2 and ERK1/2, including their mode of regulation and their role in various cellular processes and in oncogenesis. This cascade may serve as a prototype of the other MAPK cascades, and the study of this cascade is likely to contribute to the understanding of mitogenic and other processes in many cell lines and tissues.

The TNF receptor, RELT, binds SPAK and uses it to mediate p38 and JNK activation

Receptor expressed in lymphoid tissues (RELT) is a new member of the TNFR family with little known regarding its signaling. Typically, TNFRs engage TRAFs for activation of NF-kappaB and MAPK cascades. We found that RELT does not use the standard signaling pathways characteristic of other TNFRs. While overexpression of RELT in 293 cells induced p38 and JNK activation, it did not activate NF-kappaB. In addition, no binding of RELT to TRAF1,2,3,5, or 6 was detected. Using a yeast two-hybrid system, we identified a Ste20-related proline-alanine-rich kinase (SPAK) that binds RELT. Disruption of the SPAK binding motif, 349RFRV, in RELT inhibited RELT activation of p38 and JNK. In addition, a kinase-dead SPAK acted as an inhibitor of RELT signaling. Thus, we conclude that RELT does not rely on the canonical TRAF pathways for its function, but instead uses a kinase, SPAK, to mediate p38 and JNK activation.

Induction of Apoptosis by the Ste20-like Kinase SLK, a Germinal Center Kinase That Activates Apoptosis Signal-regulating Kinase and p38

Expression and activity of the germinal center kinase, Ste20-like kinase (SLK), are increased during kidney development and recovery from ischemic acute renal failure. In this study, we characterize the activation and functional role of SLK. SLK underwent dimerization via the C-terminal domain, and dimerization enhanced SLK activity. In contrast, the C-terminal domain of SLK did not dimerize with a related kinase, Mst1, and did not affect Mst1 activity. Phosphorylation/dephosphorylation of SLK were not associated with changes in kinase activity. SLK induced phosphorylation of apoptosis signal-regulating kinase-1 (ASK1) and increased ASK1 activity, indicating that ASK1 is a substrate of SLK. Moreover, SLK stimulated phosphorylation of p38 mitogen-activated protein kinase via ASK1, but not c-Jun N-terminal kinase nor extracellular signal-regulated kinase. Chemical anoxia and recovery during re-exposure to glucose (ischemia-reperfusion injury in cell culture) stimulated SLK activity. Overexpression of SLK enhanced anoxia/recovery-induced apoptosis, release of cytochrome c, and activities of caspase-8 and -9, and apoptosis was reduced significantly with p38 and caspase-9 inhibitors. Induction of the endoplasmic reticulum stress response by anoxia/recovery or tunicamycin (monitored by induction of Bip or Grp94 expression, phosphorylation of eukaryotic translation initiation factor 2alpha subunit, expression of CHOP, and activation of caspase-12) was attenuated in cells that overexpress SLK. Thus, SLK is an anoxia/recovery-dependent kinase that is activated via homodimerization and that signals via ASK1 and p38 to promote apoptosis. Attenuation of the protective aspects of the endoplasmic reticulum stress response by SLK may contribute to its proapoptotic effect.

The Ste20-like kinase SLK is required for cell cycle progression through G2

We have previously shown that the Ste20-like kinase SLK is a microtubule-associated protein that can regulate actin reorganization during cell adhesion and spreading (Wagner, S., Flood, T. A., O'Reilly, P., Hume, K., and Sabourin, L. A. (2002) J. Biol. Chem. 277, 37685-37692). Because of its association with the microtubule network, we investigated whether SLK plays a role in cell cycle progression, a process that requires microtubule dynamics during mitosis. Consistent with microtubule association in exponentially growing cells, our results showed that SLK co-localizes with the mitotic spindle in cells undergoing mitosis. Expression of a kinase-inactive mutant or SLK small interfering RNAs inhibited cell proliferation and resulted in an accumulation of quiescent cells stimulated to re-enter the cell cycle in the G2 phase. Cultures expressing the mutant SLK displayed a normal pattern of cyclin D, E, and B expression but failed to down-regulate cyclin A levels, suggesting that they cannot proceed through M phase. In addition, these cultures displayed low levels of both phospho-H3 and active p34/cdc2 kinase. Overexpression of active SLK resulted in ectopic spindle assembly and the induction of cell cycle re-entry of Xenopus oocytes, suggesting that SLK is required for progression through G2 upstream of H1 kinase activation.

Proteome-wide Identification of Cellular Targets Affected by Bisindolylmaleimide-type Protein Kinase C Inhibitors

Bisindolylmaleimide compounds such as GF109203X are potent inhibitors of protein kinase C (PKC) activity. Although bisindolylmaleimides are not entirely selective for PKC and are known to inhibit a few other protein kinases, these reagents have been extensively used to study the functional roles of PKC family enzymes in cellular signal transduction for more than a decade. Here, we establish a proteomics approach to gain further insights into the cellular effects of this compound class. Functional immobilization of suitable bisindolylmaleimide analogues in combination with the specific purification of cellular binding proteins by affinity chromatography led to the identification of several known and previously unknown enzyme targets. Subsequent in vitro binding and activity assays confirmed the protein kinases Ste20-related kinase and cyclin-dependent kinase 2 (CDK2) and the non-protein kinases adenosine kinase and quinone reductase type 2 as novel targets of bisindolylmaleimide inhibitors. As observed specifically for CDK2, minor chemical variation of the ligand by immobilizing the closely related bisindolylmaleimides III, VIII, and X dramatically affected target binding. These observed changes in affinity correlated with both the measured IC(50) values for in vitro CDK2 inhibition and results from molecular docking into the CDK2 crystal structure. Moreover, the conditions for affinity purification could be adapted in a way that immobilized bisindolylmaleimide III selectively interacted with either PKC alpha or ribosomal S6 protein kinase 1 only after activation of these kinases. Thus, we have established an efficient technique for the rapid identification of cellular bisindolylmaleimide targets and further demonstrate the comparative selectivity profiling of closely related kinase inhibitors within a cellular proteome.

Ste20-like kinase SLK displays myofiber type specificity and is involved in C2C12 myoblast differentiation

Cell growth and terminal differentiation are controlled by complex signaling cascades that regulate the expression of specific subsets of genes implicated in cell fate and morphogenic processes. We have recently cloned and characterized a novel Ste20-like kinase termed SLK that is associated with adhesion structures during cell adhesion and spreading. However, the specific function of SLK is poorly understood. To gain further insight into the role of SLK, we have characterized its activity, expression, and distribution in skeletal muscle and during the in vitro differentiation of C2C12 myoblasts. Although SLK is expressed ubiquitously in adult tissues, our results show that it is predominantly expressed in muscle masses during development. Furthermore, SLK activity is upregulated during the differentiation of C2C12 myoblasts. In addition, we have found that SLK localizes presynaptically at neuromuscular junctions and that it is preferentially expressed in types I and IIA myofibers at major myofibrillar striations. Supporting a role in myoblast function and differentiation, SLK expression is induced in Myf5- and Pax7-positive activated satellite cells during regeneration and expression of dominant negative SLK in C2C12 cultures impairs myoblast fusion, suggesting a role for SLK in muscle cell differentiation.

Renal expression and activity of the germinal center kinase SK2

Rat fetal kidney mRNA was analyzed by RT-PCR to identify protein kinases. This screening demonstrated expression of a protein kinase consistent with SK2, a group II germinal center kinase and homolog of human Ste20-like kinase (SLK). SK2 mRNA, protein expression, and kinase activity were increased in rat fetal kidney homogenates (embryonic days 17-21) compared with adult controls. In adult kidneys subjected to cross-clamping of the renal artery, followed by reperfusion, SK2 mRNA, protein expression, and kinase activity were increased compared with untreated contralateral controls. By immunohistochemistry, SK2 expression was evident mainly in the cytoplasm of tubular epithelial cells in fetal and adult kidneys. There was also some expression in developing and mature podocytes, but staining of the interstitium was negative. In cultured renal tubular epithelial cells, SK2 kinase activity was increased after incubation with serum, or after exposure to chemical anoxia plus reexposure to glucose. Stable overexpression of SLK reduced cell proliferation and increased apoptosis and exacerbated apoptosis and necrosis after chemical anoxia plus reexposure to glucose. Thus SK2 is a renal epithelial protein kinase whose expression and activity are increased during development and recovery from acute renal failure, where tubular epithelial regeneration may recapitulate developmental processes. The actions of SK2 appear to be antiproliferative and may facilitate cell injury.

PDZK1: I. A major scaffolder in brush borders of proximal tubular cells11See Editorial by Moe, p. 1916

BACKGROUND

In proximal tubular cells, PDZK1 (NaPi-Cap1) has been implicated in apical expression of the Na+-dependent phosphate cotransporter (NaPi-IIa) via interaction with its C-terminus. PDZK1 represents a multidomain protein consisting of four PDZ domains and thus is believed to have a broader specificity besides NaPi-IIa.

METHODS

We subjected single PDZ domains derived from PDZK1 either to yeast two-hybrid screens or yeast trap assays. Different pull-down assays and blot overlays were applied to corroborate the PDZK1-mediated interactions in vitro. Co-localization of interacting proteins with PDZK1 in proximal tubular cells was assessed by immunohistochemistry.

RESULTS

In the yeast screens, the most abundant candidate protein to interact with PDZK1 was the membrane-associated protein of 17 kD (MAP17). Besides MAP17, C-terminal parts of following transporters were also identified: NaPi-IIa, solute carrier SLC17A1 (NaPi-I), Na+/H+ exchanger (NHE-3), organic cation transporter (OCTN1), chloride-formate exchanger (CFEX), and urate-anion exchanger (URAT1). In addition, other regulatory factors were found among the clones, such as a protein kinase A (PKA)-anchoring protein (D-AKAP2) and N+/H+ exchanger regulator factor (NHERF-1). All interactions of itemized proteins with PDZK1 were affirmed by in vitro techniques. Apart from PDZK1, strong in vitro interactions of NHERF-1 were also observed with the solute transporters (excluding MAP17) and D-AKAP2. All identified proteins were immunolocalized in proximal tubular cells, wherein all membrane proteins co-localized with PDZK1 in brush borders.

CONCLUSION

We hypothesize that PDZK1 and NHERF-1 establish an extended network beneath the apical membrane to which membrane proteins and regulatory components are anchored.

Identification and characterization of a novel Ste20/germinal center kinase-related kinase, polyploidy-associated protein kinase

A novel protein kinase, polyploidy-associated protein kinase (PAPK), was isolated using a subtraction cDNA library approach from a mouse erythroleukemia cell line that had been induced to polyploidy after serum withdrawal. PAPK shares homology with members of the Ste20/germinal center kinase family of protein kinases and is ubiquitously expressed as two spliced forms, PAPK-A and PAPK-B, that encode for proteins of 418 and 189 amino acids, respectively. The expression of endogenous PAPK-A protein increased after growth factor withdrawal in murine hematopoietic and fibroblast cells. When tested in an in vitro kinase assay, PAPK-A was activated in response to the stress-inducing agent hydrogen peroxide and slightly by fetal calf serum. Biochemical characterization of the PAPK-A-initiated pathway revealed that this novel kinase does not affect MAP kinase activity but can stimulate both c-Jun N-terminal kinase 1 (JNK1) and ERK6/p38 gamma. The kinase activity of PAPK appears to be required for the activation of ERK6/p38 gamma but not JNK1. When an inducible construct of PAPK-A was expressed in stably transfected NIH3T3 cells, the cells exhibited distinct cytoskeletal changes and became resistant to apoptotic cell death induced by serum withdrawal, effects of PAPK that require its kinase activity. These data suggest that PAPK is a new member of the Ste20/germinal center kinase family that modulates cytoskeletal organization and cell survival.

Expression of the Ste20-like kinase SLK during embryonic development and in the murine adult central nervous system

Cell growth and terminal differentiation are controlled by complex signaling cascades that regulate the expression of specific subsets of genes controlling cell fate and morphogenic processes. We have recently cloned and characterized a novel Ste20-like kinase termed SLK (Sabourin et al., Mol. Cel. Biol. 20 (2000) 684). However, the specific function of SLK is poorly understood. To gain further insights into the role of SLK we have characterized its activity, expression and distribution in the CNS during embryonic development and in the adult brain. Although SLK is expressed ubiquitously in adult tissues, our results show that it is expressed preferentially in neuronal lineages during development. We find that SLK is preferentially expressed in the neurons and neuroepithelium of the developing embryo and can be detected at 10.5 and 12.5 days post-coitum (dpc) in the forebrain, midbrain and hindbrain of the developing CNS. At later stages (14.5 dpc), SLK is expressed in the hypothalamus region, all layers of the neural tube, dorsal root ganglion and in the proliferating ependymal layers. Surprisingly, following middle cerebral artery occlusion, SLK expressing neuronal cells are lost and SLK is localized to phagocytic macrophages/microglia. These results suggest a functional role for SLK in early neuronal development as well as in the adult CNS.

Association of the Ste20-like kinase (SLK) with the microtubule. Role in Rac1-mediated regulation of actin dynamics during cell adhesion and spreading

Cytoskeletal remodeling events are tightly regulated by signal transduction systems that impinge on adhesion components and modulators of cellular architecture. We have previously shown that the Ste20-like kinase (SLK) can induce apoptosis through the induction of actin disassembly and cellular retraction (Sabourin, L. A., Tamai, K., Seale, P., Wagner, J., and Rudnicki, M. A. (2000) Mol. Cell. Biol. 20, 684-696). Using immunofluorescence studies, we report that SLK is redistributed with adhesion components at large podosome-like adhesion sites in fibronectin-stimulated fibroblasts. However, in vitro kinase assays demonstrate that its activity is not modulated following fibronectin stimulation. Double immunofluorescence studies in exponentially growing or spreading fibroblasts show that SLK is associated with the microtubule network and can be coprecipitated with alpha-tubulin. Furthermore, the stimulation of adhesion site formation by microtubule-disrupting agents induces the relocalization of SLK with unpolymerized alpha-tubulin to large vinculin-containing adhesion complexes. Using microinjection studies, we show that ectopic expression of activated SLK induces the disassembly of actin stress fibers, a process that can be inhibited by dominant negative Rac1. Significantly, endogenous SLK can be colocalized with Rac1 in spreading cells on FN. Finally, the overexpression of SLK by adenoviral infection inhibits cell spreading on fibronectin. These results suggest that SLK is part of a microtubule-associated complex that is targeted to adhesion sites and implicated in the regulation of cytoskeletal dynamics.

Cloning of MASK, a novel member of the mammalian germinal center kinase III subfamily, with apoptosis-inducing properties

We have cloned a novel human GCK family kinase that has been designated as MASK (Mst3 and SOK1-related kinase). MASK is widely expressed and encodes a protein of 416 amino acid residues, with an N-terminal kinase domain and a unique C-terminal region. Like other GCK-III subfamily kinases, MASK does not activate any mitogen-activated protein kinase pathways. Wild type MASK, but not a form lacking the C terminus, exhibits homophilic binding in the yeast two-hybrid system and in coimmunoprecipitation experiments. Additionally, deletion of this C-terminal region of MASK leads to an increased kinase activity toward itself as well as toward an exogenous substrate, myelin basic protein. A potential caspase 3 cleavage site (DESDS) is present in the C-terminal region of MASK, and we show that MASK is cleaved in vitro by caspase 3. Finally, wild type and C-terminally truncated forms of MASK can both induce apoptosis upon overexpression in mammalian cells that is abrogated by CrmA, suggesting involvement of MASK in the apoptotic machinery in mammalian cells.

MST4, a new Ste20-related kinase that mediates cell growth and transformation via modulating ERK pathway

In this study, we report the cloning and characterization of a novel human Ste20-related kinase that we designated MST4. The 416 amino acid full-length MST4 contains an amino-terminal kinase domain, which is highly homologous to MST3 and SOK, and a unique carboxy-terminal domain. Northern blot analysis indicated that MST4 is highly expressed in placenta, thymus, and peripheral blood leukocytes. Wild-type but not kinase-dead MST4 can phosphorylate myelin basic protein in an in vitro kinase assay. MST4 specifically activates ERK but not JNK or p38 MAPK in transient transfected cells or in stable cell lines. Overexpression of dominant negative MEK1 or treatment with PD98059 abolishes MST4-induced ERK activity, whereas dominant-negative Ras or c-Raf-1 mutants failed to do so, indicating MST4 activates MEK1/ERK via a Ras/Raf-1 independent pathway. HeLa and Phoenix cell lines overexpressing wild-type, but not kinase-dead, MST4 exhibit increased growth rate and form aggressive soft-agar colonies. These phenotypes can be inhibited by PD98059. These results provide the first evidence that MST4 is biologically active in the activation of MEK/ERK pathway and in mediating cell growth and transformation.

Cloning and characterization of MST4, a novel Ste20-like kinase

MST4, a novel member of the germinal center kinase subfamily of human Ste20-like kinases, was cloned and characterized. Composed of a C-terminal regulatory domain and an N-terminal kinase domain, MST4 is most closely related to mammalian Ste20 kinase family member MST3. Both the kinase and C-terminal regulatory domains of MST4 are required for full activation of the kinase. Northern blot analysis indicates that MST4 is ubiquitously distributed, and the MST4 gene is localized to chromosome Xq26, a disease-rich region, by fluorescence in situ hybridization. Although some members of the MST4 family function as upstream regulators of mitogen-activated protein kinase cascades, expression of MST4 in 293 cells was not sufficient to activate or potentiate extracellular signal-regulated kinase, c-Jun N-terminal kinase, or p38 kinase. An alternatively spliced isoform of MST4 (MST4a) was isolated by yeast two-hybrid interaction with the catalytic domain of Raf from a human fetal brain cDNA library and also found in a variety of human fetal and adult tissues. MST4a lacks an exon encoding kinase subdomains IX-XI that stabilizes substrate binding. The existence of both MST4 isoforms suggests that the MST4 kinase activity is highly regulated, and MST4a may function as a dominant-negative regulator of the MST4 kinase.

MST, a physiological caspase substrate, highly sensitizes apoptosis both upstream and downstream of caspase activation

The human serine/threonine kinase, mammalian STE20-like kinase (MST), is considerably homologous to the budding yeast kinases, SPS1 and STE20, throughout their kinase domains. The cellular function and physiological activation mechanism of MST is unknown except for the proteolytic cleavage-induced activation in apoptosis. In this study, we show that MST1 and MST2 are direct substrates of caspase-3 both in vivo and in vitro. cDNA cloning of MST homologues in mouse and nematode shows that caspase-cleaved sequences are evolutionarily conserved. Human MST1 has two caspase-cleavable sites, which generate biochemically distinct catalytic fragments. Staurosporine activates MST either caspase-dependently or independently, whereas Fas ligation activates it only caspase-dependently. Immunohistochemical analysis reveals that MST is localized in the cytoplasm. During Fas-mediated apoptosis, cleaved MST translocates into the nucleus before nuclear fragmentation is initiated, suggesting it functions in the nucleus. Transiently expressed MST1 induces striking morphological changes characteristic of apoptosis in both nucleus and cytoplasm, which is independent of caspase activation. Furthermore, when stably expressed in HeLa cells, MST highly sensitizes the cells to death receptor-mediated apoptosis by accelerating caspase-3 activation. These findings suggest that MST1 and MST2 play a role in apoptosis both upstream and downstream of caspase activation.

The Ste20 group kinases as regulators of MAP kinase cascades

Ste20p (sterile 20 protein) is a putative yeast mitogen-activated protein kinase kinase kinase kinase (MAP4K) involved in the mating pathway. Its homologs in mammals, Drosophila, Caenorhabditis elegans and other organisms make up a large emerging group of protein kinases including 28 members in human. The Ste20 group kinases are further divided into the p21-activated kinase (PAK) and germinal center kinase (GCK) families. They are characterized by the presence of a conserved kinase domain and a noncatalytic region of great structural diversity that enables the kinases to interact with various signaling molecules and regulatory proteins of the cytoskeleton. This review describes the phylogenetic relationships of the Ste20 group kinases based on discussions with many researchers in this field. With the newly established phylogenetic relationships, crucial arguments can be advanced regarding the functions of these kinases as upstream activators of the MAPK pathways and possible activity as MAP4Ks. Their involvement in apoptosis, morphogenesis and cytoskeletal rearrangements is also discussed.

Life and death decisions: regulation of apoptosis by proteolysis of signaling molecules

Caspases are the major executioners of cell death, serving as molecular guillotines to behead many proteins required for maintenance of cellular homeostasis. Identification of caspase substrates has taken on increasing importance as we attempt to better understand the molecular mechanisms involved in regulating the struggle between life and death. Many caspase substrates have been described and include RNA binding proteins such as La and U1-70 kD, structural proteins such as keratin and nuclear lamins, and transcription factors or their regulatory proteins that include IkappaB, SP1, and SREBP. Kinases and other signaling proteins are perfectly suited to regulate life and death decisions in response to cellular stressors and have only recently been identified as important caspase substrates. Here we review the current status of signaling pathways that are activated, inactivated or dysregulated by proteases such as caspases and calpain to control entry into apoptosis. The emerging concept that some caspase pathways may be inhibited by cellular and viral apoptosis inhibitory proteins while other caspase pathways are preserved suggests that a subset of these kinases may exist as cleaved 'isoforms' in cells that are not destined to perish. By acting as executioners and as important 'molecular sensors' of the degree of cellular injury, the signaling proteins described in this review are strong candidates to mediate downstream events, both in condemned and in viable cells.