Purification, auto-activation and kinetic characterization of apoptosis signal-regulating kinase I.

Apoptosis signal-regulating kinase I (ASK1) is a mitogen-activated protein kinase kinase kinase (MAP3K) that activates the downstream MAP kinase kinases (MKKs) from two MAP kinase cascades: c-Jun N-terminal kinase (JNK) and p38. The essential physiological functions of ASK1 have attracted extensive attention. However, our understanding of the molecular mechanisms of ASK1, including the activation mechanism of ASK1 and the catalytic mechanism of ASK1-mediated MKK phosphorylation, remain unclear. The lack of purified ASK1 protein has hindered the elucidation of ASK1-initiated signal transduction mechanisms. Here, we report a one-step chromatography method for the expression and purification of functional full-length ASK1 from Escherichia coli. The purified ASK1 demonstrates auto-phosphorylation activity. The kinase activity of auto-phosphorylated ASK1 (pASK1) was also evaluated on two MKK substrates, MKK4 and 7, from the JNK cascades. Our results show that MKK7 can be phosphorylated by pASK1 more effectively than MKK4. The steady-state kinetic analysis demonstrates that MKK7 is a better ASK1 substrate than MKK4. These observations are further confirmed by direct pull-down assays which shows ASK1 binds MKK7 significantly stronger than MKK4. Furthermore, robust phospho-tyrosine signal is observed in MKK4 phosphorylation by pASK1 in addition to the phospho-serine and phospho-threonine. This study provides novel mechanistic and kinetic insights into the ASK1-initiated MAPK signal transduction via highly controlled reconstructed protein systems.

Production of recombinant human apoptosis signal-regulating kinase 1 (ASK1) in Escherichia coli.

Apoptosis signal-regulating kinase 1 (ASK1) is a mediator of the MAPK signaling cascade, which regulates different cellular processes including apoptosis, cell survival, and differentiation. The increased activity of ASK1 is associated with a number of human diseases and this protein kinase is considered as promising therapeutic target. In the present study, the kinase domain of human ASK1 was expressed in Escherichia coli (E. coli) in soluble form. The expression level of ASK1 was around 0.3-0.47 g per 1 L after using auto-induction protocol or IPTG induction. A one-step on column method for the efficient purification of recombinant ASK1 was performed. Our approach yields sufficient amount of recombinant ASK1, which can be used for inhibitor screening assays and different crystallographic studies.

Mechanism of oxidative stress-induced ASK1-catalyzed MKK6 phosphorylation.

Apoptosis signal-regulating kinase 1 (ASK1) is a serine/threonine kinase that responds to a plethora of stress-inducing signals. In turn, activation of ASK1 is associated with a number of human pathological conditions, including neurodegenerative disease, inflammation, and heart failure. In response to oxidative stress, ASK1 activates the cell death-associated p38 MAPK pathway by phosphorylating MKK6. Here, we investigated the regulation of oxidative stress-induced ASK1-catalyzed phosphorylation of MKK6. MKK6 phosphorylation levels increased immediately after H(2)O(2) treatment in intact cells and decreased following treatment for 30 min. When expressed in HEK293T cells, ASK1 was reproducibly purified within a high-molecular mass complex ( approximately 1500 kDa) known as the ASK1 signalosome. Measurement of the in vitro kinetic parameters revealed that the catalytic efficiency (k(cat)/K(m)) of ASK1 was 4000-fold greater in cells treated with H(2)O(2) for 3 min than in untreated cells. Interestingly, although the K(m(ATP)) values were found to be unchanged, the K(m(MKK6)) was dramatically decreased ( approximately 1000-fold). The increased affinity was specific for MKK6 and short-lived, as the K(m(MKK6)) returned to basal levels 30 min after treatment. Consistently, endogenous MKK6 was found within the ASK1 signalosome in intact cells and in addition copurified with ASK1 following treatment for 3 min. In contrast, proteins modulating ASK1 activity and degradation were found to interact with the ASK1 signalosome once MKK6 activation was completed. Taken together, these data suggest that oxidative stress rapidly increases ASK1 catalytic efficiency for MKK6 phosphorylation by increasing MKK6 binding affinity within the ASK1 signalosome prior to induction of inactivation and degradation of the complex.

Platelet-activating factor-induced clathrin-mediated endocytosis requires beta-arrestin-1 recruitment and activation of the p38 MAPK signalosome at the plasma membrane for actin bundle formation.

Clathrin-mediated endocytosis (CME) is a common pathway used by G protein-linked receptors to transduce extracellular signals. We hypothesize that platelet-activating factor (PAF) receptor (PAFR) ligation requires CME and causes engagement of beta-arrestin-1 and recruitment of a p38 MAPK signalosome that elicits distinct actin rearrangement at the receptor before endosomal scission. Polymorphonuclear neutrophils were stimulated with buffer or 2 microM PAF (1 min), and whole cell lysates or subcellular fractions were immunoprecipitated or slides prepared for colocalization and fluorescent resonance energy transfer analysis. In select experiments, beta-arrestin-1 or dynamin-2 were neutralized by intracellular introduction of specific Abs. PAFR ligation caused 1) coprecipitation of the PAFR and clathrin with beta-arrestin-1, 2) fluorescent resonance energy transfer-positive interactions among the PAFR, beta-arrestin-1, and clathrin, 3) recruitment and activation of the apoptosis signal-regulating kinase-1/MAPK kinase-3/p38 MAPK (ASK1/MKK3/p38 MAPK) signalosome, 4) cell polarization, and 5) distinct actin bundle formation at the PAFR. Neutralization of beta-arrestin-1 inhibited all of these cellular events, including PAFR internalization; conversely, dynamin-2 inhibition only affected receptor internalization. Selective p38 MAPK inhibition globally abrogated actin rearrangement; however, inhibition of MAPK-activated protein kinase-2 and its downstream kinase leukocyte-specific protein-1 inhibited only actin bundle formation and PAFR internalization. In addition, ASK1/MKK3/p38 MAPK signalosome assembly appears to occur in a novel manner such that the ASK1/p38 MAPK heterodimer is recruited to a beta-arrestin-1 bound MKK3. In polymorphonuclear neutrophils, leukocyte-specific protein-1 may play a role similar to fascin for actin bundle formation. We conclude that PAF signaling requires CME, beta-arrestin-1 recruitment of a p38 MAPK signalosome, and specific actin bundle formation at the PAFR for transduction before endosomal scission.