The extracellular signal-regulated kinase (ERK) cascade is a central pathway that transmits signals from many extracellular agents to regulate cellular processes such as proliferation, differentiation and cell cycle progression. The signaling via the ERK cascade is mediated by sequential phosphorylation and activation of protein kinases in the different tiers of the cascade. Although the main core phosphorylation chain of the cascade includes Raf kinases, MEK1/2, ERK1/2 (ERKs) and RSKs, other alternatively spliced forms and distinct components exist in the different tiers, and participate in ERK signaling under specific conditions. These components enhance the complexity of the ERK cascade and thereby, enable the wide variety of functions that are regulated by it. Another factor that is important for the dissemination of ERKs' signals is the multiplicity of the cascade's substrates, which include transcription factors, protein kinases and phosphatases, cytoskeletal elements, regulators of apoptosis, and a variety of other signaling-related molecules. About 160 substrates have already been discovered for ERKs, and the list of these substrates, as well as the function and mechanism of activation of representative substrates, are described in the current review. Many of these substrates are localized in the nucleus, and seem to participate in the regulation of transcription upon stimulation. However, other substrates are found in the cytosol as well as in other cellular organelles, and those are responsible for processes such as translation, mitosis and apoptosis. Understanding of these processes may provide a full picture of the distinct, and even opposing cellular processes that are regulated by the ERK cascade.

Kinase activity is known as the key biochemical property of MAPKs. Here, we report that ERK1/2 also utilizes its noncatalytic function to mediate certain signal transductions. Sustained activation of the Raf/MEK/ERK pathway induces growth arrest, accompanied by changes in cell cycle regulators (decreased retinoblastoma phosphorylation, E2F1 down-regulation, and/or p21 CIP1 up-regulation) and cell type-specific changes in morphology and expression of c-Myc or RET in the human tumor lines LNCaP, U251, and TT. Ablation of ERK1/2 by RNA interference abrogated all these effects. However, active site-disabled ERK mutants (ERK1-K71R, ERK2-K52R, and ERK2-D147A), which competitively inhibit activation of endogenous ERK1/2, could not block Raf/MEK-induced growth arrest as well as changes in the cell cycle regulators, although they effectively blocked phosphorylation of the ERK1/2 catalytic activity readouts, p90 RSK and ELK1, as well as the cell type-specific changes. Because this indicated a potential noncatalytic ERK1/2 function, we generated stable lines of the tumor cells in which both ERK1 and ERK2 were significantly knocked down, and we further investigated the possibility using rat-derived kinase-deficient ERK mutants (ERK2-K52R and ERK2-T183A/Y185F) that were not targeted by human small hairpin RNA. Indeed, ERK2-K52R selectively restored Raf-induced growth inhibitory signaling in ERK1/2-depleted cells, as manifested by regained cellular ability to undergo growth arrest and to control the cell cycle regulators without affecting c-Myc and morphology. However, ERK2-T183A/Y185F was less effective, indicating the requirement of TEY site phosphorylation. Our study suggests that functions of ERK1/2 other than its "canonical" kinase activity are also involved in the pathway-mediated growth arrest signaling. ERK12 and its homologue ERK2, the MAPK components of the Raf/MEK/ERK cascade of Ras signaling, are ubiquitously expressed serine/threonine kinases with more than 160 substrates identified to date (1). ERK1/2 interacts with a wide variety of proteins (2, 3). Upon phosphorylation by MEK1/2, the only known activator of ERK1/2, ERK1/2 phosphorylates transcription factors, other kinases, phosphatases, cytoskeletal proteins, scaffolds, receptors, and signaling components that mediate diverse cellular processes. Although kinase activity of ERK1/2 is central in activation or inactivation of these ERK targets, it was also reported that ERK, in an in vitro reaction, can mediate noncatalytic activation of DNA topoisomerase II␣, suggesting that ERK1/2 also has noncatalytic function (4). Nonetheless, the possibility that ERK1/2 has functions other than kinase has not yet been clearly addressed in cells.Many studies have shown that ERK1/2 signaling is pivotal in controlling cell survival and cell cycle progression (5). Constitutive activation of the MAPK cascade is also a central signature of many cancers with dysregulated Ras/Raf signaling (6, 7). Paradoxically, sustained activation of the Ras/Raf pathway induces growth arrest in p...

The Extracellular signal-regulated kinases (ERKs) are translocated into the nucleus in response to mitogenic stimulation. The mechanism of translocation and the residues in ERKs that govern this process are not clear as yet. Here we studied the involvement of residues in the activation loop of ERK2 in determining its subcellular localization. Substitution of residues in the activation loop to alanines indicated that residues 173-181 do not play a significant role in the phosphorylation and activation of ERK2. However, residues 176 -181 are responsible for the detachment of ERK2 from MEK1 upon mitogenic stimulation. This dissociation can be mimicked by substitution of residues 176 -178 to alanines and is prevented by deletion of these residues or by substitution of residues 179 -181 to alanines. On the other hand, residues 176 -181, as well as residues essential for ERK2 dimerization, do not play a role in the shuttle of ERK2 through nuclear pores. Thus, phosphorylation-induced conformational rearrangement of residues in the activation loop of ERK2 plays a major role in the control of subcellular localization of this protein.

Flipped learning is known to be an approach where learners take part in the learning process in a different way than in the non-flipped classroom and that in both they may deepen their knowledge and develop various competencies such as problem-solving and collaboration. Paying attention to the characteristics of flipped learning, the purpose of this study is to propose practical guidelines for the use of the flipped classroom by exploring the teaching and learning processes involved. Through a literature review, the 3 P model of teaching and learning was applied, and grit, professor support for fostering autonomy, learning engagement and perceived achievement were selected as variables for the process involved in learning via the flipped mode. In order to investigate the relationship between these variables, 121 students in a flipped classroom were surveyed. Data were analyzed by path analysis. The results showed that the support provided by the professor in terms of helping students to develop autonomy significantly predicted perceived achievement. Learning engagement mediated the relationship between grit and perceived achievement, and between the support given by the professor in terms of helping students to develop autonomy and perceived achievement. Based on the results, the specific strategies to enhance learners’ grit, their autonomy and learning engagement are discussed for the successful flipped classroom.

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