Rony Seger

The mitogen-activated protein kinase activator

The mitogen-activated protein kinase appears to be regulated by another growth factor regulated kinase, the mitogen-activated protein kinase activator. In the past year, much progress has been made in purifying and characterizing the mitogen-activated protein kinase activator, in determining its primary structure, and in identifying another protein kinase that may function upstream to regulate its activity.

The MAP kinase cascade.

Publisher Summary The discovery of a number of hormones and growth factors with receptors that are protein tyrosine kinases generated interest in discovering the pathway by which these signals are transmitted from these receptors to various parts of the cell. This chapter provides an overview of the two broad approaches that were employed to discover the pathway: (1) upstream approach and (2) downstream approach. The upstream approach shows that partially purified microtubule associated protein 2 (MAP-2) protein kinase obtained from insulin stimulates 3T3-L1 cells can phosphorylate and reactive a homogeneous phosphatase-treated S6 kinase. MAP-2 kinase, in essence a S6 kinase kinase, also appears to be regulated by serine/threonine phosphorylation because protein phosphatise 2A (PP2A) treatment inactivatsed the enzymatic activity. The chapter presents a downstream approach, which suggests that MAPKK could bind directly to p21 ras or indirectly through another protein and also MAPKK interacts with the Raf-1 C-terminal catalytic domain. These findings suggest that p21 ras and that MAPKK can form a complex with Raf-1 by binding to opposite ends of the protein. This chapter also discusses the features of MAP kinase cascade such as, inactivation of the cascade, functions and branch points, and cross talk with other kinases involved in signal transduction.

Identification of an activator of the microtubule-associated protein 2 kinases ERK1 and ERK2 in PC12 cells stimulated with nerve growth factor or bradykinin.

Treatment of PC12 pheochromocytoma cells with nerve growth factor (NGF) or bradykinin leads to the activation of extracellular signal‐regulated kinases ERK1 and ERK2, two isozymes of microtubule‐associated protein 2 (MAP) kinase that are present in numerous cell lines and regulated by diverse extracellular signals. The activation of MAP kinase is associated with its phosphorylation on tyro‐sine and threonine residues, both of which are required for activity. In the present studies, we have identified a factor in extracts of PC12 cells treated with NGF or bradykinin, named MAP kinase activator, that, when reconstituted with inactive MAP kinase from untreated cells, dramatically increased MAP kinase activity. Activation of MAP kinase in vitro by this factor required MgATP and was associated with the phosphorylation of a 42‐ (ERK1) and 44‐kDa (ERK2) polypeptide. Incorporation of 32P into ERK1 and ERK2 occurred primarily on tyrosine and threonine residues and was associated with a single tryptic peptide, which is identical to one whose phosphorylation is increased by treatment of intact PC12 cells with NGF. Thus, the MAP kinase activator identified in PC12 cells is likely to be a physiologically important intermediate in the signaling pathways activated by NGF and bradykinin. Moreover, stimulation of the activator by NGF and bradykinin suggests that tyrosine kinase receptors and guanine nucleotide‐binding protein‐coupled receptors are both capable of regulating these pathways.