Signaling by Dual Specificity Kinases
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Oncogene (1998) 17, 1447 ± 1455 1998 Stockton Press All rights reserved 0950 ± 9232/98 $12.00 http://www.stockton-press.co.uk/onc Signaling by dual speci®city kinases N Dhanasekaran and E Premkumar Reddy Fels Institute for Cancer Research and Molecular Biology and Department of Biochemistry Temple University School of Medicine Philadelphia, Pennsylvania 19140, USA Dual speci®city kinases that phosphorylate the Thr- and residues in their amino acid sequences rather than Tyr-residues within the TXY motif of MAP-kinases of those of the exogenous substrates initially identi®ed play a central role in the regulation of various processes most of the dual speci®city kinases. An earlier of cell growth. These dual speci®city kinases also known classi®cation of these kinases was based on their as MAP kinase kinases are constituents of the sequential ability to dual-phosphorylate the exogenous substrates kinase signaling modules. Seven distinct mammalian (Lindberg et al., 1992) and it divided these kinases into MAP kinases kinases have been identi®ed. Some of the three groups: (1) kinases that show `true' dual unique signaling properties of these kinases are discussed speci®city by phosphorylating Tyr- and Thr-residues here. of exogenous substrates; (2) kinases that exhibit dual speci®city only through autophosphorylation; and (3) Keywords: oncogene; apoptosis; cell transformation; kinases that possess the structural motif characteristic MAP kinase; MEKK; MEK; MKK; ERK; JNK; of dual speci®city kinases. However, it has been signal transduction observed that as the kinases belonging to group two and three get fully characterized, they turn out to be the `true dual speci®city kinases' of group one. Of the dierent dual speci®city kinases that have been Introduction identi®ed to date, the kinases that phosphorylate the family of proline directed protein kinases are of great Regulation of cell growth is mediated by a complex interest. In a prototypical kinase-signaling module, the array of signaling pathways precisely coordinated by kinase-core consists of a minimum of three kinases, an dierent families of cell surface receptors. These upstream Ser/Thr kinase, middle dual speci®city signaling pathways regulate all the critical phases of kinases and a downstream Ser/Thr kinase. An cell growth including cell proliferation, dierentiation, activated receptor stimulates a Ser/Thr kinase via Ras and apoptosis. In many instances, the signal coupling or Rho family of GTPases. The upstream kinases, in between the receptors to the nucleus is mediated by a turn stimulate the dual speci®city kinase through series of sequentially activating protein kinases. phosphorylation. Consequently, the phosphorylated Protein kinases add a phosphate group to speci®c DSK activates the downstream Ser/Thr kinase, which amino acid residues. The alcoholic group of serine and contain the TXY motif by dual phosphorylation at the threonine as well as the phenolic group of tyrosine Thr- and Tyr-residues (Figure 1). Such dual speci®city provide the major phosphorylation-sites for many of kinase kinase-dual speci®city kinase-TXY kinase these kinases. Depending on the speci®city of the (DSKK-DSK-TXY) signaling module appears to be substrate amino acids, the protein kinases are either well-conserved in eukaryotes ranging from yeast to known as serine/threonine (Ser/Thr) kinases or mammalian cells (Davis, 1994; Johnson and Vaillan- tyrosine kinases (Tyr). However, a third group of court, 1994; Cano and Mahadevan, 1995; Fanger et al., protein kinases that can phosphorylate both Ser/Thr- 1997). To date, seven such signaling modules have been as well as Tyr-residues has been identi®ed and the identi®ed in mammalian cells (Figure 1). Based on the kinases that belong to this group are known as dual number of newly discovered kinases that can speci®city kinases. Such dual-speci®city kinases play a `communicate' to these kinase-modules, it is likely central role in the `kinase cascade' that regulate cell that many more such `modules' are remaining to be proliferation, dierentiation, and apoptosis. The focus identi®ed in mammalian cells. of the review is to summarize the present state of Since a considerable degree of confusion persists knowledge of the signaling modules regulated by these regarding the nomenclature of these dual speci®city dual speci®c kinases. kinases, a clari®cation of the nomenclature is in order. The dual speci®city kinase that phosphorylates ERK is known as either MEK (Mitogen/Extracellular-signal Superfamily of dual-speci®city kinases regulated kinase kinase) or simply MAP kinase kinase The dual speci®city kinases (DSK) are unique in that (MKK). Thus MEK1/MKK1 and MEK2/MKK2 they share the consensus kinases motifs of both Ser/ denotes the dual speci®city kinase that phosphorylate Thr and Tyr-kinases (Lindberg et al., 1992). Their ERK1 and ERK2 respectively (Ahn et al., 1991; ability to autophosphorylate Ser/Thr- as well as Tyr- Crews et al., 1992; Zheng and Guan, 1993). In some instances, as in the case of a Xenopus homolog, MAP kinase kinase (MAPKK) also refers to the kinase that activates ERKs (Matsuda et al., 1992). MKK3 has been used to denote the dual speci®city kinase that Correspondence: N Dhanasekaran phosphorylate and activate p38MAPK (Derijard et al., Dual Specificity Kinases N Dhanasekaran and EP Reddy 1448 Figure 1 The mammalian kinase signaling modules. The seven known mammalian signaling modules involving the dual speci®city kinases are compared. The question marks denote the signaling components that remain to be established 1995). MKK4 refers to the dual speci®city kinase that MAP kinase kinase-1 and MAP kinase kinase-2 phosphorylates JNK (Derijard et al., 1995), and it is also known as JNK kinase I or abbreviated as In a typical signaling pathway involving tyrosine kinase JNKK1 (Lin et al., 1995). Since JNK is also known receptors, the ligand binding to the receptor leads to as stress activated protein kinase (SAPK), a mouse receptor-dimerization and subsequent autophosphory- homolog of MKK4/JNKK is known as SAPK/ERK lation. The phosphorylated tyrosine residues of the kinase-1 or SEK1 (Sanchez et al., 1994). The dual receptor act as a docking bay for various signaling speci®city kinase that phosphorylate ERK5 is known proteins that get activated by phosphorylation. One as MEK5 or MKK5 (Zhou et al., 1995; Tournier et such eector molecule that link the activated EGFR to al., 1997). The recently identi®ed dual speci®city Ras-signaling pathway is Shc. Phosphorylation of Shc kinase that speci®cally involved in the regulation of leads to its interaction with GRB2 through the SH2 p38MAPK is known as MKK6 (Raingeaud et al., domains. The EGFR-Shc-GRB2 complex recruits a 1996; Moriguchi et al., 1996; Han et al., 1996; Stein et guanine nucleotide exchange factor SOS which al., 1996). Likewise, newly characterized dual specifi- catalyzes the exchange of guanine nucleotides in Ras. city kinase that speci®cally activates JNK through The GTP-bound Ras facilitates the translocation of the phosphorylation is named MKK7 (Moriguchi et al., Ser/Thr kinase Raf-1 to the plasma membrane for 1997; Yao et al., 1997; Tournier et al., 1997; Wu et activation through phosphorylation. The activated al., 1997). It has been also referred as JNKK2 (Wu et Raf-1 stimulates the dual speci®city kinase MKK1 al., 1997). For the sake of clarity and conformity, only and MKK2 through the phosphorylation of Ser-217/ the MKK-nomenclature is being used here. 218 and Ser-221. MKK1 and MKK2 in turn activate It has been identi®ed that both the Thr- as well their respective ERKs through phosphorylation of as Tyr- phosphorylating activities of the dual speci®c Thr- and Tyr-residues within the characteristic speci®city kinases are required for the signaling of TPY motif. The phosphorylated ERKs activate other the respective signaling modules. The structure and signaling proteins such as cPLA2 (Nemeno et al., function relationship of these kinases has been 1993; Lin et al., 1993) as well as other kinases (Boulton widely studied using MKK1 and MKK2 as et al., 1991; Sturgill et al., 1988; Stokoe et al., 1992). In prototypic kinases (Wu et al., 1993; Brott et al., addition, the phosphorylated ERKs translocate to 1993; Pages et al., 1994). Selective mutations nucleus (Chen et al., 1992; Seth et al., 1992; Sanghera inhibiting either of the kinase activities have been et al., 1992; Lenormand et al., 1993) where they shown to disrupt the signaling function of the activate transcription factors such as TCFs through respective signaling module (Brott et al., 1993; phosphorylation (Alvarez et al., 1991; Pulverer et al., Pages et al., 1994; Huang et al., 1995). Of the 1991; Baker et al., 1992; Gille et al., 1992) which leads known kinase signaling modules, only those involved to the activation of speci®c genes involved in cell in the activation of the extracellular-signal regulated proliferation (Davis, 1992; Johnson and Vaillancourt, kinases 1 and 2 (ERK1 and ERK2) are more 1994). thoroughly characterized. However, the kinase MKK1 and MKK2 are of 43 ± 46 kDa proteins modules leading to the activation of c-Jun N- encoded by two distinct genes located at dierent terminal kinase (JNK) and p38-mitogen activated chromosomes (Brott et al., 1993). An analysis of the protein kinase (p38MAPK) are beginning to be primary sequence of MKK indicate that the N- understood. terminus (®rst 30 amino acids) show a low sequence Dual Specificity Kinases N Dhanasekaran and EP Reddy 1449 homology in comparison with other DSKs and this addition, the ®ndings that MAPKK and MAPK exist region has been speculated to be involved in the as a high molecular weight complex in Xenopus oocyte dierential interaction of speci®c MKKs with its (Matsuda et al., 1993) lend credence to the view that a substrates or activators.