©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
How MAP Kinases Are Regulated (*)

Melanie H. Cobb (1)(§), Elizabeth J. Goldsmith (2)

From the (1)Departments of Pharmacology and (2)Biochemistry, University of Texas Southwestern Medical Center, Dallas, Texas 75235-9041

INTRODUCTION
Control of the MAP Kinase Cascade
Parallel MAP Kinase Pathways
Activation and Inactivation of the MAP Kinases
Three-dimensional Structure of ERK2
MAP Kinase Mutants and Structural Implications
FOOTNOTES
ACKNOWLEDGEMENTS
REFERENCES


INTRODUCTION

The closely related MAP kinases,()extracellular signal-regulated protein kinases 1 and 2 (ERK1 and ERK2), are ubiquitous components of signal transduction pathways. ERK1 and ERK2 are activated by diverse extracellular stimuli and by protooncogene products that induce proliferation or enhance differentiation (reviewed in Refs. 1 and 2). MAP kinase phosphorylations have an impact on processes in the cytoplasm, the nucleus, the cytoskeleton, and the membrane. The variety of signals that conscript the MAP kinase pathway demonstrates that this cascade serves a myriad of purposes, and the consequences of its activation will depend on cellular context. Because of the pleiotropic potential of these kinases, their activation needs to be tightly controlled. This review discusses the complexity of upstream regulation of the MAP kinase pathway, parallel cascades, and concepts that are likely to apply to many MAP kinase family members developed from analysis of the crystal structure of ERK2.


Control of the MAP Kinase Cascade

Receptor Tyrosine Kinases

The best understood means of activating the MAP kinase pathway (reviewed in Refs. 1 and 3) is that used by receptor tyrosine kinases. Ligands cause receptors to autophosphorylate on tyrosine residues; the phosphotyrosine residues of autophosphorylated receptors then bind the SH2 domains of adapters, such as Grb2 (growth factor receptor-bound protein 2). The adapters recruit guanine nucleotide exchange factors with proline-rich SH3 domain-binding sites to the membrane in proximity to the isoprenylated small G proteins they activate. Exchange factors promote the association of Ras with GTP. The GTP-bound form of Ras binds the protein kinases Raf-1 and B-Raf, thereby targeting one or both Raf isoforms to the membrane where Raf protein kinase activity is increased. MAP kinase kinases 1 and 2 (MKK), also called MAP/ERK kinases (MEK)(4, 5, 6) , are phosphorylated and activated by Raf-1 and B-Raf and are the upstream activators of the MAP kinases. Receptor tyrosine kinases have also been reported to activate the cascade in rat fibroblasts via a Ca-dependent but protein kinase C (PKC)- and Ras-independent pathway(7) . Receptors that do not contain intrinsic tyrosine kinase activity but that harbor sites for tyrosine phosphorylation may also activate the cascade via association of phosphotyrosine residues on the receptors or the activated tyrosine kinases with adapters(8) .

G Protein-coupled Receptors

The MAP kinase cascade can also be activated by certain heterotrimeric G proteins(9, 10) . Most require Ras and are believed to exploit the steps described for tyrosine kinases, but Ras-independent activation has been reported (9-12).

PKC

PKC is used by many receptors types to regulate the MAP kinase pathway, alone or with other mechanisms(13, 14) , and may act at several steps in the cascade. The effects of phorbol esters are Ras-dependent in PC12 cells (11) and Jurkat cells (15) but Ras-independent in fibroblasts(16) , consistent with multiple sites of action of PKC. PKC may directly activate Raf-1(17) , but mutation of the site phosphorylated by PKC does not interfere with activation of Raf by many stimuli including phorbol ester(18) . Other sites of action of PKC are likely to be either farther upstream or at the level of MAP kinase inactivation.

Regulation and Specificity of MEKs

All known signaling pathways are believed to use the two dual specificity protein kinases MEK1 and MEK2 to phosphorylate and activate MAP kinase(6) . MEK1 and -2 are activated not only by Raf-1 and B-Raf (19, 20) but also by the Mos protooncogene product(21, 22) , MEK kinase 1 (MEKK1)()(23) , and other probably distinct, growth factor-stimulated activities(24, 25) . The mechanisms controlling MEKK1 are unknown, although Ras may be required(26) . In oocytes, Mos is believed to be controlled by its synthesis and degradation.

Because no other MEK substrates have been identified, MEKs are viewed as dedicated kinases that phosphorylate only the MAP kinases. Kinases related to ERK1 and ERK2, in spite of retaining a similar arrangement of activating phosphorylation sites (Fig. 1), are poor in vitro substrates of MEK1(27) . Thus, the marked specificity of MEKs contributes to the selective activation of their downstream targets.


Figure 1: Alignment of phosphorylation lip sequences of ERK/MAP kinase family members. ERK1, ERK2, ERK3, HOG1, and JNK1 are mammalian enzymes. MPK1, KSS1, and FUS3 are from budding yeast and SPK1 is from fission yeast. The lip sequence of the Drosophila rolled gene product is identical to ERK1. Dots indicate identities; dashes indicate deletions. The phosphorylation sites are denoted by an asterisk. The 17 residues disordered in the ERK2 Tyr-185 mutants extend from the Asp preceding the FUS3 insertion to the conserved Arg preceding the sequence WYRAPE.




Parallel MAP Kinase Pathways

The ERK Protein Kinase Subfamily

ERK1 and ERK2 were the first of the ERK/MAP kinase subfamily to be cloned(28, 29, 30) . Other related mammalian enzymes have been detected including: two ERK3 isoforms(29, 31, 32) , ERK4(33) , Jun N-terminal kinases/stress-activated protein kinases (JNK/SAPKs)(34, 35) , p38/HOG1(36, 37) , and p57 MAP kinases (38). The presence of at least six MAP kinases in yeast suggests that there are more in mammals. Sequence signatures of the ERK family are most apparent in subdomains V, VII, IX, and XI (39) and include a long insert between subdomains X and XI. The sequences of the regulatory phosphorylation lip (surface loop between subdomains VII and VIII, see below) are also related, with conserved dual phosphorylation sites (Fig. 1).

The MEK Protein Kinase Subfamily

Several laboratories have uncovered additional MEKs, for which some substrates have been defined. A mammalian homolog of a MEK first identified in Xenopus (40) is called MAP kinase kinase 4 (MKK4), SAPK/ERK kinase (SEK), or JNK kinase (JNKK), because in vitro it activates JNK/SAPK and p38/HOG1 (27, 41, 42) but not ERK1 or ERK2(27) . Yet another newly cloned MEK, MKK3, selectively activates p38/HOG1 in transfected cells (42).

MAP Kinase Modules Mediate Distinct Signaling Events

The consistent appearance of 3-kinase cascades, first recognized in yeast, has engendered the concept of distinct MAP kinase modules(43) (Fig. 2). The modules convey information to target effectors and coordinate incoming information from parallel signaling pathways. A canonical MAP kinase module consists of three protein kinases that act sequentially within one pathway: a MEKK (a MEK activator), a MEK (a MAP kinase activator), and a MAP kinase (any ERK homolog). Raf-1 (or B-Raf), MEK1 (or MEK2), and ERK2 (or ERK1) constitute the best known mammalian MAP kinase module. The second mammalian MAP kinase module to be defined apparently consists of MEKK1, MKK4 (the MEK), and SAPK/JNK (the ERK)(44, 45) . MKK3 and p38/HOG1 appear to define yet a third cascade. MEKK1 can activate MKK4, MKK3, MEK1, and MEK2(27, 46, 47) , suggesting that MEKKs have a broader substrate specificity than MEKs. Thus, enzymatic specificity of the MEK, not the MEKK, may limit cross-cascade noise. Additional contributions to specificity may be provided through subcellular targeting of the enzymes(48) .


Figure 2: Mammalian MAP kinase modules. There are multiple MAP kinase modules in mammalian cells. Three that can be distinguished at present are the MAP kinase pathway, the JNK/SAPK pathway, and the HOG/p38 pathway. A MAP kinase module is a 3-kinase cascade consisting of an ERK or MAP kinase, which is activated by a MEK or MAP kinase kinase that in turn is activated by a MAP kinase kinase kinase or MEKK.




Activation and Inactivation of the MAP Kinases

Phosphorylation by MEK on two sites is required for MAP kinase activation. The two activating phosphorylation sites, a tyrosine and a threonine (Tyr-185 and Thr-183 of ERK2, Fig. 2and 3), lie 1 residue apart on the MAP kinases (49) in the phosphorylation lip. In vivo and in vitro, phosphorylation of tyrosine precedes phosphorylation of threonine(50, 51) , although phosphorylation of either residue can occur in the absence of the other(52, 53) . Because both tyrosine and threonine phosphorylations are required to activate the MAP kinases, phosphatases that remove phosphate from either site will inactivate them. Certain dual specificity phosphatases selectively inactivate MAP kinases by dephosphorylating both sites (reviewed in Ref. 54).


Three-dimensional Structure of ERK2

General Features

The three-dimensional structure of the unphosphorylated form of ERK2 provides a picture of its low activity state(55) . It consists of a smaller N-terminal domain and a larger C-terminal domain connected by a linker or crossover region (Fig. 3), similar to other protein kinases. ATP binds at a site deep in the catalytic cleft, formed at the interface between the two domains, whereas protein substrates bind on the surface.


Figure 3: The positions of gain-of-function mutations of MAP kinase mapped onto the three-dimensional structure of ERK2. Red denotes oxygen atoms; all other atoms (C, N, S, H) except as noted below are shown in purple. Yellow indicates Thr-183 and Tyr-185, the phosphorylated side chains; darkblue denotes basic residues likely to be involved in binding the phosphorylated side chains. Brightgreen and turquoise indicate dominant and recessive mutations, respectively. The residue numbers of ERK2 corresponding to the mutations are indicated. A, standard kinase view (profile); B, a second view rotated 80° looking into the phosphorylation lip.



Conformational Changes

Phosphorylation probably activates ERK2 by causing both global and local conformational changes. The two domains of ERK2 are rotated 17° farther apart than these domains in the structure of cAMP-dependent protein kinase (cAPK) (56). Therefore, a rotation of the N- and C-terminal domains must occur to cause closure of the active site and align the catalytic residues.

In cAPK, a phosphothreonine residue located in the phosphorylation lip interacts with basic residues, one of which is located in the N-terminal domain, to stabilize the closed domain structure. Similar interactions are likely to stabilize the closed state of ERK2. A domain rotation within ERK2 would bring homologous basic residues, including Arg-65 in the N-terminal domain (Fig. 3), into position to bind the phosphate group on Thr-183.

The phosphorylation lip, which contains the Thr-183 and Tyr-185 phosphorylation sites, blocks access of substrates to the active site. The side chain of Tyr-185 lies buried near the active site, and its main chain occupies the substrate binding site. A local conformational change occurs upon phosphorylation, displacing Tyr-185 and creating a lip structure compatible with high catalytic activity.

A Possible Binding Site for Phosphate on Tyr-185

Arg-189 and -192, residues not highly conserved among the protein kinases, create an anion binding site (Fig. 3) on the surface of ERK2 near the phosphorylation lip(57) . In the refined, low activity structure this site was filled with a sulfate ion acquired during crystallization. Interaction of the phosphate group of Tyr-185 with these residues may help to stabilize the conformation of the lip in the active structure.


MAP Kinase Mutants and Structural Implications

The Phosphorylation Lip Controls the Activity of the MAP Kinases

Biochemical and structural analyses of mutations of the activating phosphorylation sites suggest how phosphorylation increases ERK2 activity. The structure of the ERK2 mutant T183E and its basal activity are similar to wild type, but it is activated 100-fold following a single phosphorylation on Tyr-185(53) , suggesting that glutamate in part mimics the negative charge of threonine phosphate. The crystal structures of three ERK2 mutants at Tyr-185 (57) suggest changes in local conformation upon ERK2 activation. In these mutants, 15 residues of the phosphorylation lip from Asp-173 to Ala-187 (Fig. 1) are disordered(57) . Because any change to Tyr-185 introduces disorder into the low activity structure, Tyr-185 likely has an essential role in creating the low activity conformation.

The findings of these structural studies have important implications for the regulation of ERK2 and related kinases. The disorder observed in the mutants indicates that the phosphorylation lip is not a stable structure and suggests that modest amounts of binding energy are sufficient to induce conformational changes in this region(57) . The phosphorylation lip must acquire a different conformation to be phosphorylated by MEK and, after phosphorylation, another conformation that is compatible with high catalytic activity. Tyr-185 is buried in the low activity conformation of ERK2, yet in the activation process it is phosphorylated first. The binding energy provided by interaction of ERK2 with MEK may be sufficient to dislodge Tyr-185 from its buried position allowing it access to the active site of MEK.

Locations of Mutations Identified in Genetic Selections for Activated MAP Kinases

Thus far, no mutations have been identified that greatly increase MAP kinase activity in vitro; however, gain-of-function mutations have been found in two MAP kinases, the product of the Drosophila rolled gene (58) and FUS3(59) , a component of the pheromone response pathway in budding yeast. The mutations and the corresponding residues in ERK2 are listed in and displayed on the ERK2 structure in Fig. 3. The mutations are characterized as dominant or recessive.

The three recessive FUS3 mutations are buried in the N-terminal domain (Fig. 3). These are the least likely to affect interactions with other molecules. In ERK2 these residues are in close proximity and are involved in packing the -ribbon (residues 6-18) that replaces the A helix found in cAPK. This ribbon contributes to the positioning and rigidity of the core -sheet of the N-terminal domain and may influence the open conformation of the two domains in the inactive enzyme. These mutations may increase flexibility of this part of the molecule.

The dominant mutations lie on the surface and could involve interactions with other molecules. Here we have analyzed other possible effects of these dominant mutations. One FUS3 mutation (His-230 of ERK2) results in a loss of charge on the substrate binding face near the putative phosphotyrosine binding site and most likely affects interactions with substrates or regulators. A second FUS3 mutant, Glu-58 of ERK2, lies in a part of the structure unique to ERKs that replaces the B helix of cAPK. This region, near the putative phosphothreonine binding site, may be important for interactions in the activated structure(57) . A Val to Leu mutation (V171L in ERK2) in FUS3 lies at the beginning of the phosphorylation lip. The mutation may release steric constraint associated with a -branched residue, influencing refolding of the lip. The mutation identified in the rolled gene product (Asp-319 in ERK2) is just C-terminal to the conserved protein kinase core near the crossover region between the N- and C-terminal domains. Asp-319 forms a network of ionic interactions with residues conserved among MAP kinases to create a hinge bridging the N- and C-terminal domains. Thus, this mutation may affect the domain structure or orientation.

Conclusion

Thus far, no constitutively active MAP kinases are known, despite attempts at their genetic selection and site-directed mutagenesis. Such failure suggests that cells cannot tolerate the continuous activity of MAP kinase. Constitutively active mutants of MEK transform cells and generate tumors in nude mice(60) . However, effects of activated MEKs could be compensated for in a regulatable fashion by increasing phosphatase activity to inactivate MAP kinases. Perhaps the catastrophe that a cell might encounter if MAP kinases were constitutively active accounts for the diabolically complex mechanisms to activate these protein kinases and the multiplicity of mechanisms to inactivate them.

  
Table: Gain-of-function mutations in MAP kinases

The gain-of-function mutations in FUS3 (59) and the rolled gene product (58) are listed with the corresponding residue numbers in ERK2 and are grouped as dominant or recessive.



FOOTNOTES

*
This minireview will be reprinted in the 1995 Minireview Compendium, which will be available in December, 1995. Work from the authors' laboratories was supported by grants from the Welch Foundation (I1128 to E. J. G. and I1243 to M. H. C.), the National Institutes of Health (DK34128 to M. H. C. and DK46993 to E. J. G.), the Texas Advanced Research Program, and the Tobacco Research Council.

§
To whom correspondence should be addressed: Dept. of Pharmacology, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75235-9041. Tel.: 214-648-3627; Fax: 214-648-2971.

The abbreviations used are: MAP kinase, mitogen-activated protein kinase; ERK, extracellular signal-regulated protein kinase; MKK, MAP kinase/ERK kinase, MEK (the same as MAP kinase kinase; MEKK, MEK kinase; JNK/SAPK, Jun-N-terminal kinase/stress-activated protein kinase; PKC, protein kinase C; cAPK, cyclic AMP-dependent protein kinase.

Because multiple isoforms of MEKK are likely to exist, the first isoform cloned will be referred to throughout as MEKK1.


ACKNOWLEDGEMENTS

We would like to thank Elliott Ross, Jessie Hepler, Meg Phillips, and Megan Robinson (UT Southwestern) for critical reading of the manuscript and suggestions about figures, members of the Goldsmith and the Cobb laboratories for their efforts and insights, and Jo Hicks for preparation of the manuscript.


REFERENCES
  1. Blenis, J.(1993) Proc. Natl. Acad. Sci. U. S. A.90, 5889-5892 [Abstract]
  2. Cobb, M. H., Hepler, J. E., Cheng, M., and Robbins, D.(1994) Seminars in Cancer Biol.5, 261-268
  3. Schlessinger, J.(1994) Curr. Opin. Genet. & Dev.4, 25-30 [Medline] [Order article via Infotrieve]
  4. Ahn, N. G., Seger, R., Bratlien, R. L., Diltz, C. D., Tonks, N. K., and Krebs, E. G.(1991) J. Biol. Chem.266, 4220-4227 [Abstract/Free Full Text]
  5. Seger, R., Ahn, N. G., Boulton, T. G., Yancopoulos, G. D., Panayotatos, N., Radziejewska, E., Ericsson, L., Bratlien, R. L., Cobb, M. H., and Krebs, E. G.(1991) Proc. Natl. Acad. Sci. U. S. A.88, 6142-6146 [Abstract]
  6. Campbell, J. S., Seger, R., Graves, J. D., Graves, L. M., Jensen, A. M., and Krebs, E. G.(1994) Recent Prog. Horm. Res.50, 131-159
  7. Burgering, B. M. T., de Vries-Smits, A. M. M., Medema, R. H., van Weeren, P. C., Tertoolen, L. G. J., and Bos, J. L.(1993) Mol. Cell. Biol.13, 7248-7256 [Abstract]
  8. VanderKuur, J., Allevato, G., Billestrup, N., Norstedt, G., and Carter-Su, C.(1995) J. Biol. Chem.270, 7587-7593 [Abstract/Free Full Text]
  9. Winitz, S., Russell, M., Qian, N.-X., Gardner, A., Dwyer, L., and Johnson, G. L.(1993) J. Biol. Chem.268, 19196-19199 [Abstract/Free Full Text]
  10. Alblas, J., vanCorven, E. J., Hordijk, P. L., Milligan, G., and Moolenaar, W. H.(1993) J. Biol. Chem.268, 22235-22238 [Abstract/Free Full Text]
  11. Robbins, D. J., Cheng, M., Zhen, E., Vanderbilt, C., Feig, L. A., and Cobb, M. H.(1992) Proc. Natl. Acad. Sci. U. S. A.89, 6924-6928 [Abstract]
  12. Howe, L. R., and Marshall, C. J.(1993) J. Biol. Chem.268, 20717-20720 [Abstract/Free Full Text]
  13. L'Allemain, G., Sturgill, T. W., and Weber, M. J.(1991) Mol. Cell. Biol.11, 1002-1008 [Medline] [Order article via Infotrieve]
  14. Kazlauskas, A., and Cooper, J. A.(1988) J. Cell Biol.106, 1395-1402 [Abstract]
  15. Rayter, S. I., Woodrow, M., Lucas, S. C., Cantrell, D. A., and Downward, D. A.(1992) EMBO J.11, 4549-4556 [Abstract]
  16. Levers, S. J., and Marshall, C. J.(1992) EMBO J.11, 569-574 [Abstract]
  17. Kolch, W., Heldecker, G., Kochs, G., Hummel, R., Vahidi, H., Mischak, H., Finkenzeller, G., Marmé, D., and Rapp, U. R.(1993) Nature364, 249-255 [CrossRef][Medline] [Order article via Infotrieve]
  18. Whitehurst, C. E., Owaki, H., Bruder, J. T., Rapp, U. R., and Geppert, T. D.(1995) J. Biol. Chem.270, 5594-5599 [Abstract/Free Full Text]
  19. Kyriakis, J. M., App, H., Zhang, X.-F., Banerjee, P., Brautigan, D. L., Rapp, U. R., and Avruch, J.(1992) Nature358, 417-421 [CrossRef][Medline] [Order article via Infotrieve]
  20. Dent, P., Haser, W., Haystead, T. A. J., Vincent, L. A., Roberts, T. M., and Sturgill, T. W.(1992) Science257, 1404-1407 [Medline] [Order article via Infotrieve]
  21. Posada, J., Yew, N., Ahn, N. G., Vande Woude, G. F., and Cooper, J. A. (1993) Mol. Cell. Biol.13, 2546-2553 [Abstract]
  22. Nebreda, A. R., and Hunt, T.(1993) EMBO J.12, 1979-1986 [Abstract]
  23. Lange-Carter, C. A., Pleiman, C. M., Gardner, A. M., Blumer, K. J., and Johnson, G. L.(1993) Science260, 315-319 [Medline] [Order article via Infotrieve]
  24. Haystead, C. M. M., Gregory, P., Shirazi, A., Fadden, P., Mosse, C., Dent, P., and Haystead, T. A. J.(1994) J. Biol. Chem.269, 12804-12808 [Abstract/Free Full Text]
  25. Zheng, C. F., Ohmichi, M., Saltiel, A. R., and Guan, K.-L.(1994) Biochemistry33, 5595-5599 [Medline] [Order article via Infotrieve]
  26. Lange-Carter, C. A., and Johnson, G. L.(1994) Science265, 1458-1461 [Medline] [Order article via Infotrieve]
  27. Lin, A., Minden, A., Martinetto, H., Claret, F.-X., Lange-Carter, C., Mercurio, F., Johnson, G., and Karin, M.(1995) Science268, 286-296 [Medline] [Order article via Infotrieve]
  28. Boulton, T. G., Yancopoulos, G. D., Gregory, J. S., Slaughter, C., Moomaw, C., Hsu, J., and Cobb, M. H.(1990) Science249, 64-67 [Medline] [Order article via Infotrieve]
  29. Boulton, T. G., Nye, S. H., Robbins, D. J., Ip, N. Y., Radziejewska, E., Morgenbesser, S. D., DePinho, R. A., Panayotatos, N., Cobb, M. H., and Yancopoulos, G. D.(1991) Cell65, 663-675 [Medline] [Order article via Infotrieve]
  30. Gotoh, Y., Moriyama, K., Matsuda, S., Okumura, E., Kishimoto, T., Kawasaki, H., Suzuki, K., Yahara, I., Sakai, H., and Nishida, E.(1991) EMBO J.10, 2661-2668 [Abstract]
  31. Gonzalez, F. A., Raden, D. L., Rigby, M. R., and Davis, R. J.(1992) FEBS Lett.304, 170-178 [CrossRef][Medline] [Order article via Infotrieve]
  32. Zhu, A. X., Zhao, Y. I., Moller, D. E., and Flier, J. S.(1994) Mol. Cell. Biol.14, 8202-8211 [Abstract]
  33. Boulton, T. G., and Cobb, M. H.(1991) Cell Regul.2, 357-371 [Medline] [Order article via Infotrieve]
  34. Kyriakis, J. M., Banerjee, P., Nikolakaki, E., Dai, T., Rubie, E. A., Ahmad, M. F., Avruch, J., and Woodgett, J. R.(1994) Nature369, 156-160 [CrossRef][Medline] [Order article via Infotrieve]
  35. Dérijard, B., Hibi, M., Wu, I.-H., Barrett, T., Su, B., Deng, T., Karin, M., and Davis, R. J.(1994) Cell76, 1025-1037 [Medline] [Order article via Infotrieve]
  36. Han, J., Lee, J.-D., Bibbs, L., and Ulevitch, R. J.(1994) Science265, 808-811 [Medline] [Order article via Infotrieve]
  37. Rouse, J., Cohen, P., Trigon, S., Morange, M., Alonso-Llamazares, A., Zamanillo, D., Hunt, T., and Nebreda, A. R.(1994) Cell78, 1027-1037 [Medline] [Order article via Infotrieve]
  38. Lee, H., Ghose-Dastidar, J., Winawer, S., and Friedman, E.(1993) J. Biol. Chem.268, 5255-5263 [Abstract/Free Full Text]
  39. Hanks, S. K., Quinn, A. M., and Hunter, T.(1988) Science241, 42-52 [Medline] [Order article via Infotrieve]
  40. Yashar, B. M., Kelley, C., Yee, K., Errede, B., and Zon, L. I.(1993) Mol. Cell. Biol.13, 5738-5748 [Abstract]
  41. Sánchez, I., Hughes, R. T., Mayer, B. J., Yee, K., Woodgett, J. R., Avruch, J., Kyriakis, J. M., and Zon, L. I.(1994) Nature372, 794-798 [Medline] [Order article via Infotrieve]
  42. Dérijard, B., Raingeaud, J., Barrett, T., Wu, I-H., Han, J., Ulevitch, R. J., and Davis, R. J.(1995) Science267, 682-685 [Medline] [Order article via Infotrieve]
  43. Neiman, A. M., Stevenson, B. J., Xu, H.-P., Sprague, G. F., Jr., Herskowitz, I., Wigler, M., and Marcus, S.(1993) Mol. Biol. Cell4, 107-120 [Abstract]
  44. Yan, M., Dal, T., Deak, J. C., Kyriakis, J. M., Zon, L. I., Woodgett, J. R., and Templeton, D. J.(1994) Nature372, 798-800 [Medline] [Order article via Infotrieve]
  45. Minden, A., Lin, A., McMahon, M., Lange-Carter, C., Dérijard, B., Davis, R. J., Johnson, G. L., and Karin, M.(1994) Science266, 1719-1723 [Medline] [Order article via Infotrieve]
  46. Thomas, J. E., Soriano, P., and Brugge, J. S.(1991) Science254, 568-571 [Medline] [Order article via Infotrieve]
  47. Xu, S., Robbins, D., Frost, J., Dang, A., Lange-Carter, C., and Cobb, M. H.(1995) Proc. Natl. Acad. Sci. U. S. A., in press
  48. Jelinek, T., Catling, A. D., Reuter, C. W. M., Moodie, S. A., Wolfman, A., and Weber, M. J.(1994) Mol. Cell. Biol.14, 8212-8218 [Abstract]
  49. Payne, D. M., Rossomando, A. J., Martino, P., Erickson, A. K., Her, J.-H., Shananowitz, J., Hunt, D. F., Weber, M. J., and Sturgill, T. W. (1991) EMBO J.10, 885-892 [Abstract]
  50. Robbins, D. J., and Cobb, M. H.(1992) Mol. Biol. Cell3, 299-308 [Abstract]
  51. Haystead, T. A. J., Dent, P., Wu, J., Haystead, C. M. M., and Sturgill, T. W.(1992) FEBS Lett.306, 17-22 [CrossRef][Medline] [Order article via Infotrieve]
  52. Posada, J., and Cooper, J. A.(1992) Science255, 212-215 [Medline] [Order article via Infotrieve]
  53. Robbins, D. J., Zhen, E., Okami, H., Vanderbilt, C., Ebert, D., Geppert, T. D., and Cobb, M. H.(1993) J. Biol. Chem.268, 5097-5106 [Abstract/Free Full Text]
  54. Hunter, T.(1995) Cell80, 225-236 [Medline] [Order article via Infotrieve]
  55. Zhang, F., Strand, A., Robbins, D., Cobb, M. H., and Goldsmith, E. J. (1994) Nature367, 704-710 [CrossRef][Medline] [Order article via Infotrieve]
  56. Knighton, D. R., Zheng, J., Ten Eyck, L. F., Ashford, V. A., Xuong, N.-H., Taylor, S. S., and Sowadski, J. M.(1991) Science253, 407-413 [Medline] [Order article via Infotrieve]
  57. Zhang, J., Zhang, F., Ebert, D., Cobb, M. H., and Goldsmith, E. J. (1995) Nature Structure3, 299-307
  58. Brunner, D., Oellers, N., Szabad, J., Biggs, W., Zipursky, L., and Hafen, E.(1994) Cell76, 875-888 [Medline] [Order article via Infotrieve]
  59. Brill, J. A., Elion, E. A., and Fink, G. R.(1994) Mol. Cell. Biol.5, 297-312
  60. Mansour, S. J., Matten, W. T., Hermann, A. S., Candia, J. M., Rong, S., Fukasawa, K., Vande Woude, G. F., and Ahn, N. G.(1994) Science265, 966-970 [Medline] [Order article via Infotrieve]

©1995 by The American Society for Biochemistry and Molecular Biology, Inc.