Signaling from the Small GTP-binding Proteins Rac1 and Cdc42 to the c-Jun N-terminal Kinase/Stress-activated Protein Kinase Pathway

doi:10.1074/jbc.271.44.27225

Signaling from the Small GTP-binding Proteins Rac1 and Cdc42 to the c-Jun N-terminal Kinase/Stress-activated Protein Kinase Pathway

A ROLE FOR MIXED LINEAGE KINASE 3/PROTEIN-TYROSINE KINASE 1, A NOVEL MEMBER OF THE MIXED LINEAGE KINASE FAMILY*

From the Laboratory of Cellular Development and Oncology, NIDR, National Institutes of Health, Bethesda, Maryland 20892-4330

^‡To whom correspondence should be addressed:
Molecular Signaling Unit, Laboratory of Cellular Development and Oncology, NIDR, NIH, 9000 Rockville Pike, Bldg. 30, Rm. 212, Bethesda, MD 20892-4330
. Tel.: 301-496-6259; Fax: 301-402-0823.

Abstract

Certain small GTP-binding proteins control the enzymatic activity of a family of closely related serine-threonine kinases known as mitogen-activated protein kinases (MAPKs). In turn, these MAPKs, such as p44^mapk and p42^mapk, referred to herein as MAPKs, and stress-activated protein kinases, also termed c-Jun N-terminal kinases (JNKs), phosphorylate and regulate the activity of key molecules that ultimately control the expression of genes essential for many cellular processes. Whereas Ras controls the activation of MAPK, we and others have recently observed that two members of the Rho family of small GTP-binding proteins, Rac1 and Cdc42, regulate the activity of JNKs. The identity of molecules communicating Rac1 and Cdc42 to JNK is still poorly understood. It has been suggested that Pak1 is the most upstream kinase connecting these GTPases to JNK; however, we have observed that coexpression of Pak1 with activated forms of Cdc42 or Rac1 diminishes rather than enhances JNK activation. This prompted us to explore the possibility that kinases other than Pak might participate in signaling from GTP-binding proteins to JNK. In this regard, a computer-assisted search for proteins containing areas of homology to that in Pak1 that is involved in binding to Rac1 and Cdc42 led to the identification of mixed lineage kinase 3 (MLK3), also known as protein-tyrosine kinase 1, as a potential candidate for this function. In this study, we found that MLK3 overexpression is sufficient to activate JNK potently without affecting the phosphorylating activity of MAPK or p38. Furthermore, we present evidence that MLK3 binds the GTP-binding proteins Cdc42 and Rac1 in vivo and that MLK3 mediates activation of MEKK-SEK-JNK kinase cascade by Rac1 and Cdc42. Taken together, these findings strongly suggest that members of the novel MLK family of highly related kinases link small GTP-binding proteins to the JNK signaling pathway.

Footnotes

↵* The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
↵¹ The abbreviations used are:

MAPK

mitogen-activated protein kinases

ERK

extracellular signal-regulated kinase

SAPK

stress-activated protein kinase

JNK

c-jun N-terminal kinase

CRIB

Cdc42/Rac interactive binding

MLK3

mixed lineage kinase 3

PTK1

protein-tyrosine kinase 1

GST

glutathione S-transferase.
↵²H. Teramoto, O. A. Coso, H. Miyata, T. Igishi, and J. S. Gutkind, unpublished observations.
↵³L. Vitale and J. S. Gutkind, manuscript in preparation.
- Received July 1, 1996.
- Revision received August 15, 1996.