The Mixed Lineage Kinase SPRK Phosphorylates and Activates the Stress-activated Protein Kinase Activator, SEK-1

Abstract

SPRK (also called PTK-1 and MLK-3), a member of the mixed lineage kinase subfamily of (Ser/Thr) protein kinases, encodes an amino-terminal SH 3 domain followed by a kinase catalytic domain, two leucine zippers interrupted by a short spacer, a Rac/Cdc42 binding domain, and a long carboxyl-terminal proline-rich region. We report herein that SPRK activates the stress-activated protein kinases (SAPKs) but not ERK-1 during transient expression in COS cells; the p38 kinase is activated modestly (1.3-2 fold) but consistently. SPRK also activates cotransfected SEK-1/MKK-4, a dual specificity kinase which phosphorylates and activates SAPK. Reciprocally, expression of mutant, inactive SEK-1 inhibits completely the basal and SPRK-activated SAPK activity. Immunoprecipitated recombinant SPRK is able to phosphorylate and activate recombinant SEK-1 in vitro to an extent comparable to that achieved by MEK kinase-1. These results identify SPRK as a candidate upstream activator of the stress-activated protein kinases, acting through the phosphorylation and activation of SEK-1.

A novel human SPS1/STE20 homologue, KHS, activates Jun N-terminal kinase

“…Recently, mouse PAK3, another STE20 family member, has been shown to bind the SH3 domains of phospholipase Cg and Nck in vitro . Finally, the mixed lineage kinases, SPRK contains SH3 domains and functions as an activator of the JNK/SEK pathway (Rana et al, 1996). Examination of KHS's ability to bind various SH3-containing proteins is under way.…”

Signaling by dual specificity kinases

“…Similarly, The small GTPases such as Rac and CDC 42 fail to activate MKK3 (Han et al, 1996). However, the recently identi®ed MKKKs such as apoptosis signal regulating kinase or ASK1 (Ichijo et al, 1997), MEKK4/MTK1 (Takekawa et al, 1997), TGFbactivated kinase-1 or TAK1 (Shirakabe et al, 1997) and mixed lineage kinase-3 or MLK3 (Rana et al, 1996;Tibbles et al, 1996) appear to activate MKK3. The identity of the upstream small GTPases and kinase(s) involved in the activation of MKK3 is yet to be de®ned.…”

From receptors to stress-activated MAP kinases

“…In contrast to MAPKs and MAPKKs, the MAPKKKs in the JNK and p38 modules are highly divergent in structure and gene number. To date, eleven di erent MAPKKKs have been identi®ed as upstream activators of JNK pathway (Widmann et al, 1999); MEKK1 (Lange et al, 1993), MEKK2 (Blank et al, 1996), MEKK3 (Blank et al, 1996), MTK1/ MEKK4 (Takekawa et al, 1997;Gerwins et al, 1997), Tpl-2/Cot (Aoki et al, 1991;Salmeron et al, 1996), MUK/DLK/ZPK Holzman et al, 1994;Reddy and Pleasure, 1994), MLK-2/MST (Dorow et al, 1995;Hirai et al, 1997), MLK-3/SPRK/ PTK-1 (Rana et al, 1996;Gallo et al, 1994;Ing et al, 1994), TAK1 (Yamaguchi et al, 1995), ASK1/ MAPKKK5 Ichijo et al, 1997) and ASK2 (Saitoh and Ichijo, unpublished observation)/MAPKKK6 (Wang et al, 1998) have been shown to activate JNKs by overexpression (Figure 1). Of these, MEKK1, MEKK2, MEKK3 and Tpl-2 can also activate the ERK pathway, while only TAK1, ASK1 and MTK1 have been shown to strongly activate p38s as well.…”