The mitogen-activated protein kinase (MAPK) pathway that activates extracellular signal–regulated kinase (ERK) is canonically shown as involving activation of the guanosine triphosphatase (GTPase) Ras, which activates a Raf (a MAPKKK), which activates MEK (a MAPKK), which activates ERK (a MAPK). GTPases are also considered essential components in the activation of the members of the Pak family of kinases, which exist in an autoinhibited dimeric state that is disrupted by the GTPases Rac and Cdc42. Roquette-Jazdanian et al. show a pathway in T cells involving Pak1 that activates ERK without requiring any GTPase input. Bam32 is an adaptor protein present in B and T cells. Knockout of Bam32 in mice compromised ERK activation in T cells in response to low concentrations of activating antibody; whereas its overexpression in the Jurkat T cell line increased Raf-1, MEK1, and ERK1/2 phosphorylation and did not induce the formation of GTP-loaded Ras. Coimmunoprecipitation studies with Jurkat cells expressing tagged proteins or human primary T cells showed that Bam32, phospholipase C (PLC)–γ1, and Pak1 formed a complex detected in resting and stimulated cells. This complex was important for Bam32-mediated ERK activation, because knockdown or genetic knockout or introduction of mutants that disrupted the complex compromised the induction of ERK activity by Bam32 overexpression. Linker of activated T cells (LAT) and SLP-76 are other T cell adaptors that bind to different regions of PLC-γ1 and form a complex that triggers downstream signaling. Overexpression of Bam32 in Jurkat cells deficient in LAT or SLP-76 stimulated ERK activation. However, the two complexes (Bam32-PLC-γ1-Pak1 and LAT-SLP-76-PLC-γ1) competed: Overexpression of LAT reduced the coimmunoprecipitation of Bam32 and PLC-γ1, whereas overexpression of SLP-76 reduced the coimmunoprecipitation of Pak1 with PLC-γ1. Overexpression of Bam32 reduced the amount of PLC-γ1 that coimmunoprecipitated with LAT and also reduced the amount of catalytically active PLC-γ1. Indeed, a catalytically inactive PLC-γ1 mutant promoted ERK activity through the Bam32-mediated pathway. Phosphorylation of Pak1 in cells overexpressing Bam32 was not affected by loss of Rac1 or Cdc42 function, and in vitro experiments indicated that PLC-γ1 disrupted the autoinhibited Pak1 dimers to promote Pak1 activity in the Bam32-PLC-γ1-Pak1 complex. Consistent with the hypothesis that Bam32 functions as a parallel and competing pathway that diverts PLC-γ1 from LAT-mediated signaling, knockdown of Bam32 in human or mouse T cells enhanced PLC-γ1 phosphorylation (and thus lipase activity) while reducing Pak1, MEK, and ERK activation. Thus, the abundance of Bam32 may serve to regulate PLC-γ1–mediated signaling by diverting it away from pathways involving its lipase activity and instead directing it into a pathway independent of its catalytic activity.
A. K. Roquette-Jazdanian, C. L. Sommers, R. L. Kortum, D. K. Morrison, L. E. Samelson, LAT-independent Erk activation via Bam32-PLC-γ1-Pak1 complexes: GTPase-independent Pak1 activation. Mol. Cell 48, 298–312 (2012). [PubMed]