Pak Functions Downstream of Dock to Regulate Photoreceptor Axon Guidance in Drosophila

Abstract

The SH2/SH3 adaptor protein Dock has been proposed to transduce signals from guidance receptors to the actin cytoskeleton in Drosophila photoreceptor (R cell) growth cones. Here, we demonstrate that Drosophila p21-activated kinase (Pak) is required in a Dock pathway regulating R cell axon guidance and targeting. Dock and Pak colocalize to R cell axons and growth cones, physically interact, and their loss-of-function phenotypes are indistinguishable. Normal patterns of R cell connectivity require Pak's kinase activity and binding sites for both Dock and Cdc42/Rac. A membrane-tethered form of Pak (Pak(myr) acts as a dominant gain-of-function protein. Retinal expression of Pak(myr) rescues the R cell connectivity phenotype in dock mutants. These data establish Pak as a critical regulator of axon guidance and a downstream effector of Dock in vivo.

Nck/Dock: an adapter between cell surface receptors and the actin cytoskeleton

“…Among other structural features, the Drosophila Trio contains a motif of DH-PH ± DH-PH tandem domains, the characteristic feature of GEFs speci®c for the Rho family GTPases. It was shown that Trio localizes at the growth cone membrane and that the trio mutant disrupted the photoreceptor axon projection patterns (Newsome et al, 2000), defects similar to those seen in both Pak and dock mutants (Garrity et al, 1996;Hing et al, 1999). This function of Trio in axon guidance depended upon its GEF activity, since mutation in one of two GEF domains of Trio completely abolished the Trio function.…”

“…These studies suggested that these Ste-20-related protein kinases are important downstream targets for Nck. In collaboration with Louis Lim's group, Zipursky and colleagues reported that, Pak, one of the three Drosophila Pakrelated molecules (Pak, Mbt and Dpak2), is required for the Dock pathway regulating R cell axon guidance and targeting (Hing et al, 1999). Mbt and Dpak2 did not bind Dock and had no e ect on the R cell axon guidance.…”

“…Their studies showed that the loss-offunction phenotypes of Pak and Dock were indistinguishable, i.e. there was disruption of R cell axon guidance and targeting but not R cell di erentiation (Hing et al, 1999). Drosophila Pak and Dock colocalize to axons and growth cones and, similar to the mammalian Nck and Pak, physically interact with each other, via the middle SH3 of Dock and the N-terminal PxxP site of Pak.…”

“…Drosophila Pak and Dock colocalize to axons and growth cones and, similar to the mammalian Nck and Pak, physically interact with each other, via the middle SH3 of Dock and the N-terminal PxxP site of Pak. Furthermore, rescue of the dock phenotype by Pak requires its protein kinase activity, the Dock-binding and the Cdc42/Rac-binding sites (Hing et al, 1999). Also, consistent with the notion that recruitment of Pak to the membrane by Nck is an essential regulatory step for Pak function (Manser et al, 1997;Lu et al, 1997), a membrane-attached Pak, but not the kinase-dead (GMR-Pak myr-k459A ) or the nonmembrane-bound but constitutively active (GMRPak L115F ) Pak, was able to rescue the dock phenotype (Hing et al, 1999).…”

Les protéines Rho : leur rôle dans les neurones

“…L'un des effecteurs de Cdc42, N-WASP (Wiskott-Aldrich syndrome protein), essentiel pour la formation des filopodes induite par [16]. Dans les photorécep-teurs de drosophile, des mutations entraînant une perte de fonction de PAK1 provoquent des défauts dans le guidage axonal [17,18]. PAK possède plusieurs cibles, dont la LIM kinase (Lim-11, Isl-1, Mec-3 domains kinase) et la MLCK (myosin light chain kinase).…”