Avoiding a Traffic Jam

Abstract

Proteins destined for secretion are delivered from the endoplasmic reticulum (ER) to the Golgi complex, which they must traverse en route to the plasma membrane. Noting that exit from the ER of procollagen-I is temperature-dependent, Pulvirenti et al . shifted the temperature of human skin fibroblasts from 40 o C to 32 o C to deliver a "pulse" of cargo to the Golgi and found that tyrosine phosphorylation in the Golgi region increased in parallel with the arrival and accumulation of procollagen-I after relief of ER export block. A combination of immunoblot and pharmacological analysis of cells homogenized during or after block indicated that trafficking was associated with the phosphorylation and activation of Src-family tyrosine kinases (SFKs). Immunofluorescence and immunoelectron microscopy confirmed localization of trafficking-induced phosphorylated SFK to the Golgi; trafficking did not, however, affect the total amount of Golgi-SFK. ER chaperones that accompany cargo proteins bind to the transmembrane KDEL receptor (KDEL-R) in the Golgi, triggering retrograde movement of KDEL-R and thereby recycling chaperones to the ER (see Asp and Nilsson), and the authors explored the role of KDEL-R in SFK activation in various cell types. Introduction of KDEL-R ligands (through endocytosis or transfection) stimulated phosphorylation of Golgi-associated SFKs, as did KDEL-R overexpression. In contrast, transfection of a KDEL-R dominant-negative mutant, microinjection of antibody directed against the KDEL-R cytosolic C-terminal tail, or depletion of KDEL-R with siRNA inhibited trafficking-induced activation of Golgi-localized SFKs. The KDEL-R C-terminal domain bound Src in a yeast two-hybrid assay; moreover, SFK inhibitors, depletion of Src with siRNA, and a Src dominant-negative mutant all inhibited intra-Golgi trafficking of a cargo protein. In contrast, a constitutively active Src mutant antagonized inhibition of trafficking by the KDEL-R dominant-negative. The authors thus conclude that KDEL-R binding by chaperones that accompany cargo initiates an SFK-mediated signaling cascade that stimulates Golgi trafficking. T. Pulvirenti, M. Giannotta, M. Capestrano, M. Capitani, A. Pisanu, R. S. Polishchuk, E. San Pietro, G. V. Beznoussenko, A. A. Mironov, G. Turacchio, V. W. Hsu, M. Sallese, A. Luini, A traffic-activated Golgi-based signalling circuit coordinates the secretory pathway. Nat. Cell Biol. 10 , 912-922 (2008). [PubMed] L. Asp, T. Nilsson, Golgi gets wired up. Nat. Cell Biol. 10 , 885-887 (2008). [PubMed]