Furin catalyses a simple biochemical reaction--the proteolytic maturation of proprotein substrates in the secretory pathway. But the simplicity of this reaction belies furin's broad and important roles in homeostasis, as well as in diseases ranging from Alzheimer's disease and cancer to anthrax and Ebola fever. This review summarizes various features of furin--its structural and enzymatic properties, intracellular localization, trafficking, substrates, and roles in vivo.

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Furin at the cutting edge: from protein traffic to embryogenesis and disease.

The proprotein convertases.

HUMAN stromelysin-3, a new member of the matrix metalloproteinase family, is expressed in tissues undergoing the active remodelling associated with embryonic development, wound healing and tumour invasion1–3. But like all other members of the matrix metalloproteinase gene family, stromelysin-3 is synthesized as an inactive precursor that must be processed to its mature form in order to express enzymic activity4,5. Here we identify stromelysin-3 as the first matrix metalloproteinase to be discovered that can be processed directly to its enzymically active form by an obligate intracellular proteolytic event that occurs within the constitutive secretory pathway. Intracellular activation is regulated by an unusual 10-amino-acid insert sandwiched between the pro-and catalytic-domains of stromelysin-3, which is encrypted with an Arg-X-Arg-X-Lys-Arg recognition motif for the Golgi-associated proteinase, furin, a mammalian homologue of the yeast Kex2 pher-omone convertase6,7. A furin–stromelysin-3 processing axis not only differentiates the regulation of this enzyme from all previously characterized matrix metalloproteinases, but also identifies pro-protein convertases as potential targets for therapeutic intervention in matrix-destructive disease states.

/pdf/furin-dependent-intracellular-activation-of-the-human-51u84s0vit.pdf

Furin-dependent intracellular activation of the human stromelysin-3 zymogen

The Menkes P-type ATPase (MNK), encoded by the Menkes gene (MNK; ATP7A), is a transmembrane copper-translocating pump which is defective in the human disorder of copper metabolism, Menkes disease Recent evidence that the MNK P-type ATPase has a role in copper efflux has come from studies using copper-resistant variants of cultured Chinese hamster ovary (CHO) cells These variants have MNK gene amplification and consequently overexpress MNK, the extents of which correlate with the degree of elevated copper efflux Here, we report on the localization of MNK in these copper-resistant CHO cells when cultured in different levels of copper Immunofluorescence studies demonstrated that MNK is predominantly localized to the Golgi apparatus of cells in basal medium In elevated copper conditions there was a rapid trafficking of MNK from the Golgi to the plasma membrane This shift in steady-state distribution of MNK was reversible and not dependent on new protein synthesis In media containing basal copper, MNK accumulated in cytoplasmic vesicles after treatment of cells with a variety of agents that inhibit endosomal recycling We suggest that MNK continuously recycles between the Golgi and the plasma membrane and elevated copper shifts the steady-state distribution from the Golgi to the plasma membrane These data reveal a novel system of regulated protein trafficking which ultimately leads to the efflux of an essential yet potentially toxic ligand, where the ligand itself appears directly and specifically to stimulate the trafficking of its own transporter

Ligand-regulated transport of the Menkes copper P-type ATPase efflux pump from the Golgi apparatus to the plasma membrane: a novel mechanism of regulated trafficking.

Intracellular cleavage of immature flaviviruses is a critical step in assembly that generates the membrane fusion potential of the E glycoprotein. With cryo-electron microscopy we show that the immature dengue particles undergo a reversible conformational change at low pH that renders them accessible to furin cleavage. At a pH of 6.0, the E proteins are arranged in a herringbone pattern with the pr peptides docked onto the fusion loops, a configuration similar to that of the mature virion. After cleavage, the dissociation of pr is pH-dependent, suggesting that in the acidic environment of the trans-Golgi network pr is retained on the virion to prevent membrane fusion. These results suggest a mechanism by which flaviviruses are processed and stabilized in the host cell secretory pathway.

https://science.sciencemag.org/content/sci/319/5871/1834.full.pdf

Structure of the Immature Dengue Virus at Low pH Primes Proteolytic Maturation

Furin is a membrane-associated endoprotease that efficiently cleaves precursor proteins on the C-terminal side of the consensus sequence, Arg-X-Lys/Arg-Arg1, and has been proposed to catalyze these reactions in both exocytic and endocytic compartments. To study its biosynthesis and routing, a furin construct (designated fur/f) containing the FLAG epitope tag inserted on the C-terminal side of the enzyme's autoproteolytic maturation site was used. Introduction of the epitope tag had no effect on the expression, proteolytic maturation or activity of furin. Analysis of the localization of fur/f by immunofluorescence microscopy showed that its staining pattern largely overlapped with those of several Golgi-associated markers. Treatment of cells with brefeldin A caused the fur/f distribution to collapse around the microtubule organizing center, indicating that furin is concentrated in the trans-Golgi network (TGN). Immunoelectron microscopy showed unequivocally that furin resides in the TGN where it colocalized with TGN38. In agreement with its proposed activity in multiple compartments, antibody uptake studies showed that fur/f cycles between the cell surface and TGN. Furthermore, targeting to the TGN requires sequences in the cytoplasmic tail of the enzyme. Pulse-chase and immunofluorescence analyses demonstrated that proregion removal occurs in the endoplasmic reticulum and that cleavage may be required for exist from this compartment. Finally, we show that proregion removal is necessary but not sufficient for enzyme activation.

https://www.embopress.org/doi/pdf/10.1002/j.1460-2075.1994.tb06231.x

P.E. Stenberg

Papers

Intracellular trafficking and activation of the furin proprotein convertase: localization to the TGN and recycling from the cell surface.