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Regulation of ubiquitin ligase dynamics by the nucleolus.

Mekhail K, Khacho M, Carrigan A, Hache RR, Gunaratnam L, Lee S - J. Cell Biol. (2005)

Bottom Line: Photobleaching experiments reveal that MDM2 and VHL are highly mobile proteins in settings where their substrates are efficiently degraded.The nucleolar architecture converts MDM2 and VHL to a static state in response to regulatory cues that are associated with substrate stability.Data shown here provide the first evidence that cells have evolved a mechanism to regulate molecular networks by reversibly switching proteins between a mobile and static state.

View Article: PubMed Central - PubMed

Affiliation: Department of Cellular and Molecular Medicine, Faculty of Medicine, University of Ottawa, Ottawa, Ontario K1H 8M5, Canada.

ABSTRACT
Cellular pathways relay information through dynamic protein interactions. We have assessed the kinetic properties of the murine double minute protein (MDM2) and von Hippel-Lindau (VHL) ubiquitin ligases in living cells under physiological conditions that alter the stability of their respective p53 and hypoxia-inducible factor substrates. Photobleaching experiments reveal that MDM2 and VHL are highly mobile proteins in settings where their substrates are efficiently degraded. The nucleolar architecture converts MDM2 and VHL to a static state in response to regulatory cues that are associated with substrate stability. After signal termination, the nucleolus is able to rapidly release these proteins from static detention, thereby restoring their high mobility profiles. A protein surface region of VHL's beta-sheet domain was identified as a discrete [H+]-responsive nucleolar detention signal that targets the VHL/Cullin-2 ubiquitin ligase complex to nucleoli in response to physiological fluctuations in environmental pH. Data shown here provide the first evidence that cells have evolved a mechanism to regulate molecular networks by reversibly switching proteins between a mobile and static state.

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Regulation of ubiquitylation networks by the nucleolus. (A) Ubiquitin ligases follow a stochastic model of molecular mobility to ensure functional interactions such as those with the functional nuclear pore architecture at the nuclear envelope. (B) Complete static detention of ligases within the nucleolar space abolishes these required molecular interactions. (C) Detention of a fraction of the protein population results in a static nucleolar pool while a second pool sustains dynamic functions in the nucleoplasm or cytoplasm. (D) Dynamic change in the steady-state distribution of a protein from mainly nucleo-cytoplasmic to mainly nucleolar allows the protein to assume dynamic functions in the nucleolus and other cellular compartments.
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fig10: Regulation of ubiquitylation networks by the nucleolus. (A) Ubiquitin ligases follow a stochastic model of molecular mobility to ensure functional interactions such as those with the functional nuclear pore architecture at the nuclear envelope. (B) Complete static detention of ligases within the nucleolar space abolishes these required molecular interactions. (C) Detention of a fraction of the protein population results in a static nucleolar pool while a second pool sustains dynamic functions in the nucleoplasm or cytoplasm. (D) Dynamic change in the steady-state distribution of a protein from mainly nucleo-cytoplasmic to mainly nucleolar allows the protein to assume dynamic functions in the nucleolus and other cellular compartments.

Mentions: We provide evidence that the nucleolar architecture serves as a scaffold to convert highly mobile ubiquitin ligases to static participants of their molecular networks in response to physiological cues. This has various implications for our understanding of the role of nuclear compartments in regulating the output of dynamic molecular networks. Unlike certain core histones, which ensure chromatin stability by adopting a constitutive profile of relative immobility (Abney et al., 1997; Kimura and Cook, 2001), most proteins, including heterochromatin protein-1 (Cheutin et al., 2003; Festenstein et al., 2003; Maison and Almouzni, 2004), follow a stochastic model of high molecular mobility to ensure efficient functional interactions. We propose a model by which dynamic molecular networks, such as the ubiquitylation system, are built on complex interactions between mobile and relatively static participants. According to this model, modulation of these interactions through regulation of the dynamic state of the participants alters the output of the network. It is known that the interaction of the VBC/Cul-2 and MDM2 ubiquitin ligases with the functional nuclear pore architecture is required for nuclear export and subsequent degradation of their substrates (Fig. 10 A) (Momand et al., 1992; Oliner et al., 1993; Freedman and Levine, 1998; Roth et al., 1998; Lee et al., 1999; Groulx and Lee, 2002). Although constituents of the nuclear pore can move between subcellular compartments, functional pore architecture is confined to the nuclear envelope and persists for long periods of time within well-defined spatial regions (Rabut et al., 2004). Therefore, eliminating the physical interaction between an immobile and a mobile participant only requires the immobilization of the dynamic participant at a different spatial coordinate. In the herein described system, key interactions are abolished after static detention of the ubiquitin ligases within the nucleolar space, a phenomenon that alters network output (i.e., degradation of substrates) (Fig. 10 B) as previously shown by work from our and other groups (Tao and Levine, 1999; Weber et al., 1999; Lohrum et al., 2003; Mekhail et al., 2004a). These data suggest that static nucleolar detention selectively abolishes ubiquitin ligase functions requiring interactions with immobile constituents of the ubiquitylation networks. Whether VHL or MDM2 retain other functions when sequestered in nucleoli, or assume new roles, remains unknown.


Regulation of ubiquitin ligase dynamics by the nucleolus.

Mekhail K, Khacho M, Carrigan A, Hache RR, Gunaratnam L, Lee S - J. Cell Biol. (2005)

Regulation of ubiquitylation networks by the nucleolus. (A) Ubiquitin ligases follow a stochastic model of molecular mobility to ensure functional interactions such as those with the functional nuclear pore architecture at the nuclear envelope. (B) Complete static detention of ligases within the nucleolar space abolishes these required molecular interactions. (C) Detention of a fraction of the protein population results in a static nucleolar pool while a second pool sustains dynamic functions in the nucleoplasm or cytoplasm. (D) Dynamic change in the steady-state distribution of a protein from mainly nucleo-cytoplasmic to mainly nucleolar allows the protein to assume dynamic functions in the nucleolus and other cellular compartments.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2171338&req=5

fig10: Regulation of ubiquitylation networks by the nucleolus. (A) Ubiquitin ligases follow a stochastic model of molecular mobility to ensure functional interactions such as those with the functional nuclear pore architecture at the nuclear envelope. (B) Complete static detention of ligases within the nucleolar space abolishes these required molecular interactions. (C) Detention of a fraction of the protein population results in a static nucleolar pool while a second pool sustains dynamic functions in the nucleoplasm or cytoplasm. (D) Dynamic change in the steady-state distribution of a protein from mainly nucleo-cytoplasmic to mainly nucleolar allows the protein to assume dynamic functions in the nucleolus and other cellular compartments.
Mentions: We provide evidence that the nucleolar architecture serves as a scaffold to convert highly mobile ubiquitin ligases to static participants of their molecular networks in response to physiological cues. This has various implications for our understanding of the role of nuclear compartments in regulating the output of dynamic molecular networks. Unlike certain core histones, which ensure chromatin stability by adopting a constitutive profile of relative immobility (Abney et al., 1997; Kimura and Cook, 2001), most proteins, including heterochromatin protein-1 (Cheutin et al., 2003; Festenstein et al., 2003; Maison and Almouzni, 2004), follow a stochastic model of high molecular mobility to ensure efficient functional interactions. We propose a model by which dynamic molecular networks, such as the ubiquitylation system, are built on complex interactions between mobile and relatively static participants. According to this model, modulation of these interactions through regulation of the dynamic state of the participants alters the output of the network. It is known that the interaction of the VBC/Cul-2 and MDM2 ubiquitin ligases with the functional nuclear pore architecture is required for nuclear export and subsequent degradation of their substrates (Fig. 10 A) (Momand et al., 1992; Oliner et al., 1993; Freedman and Levine, 1998; Roth et al., 1998; Lee et al., 1999; Groulx and Lee, 2002). Although constituents of the nuclear pore can move between subcellular compartments, functional pore architecture is confined to the nuclear envelope and persists for long periods of time within well-defined spatial regions (Rabut et al., 2004). Therefore, eliminating the physical interaction between an immobile and a mobile participant only requires the immobilization of the dynamic participant at a different spatial coordinate. In the herein described system, key interactions are abolished after static detention of the ubiquitin ligases within the nucleolar space, a phenomenon that alters network output (i.e., degradation of substrates) (Fig. 10 B) as previously shown by work from our and other groups (Tao and Levine, 1999; Weber et al., 1999; Lohrum et al., 2003; Mekhail et al., 2004a). These data suggest that static nucleolar detention selectively abolishes ubiquitin ligase functions requiring interactions with immobile constituents of the ubiquitylation networks. Whether VHL or MDM2 retain other functions when sequestered in nucleoli, or assume new roles, remains unknown.

Bottom Line: Photobleaching experiments reveal that MDM2 and VHL are highly mobile proteins in settings where their substrates are efficiently degraded.The nucleolar architecture converts MDM2 and VHL to a static state in response to regulatory cues that are associated with substrate stability.Data shown here provide the first evidence that cells have evolved a mechanism to regulate molecular networks by reversibly switching proteins between a mobile and static state.

View Article: PubMed Central - PubMed

Affiliation: Department of Cellular and Molecular Medicine, Faculty of Medicine, University of Ottawa, Ottawa, Ontario K1H 8M5, Canada.

ABSTRACT
Cellular pathways relay information through dynamic protein interactions. We have assessed the kinetic properties of the murine double minute protein (MDM2) and von Hippel-Lindau (VHL) ubiquitin ligases in living cells under physiological conditions that alter the stability of their respective p53 and hypoxia-inducible factor substrates. Photobleaching experiments reveal that MDM2 and VHL are highly mobile proteins in settings where their substrates are efficiently degraded. The nucleolar architecture converts MDM2 and VHL to a static state in response to regulatory cues that are associated with substrate stability. After signal termination, the nucleolus is able to rapidly release these proteins from static detention, thereby restoring their high mobility profiles. A protein surface region of VHL's beta-sheet domain was identified as a discrete [H+]-responsive nucleolar detention signal that targets the VHL/Cullin-2 ubiquitin ligase complex to nucleoli in response to physiological fluctuations in environmental pH. Data shown here provide the first evidence that cells have evolved a mechanism to regulate molecular networks by reversibly switching proteins between a mobile and static state.

Show MeSH
Related in: MedlinePlus