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Genetic Interactions between the Members of the SMN-Gemins Complex in Drosophila.

Borg RM, Bordonne R, Vassallo N, Cauchi RJ - PLoS ONE (2015)

Bottom Line: Despite multiple genetic studies, the Gemin proteins have not been identified as prominent modifiers of SMN-associated mutant phenotypes.We show a modifier effect by all three members of the minimalistic fly SMN-Gemins complex within the muscle compartment of the motor unit.The toxicity associated with increased Gemin2 levels is conserved in the yeast S. pombe in which we find that the cytoplasmic retention of Sm proteins, likely reflecting a block in the snRNP assembly pathway, is a contributing factor.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biochemistry, Faculty of Medicine and Surgery, University of Malta, Msida, Malta GC; Institut de Génétique Moléculaire de Montpellier, CNRS-UMR5535, Université Montpellier 1 and 2, Montpellier, France.

ABSTRACT
The SMN-Gemins complex is composed of Gemins 2-8, Unrip and the survival motor neuron (SMN) protein. Limiting levels of SMN result in the neuromuscular disorder, spinal muscular atrophy (SMA), which is presently untreatable. The most-documented function of the SMN-Gemins complex concerns the assembly of spliceosomal small nuclear ribonucleoproteins (snRNPs). Despite multiple genetic studies, the Gemin proteins have not been identified as prominent modifiers of SMN-associated mutant phenotypes. In the present report, we make use of the Drosophila model organism to investigate whether viability and motor phenotypes associated with a hypomorphic Gemin3 mutant are enhanced by changes in the levels of SMN, Gemin2 and Gemin5 brought about by various genetic manipulations. We show a modifier effect by all three members of the minimalistic fly SMN-Gemins complex within the muscle compartment of the motor unit. Interestingly, muscle-specific overexpression of Gemin2 was by itself sufficient to depress normal motor function and its enhanced upregulation in all tissues leads to a decline in fly viability. The toxicity associated with increased Gemin2 levels is conserved in the yeast S. pombe in which we find that the cytoplasmic retention of Sm proteins, likely reflecting a block in the snRNP assembly pathway, is a contributing factor. We propose that a disruption in the normal stoichiometry of the SMN-Gemins complex depresses its function with consequences that are detrimental to the motor system.

No MeSH data available.


Related in: MedlinePlus

Possible mechanisms responsible for the Gemin2 overexpression phenotypes.Mechanism A: Gemin2 makes multiple contacts within the multi-protein SMN-Gemins complex so that its overexpression can destabilise the intact complex, thereby resulting in the formation of non-functional sub-complexes. Mechanism B: Sm proteins are shared subunits of snRNPs and SMN-Gemins complexes. Overexpression of Gemin2 competes for limiting amounts of Sm proteins, hence reducing their presence in other complexes. Both mechanisms predict a dysfunction in Sm core assembly on snRNAs. Abbreviations: G2, Gemin2; G3, Gemin3; and, G5, Gemin5.
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pone.0130974.g009: Possible mechanisms responsible for the Gemin2 overexpression phenotypes.Mechanism A: Gemin2 makes multiple contacts within the multi-protein SMN-Gemins complex so that its overexpression can destabilise the intact complex, thereby resulting in the formation of non-functional sub-complexes. Mechanism B: Sm proteins are shared subunits of snRNPs and SMN-Gemins complexes. Overexpression of Gemin2 competes for limiting amounts of Sm proteins, hence reducing their presence in other complexes. Both mechanisms predict a dysfunction in Sm core assembly on snRNAs. Abbreviations: G2, Gemin2; G3, Gemin3; and, G5, Gemin5.

Mentions: Gemin2’s key role in snRNP assembly was only revealed recently through structural and biochemical studies. To this end, Gemin2 is thought to serve as the arm of the SMN-Gemins complex that captures select Sm proteins and holds them in an ordered form prior to their coupling with snRNAs. Importantly, as part of its job it prevents their assembly on unintended RNAs until the joining of an snRNA [10, 11]. Interestingly, similar to SMN and Gemin8, Gemin2 is capable of self-association, a likely requirement for its role in stabilising the SMN-Gemins complex [62]. In this context, two hypotheses can explain the toxicity associated with excess Gemin2. Since Gemin2 binds to itself and makes multiple contacts within the SMN-Gemins complex, a surplus of Gemin2 can result in partial complexes, thereby reducing the amount of the intact functional SMN-Gemins complex (Fig 9). Alternatively, an overabundance of Gemin2 could hijack Sm proteins, consequently reducing their capture by bona-fide SMN-Gemins complexes (Fig 9). Both models predict reduced cytoplasmic coupling of Sm proteins with snRNAs to form snRNPs that following assembly are normally imported in the nucleus where they function. Lending support to this prediction, we report a surplus of Sm proteins within the cytoplasm of S. pombe overexpressing Gemin2, a phenotype that is reminiscent of that reported for the dominant-negative mutant SMNΔN27 [63], and most likely indicates a cytoplasmic block in the snRNP assembly pathway. Future studies confirming that snRNP biogenesis is disrupted as well as those that distinguish between the two proposed mechanisms for a Gemin2 gain-of-function, including attempts at increasing the levels of other SMN-Gemins complex members simultaneously with Gemin2 to overturn the imbalance, are warranted.


Genetic Interactions between the Members of the SMN-Gemins Complex in Drosophila.

Borg RM, Bordonne R, Vassallo N, Cauchi RJ - PLoS ONE (2015)

Possible mechanisms responsible for the Gemin2 overexpression phenotypes.Mechanism A: Gemin2 makes multiple contacts within the multi-protein SMN-Gemins complex so that its overexpression can destabilise the intact complex, thereby resulting in the formation of non-functional sub-complexes. Mechanism B: Sm proteins are shared subunits of snRNPs and SMN-Gemins complexes. Overexpression of Gemin2 competes for limiting amounts of Sm proteins, hence reducing their presence in other complexes. Both mechanisms predict a dysfunction in Sm core assembly on snRNAs. Abbreviations: G2, Gemin2; G3, Gemin3; and, G5, Gemin5.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4476591&req=5

pone.0130974.g009: Possible mechanisms responsible for the Gemin2 overexpression phenotypes.Mechanism A: Gemin2 makes multiple contacts within the multi-protein SMN-Gemins complex so that its overexpression can destabilise the intact complex, thereby resulting in the formation of non-functional sub-complexes. Mechanism B: Sm proteins are shared subunits of snRNPs and SMN-Gemins complexes. Overexpression of Gemin2 competes for limiting amounts of Sm proteins, hence reducing their presence in other complexes. Both mechanisms predict a dysfunction in Sm core assembly on snRNAs. Abbreviations: G2, Gemin2; G3, Gemin3; and, G5, Gemin5.
Mentions: Gemin2’s key role in snRNP assembly was only revealed recently through structural and biochemical studies. To this end, Gemin2 is thought to serve as the arm of the SMN-Gemins complex that captures select Sm proteins and holds them in an ordered form prior to their coupling with snRNAs. Importantly, as part of its job it prevents their assembly on unintended RNAs until the joining of an snRNA [10, 11]. Interestingly, similar to SMN and Gemin8, Gemin2 is capable of self-association, a likely requirement for its role in stabilising the SMN-Gemins complex [62]. In this context, two hypotheses can explain the toxicity associated with excess Gemin2. Since Gemin2 binds to itself and makes multiple contacts within the SMN-Gemins complex, a surplus of Gemin2 can result in partial complexes, thereby reducing the amount of the intact functional SMN-Gemins complex (Fig 9). Alternatively, an overabundance of Gemin2 could hijack Sm proteins, consequently reducing their capture by bona-fide SMN-Gemins complexes (Fig 9). Both models predict reduced cytoplasmic coupling of Sm proteins with snRNAs to form snRNPs that following assembly are normally imported in the nucleus where they function. Lending support to this prediction, we report a surplus of Sm proteins within the cytoplasm of S. pombe overexpressing Gemin2, a phenotype that is reminiscent of that reported for the dominant-negative mutant SMNΔN27 [63], and most likely indicates a cytoplasmic block in the snRNP assembly pathway. Future studies confirming that snRNP biogenesis is disrupted as well as those that distinguish between the two proposed mechanisms for a Gemin2 gain-of-function, including attempts at increasing the levels of other SMN-Gemins complex members simultaneously with Gemin2 to overturn the imbalance, are warranted.

Bottom Line: Despite multiple genetic studies, the Gemin proteins have not been identified as prominent modifiers of SMN-associated mutant phenotypes.We show a modifier effect by all three members of the minimalistic fly SMN-Gemins complex within the muscle compartment of the motor unit.The toxicity associated with increased Gemin2 levels is conserved in the yeast S. pombe in which we find that the cytoplasmic retention of Sm proteins, likely reflecting a block in the snRNP assembly pathway, is a contributing factor.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biochemistry, Faculty of Medicine and Surgery, University of Malta, Msida, Malta GC; Institut de Génétique Moléculaire de Montpellier, CNRS-UMR5535, Université Montpellier 1 and 2, Montpellier, France.

ABSTRACT
The SMN-Gemins complex is composed of Gemins 2-8, Unrip and the survival motor neuron (SMN) protein. Limiting levels of SMN result in the neuromuscular disorder, spinal muscular atrophy (SMA), which is presently untreatable. The most-documented function of the SMN-Gemins complex concerns the assembly of spliceosomal small nuclear ribonucleoproteins (snRNPs). Despite multiple genetic studies, the Gemin proteins have not been identified as prominent modifiers of SMN-associated mutant phenotypes. In the present report, we make use of the Drosophila model organism to investigate whether viability and motor phenotypes associated with a hypomorphic Gemin3 mutant are enhanced by changes in the levels of SMN, Gemin2 and Gemin5 brought about by various genetic manipulations. We show a modifier effect by all three members of the minimalistic fly SMN-Gemins complex within the muscle compartment of the motor unit. Interestingly, muscle-specific overexpression of Gemin2 was by itself sufficient to depress normal motor function and its enhanced upregulation in all tissues leads to a decline in fly viability. The toxicity associated with increased Gemin2 levels is conserved in the yeast S. pombe in which we find that the cytoplasmic retention of Sm proteins, likely reflecting a block in the snRNP assembly pathway, is a contributing factor. We propose that a disruption in the normal stoichiometry of the SMN-Gemins complex depresses its function with consequences that are detrimental to the motor system.

No MeSH data available.


Related in: MedlinePlus