Recombinant Saccharomyces cerevisiae GTP-binding protein RHO4 (RHO4)

Basic Research Questions

• What is the functional role of RHO4 in Saccharomyces cerevisiae?

  RHO4 is a member of the Ras superfamily of GTPases in Saccharomyces cerevisiae that plays a critical role in the maintenance of cell polarity. Unlike other Rho GTPases such as CDC42 that are essential for bud emergence, RHO4 functions primarily after bud formation to maintain cell polarity during daughter cell maturation . RHO4 exhibits functional redundancy with RHO3, as deletion of both genes causes lethality and loss of cell polarity at 30°C, while deletion of RHO4 alone produces no obvious phenotype . Cells with depleted RHO3 and RHO4 typically lyse at the small-budded stage and display delocalized chitin deposition and a depolarized actin cytoskeleton . This indicates that these proteins function downstream of the bud site assembly process, specifically during bud maturation.

  Experimental approaches to study RHO4 function include:

  • Generating conditional mutants (temperature-sensitive alleles)

  • Creating strain backgrounds with specific gene deletions

  • Analyzing cell morphology and polarity markers

  • Examining actin cytoskeleton organization using fluorescence microscopy

• What are the unique structural features of RHO4 compared to other Rho GTPases?

  The most distinctive structural feature of RHO4 is its unusually long N-terminal extension upstream of the conserved G1 box. This extension is 69 amino acids longer than those found in Cdc42 and Rho5, making it unique among the six Rho GTPases in S. cerevisiae . The N-terminal extension contains:

  • Two predicted short α-helices at amino acid residues 49-54 and 59-65

  • A potential PEST motif at residues 9-40, which might be involved in protein degradation regulation

  • Functionally important regions as evidenced by temperature-sensitive mutations

  This N-terminal extension is evolutionarily conserved in RHO4 homologs across many yeast species and filamentous fungi, suggesting functional importance . Truncation experiments have shown that removing amino acids 1-61 causes morphological defects at 24°C and growth defects at 37°C in rho3Δ rho4Δ cells expressing the truncated protein, confirming the extension's significance for proper RHO4 function .

  Region	Location (aa)	Potential Function
  PEST motif	9-40	Protein degradation regulation
  α-helix 1	49-54	Structural/functional role
  α-helix 2	59-65	Structural/functional role
  G1 box	After N-terminal extension	GTP/GDP binding

Intermediate Research Questions

• What methods are effective for investigating RHO4 protein interactions?

  Several complementary techniques have proven effective for studying RHO4 protein interactions:

  1. Yeast Two-Hybrid (Y2H) Screening:

     • Particularly useful for identifying novel binding partners

     • Bem2 (a RhoGAP for Cdc42 and Rho1) was identified as a RHO4 interactor using Y2H

     • For RHO4, constitutively active mutants like RHO4 Q131L can be used as bait to identify effectors

     • Different Y2H systems have been developed to overcome limitations of the classical system

  2. GST Pulldown Assays:

     • Effective for confirming direct interactions and determining nucleotide dependence

     • By preloading GST-RHO4 with different nucleotides (GTP, GDP, or nucleotide-free), researcher can determine binding preferences of interacting proteins

     • Bem2 specifically interacts with the GTP-bound form of RHO4 through its RhoGAP domain

  3. Bimolecular Fluorescence Complementation (BiFC):

     • Allows visualization of protein interactions in living cells

     • Has been successfully used to confirm RHO4 interactions with proteins like Bem2

  4. Active RHO4 Pull-down:

     • The Rho-binding domain (RBD) of human Rhotekin binds preferentially to GTP-bound Rho4

     • This assay can measure the amount of active RHO4 in cellular extracts

  Technique	Advantages	Limitations	Examples from RHO4 Research
  Y2H	Detects binary interactions in vivo	May miss interactions of membrane proteins	Identified Bem2 as RHO4 interactor
  GST Pulldown	Tests direct interactions; can test nucleotide dependence	In vitro approach	Showed Bem2 interacts with GTP-bound RHO4
  BiFC	Visualizes interactions in cellular context	Irreversible complex formation	Confirmed RHO4-Bem2 interaction
  RBD Pulldown	Quantifies active RHO4 levels	Indirect measure of activation	Used to show Gef3 activates Rho4 in fission yeast

Advanced Research Questions

• How does the N-terminal extension of RHO4 influence its function and regulation?

  The unique N-terminal extension of RHO4 plays crucial roles in its function and regulation, as evidenced by multiple experimental approaches:

  Temperature-sensitive mutations:
  Two temperature-sensitive rho4 alleles (rho4-2 and rho4-3) contain amino acid substitutions in the N-terminal region, demonstrating its functional importance . These mutants show growth and morphological defects at 37°C in a rho3Δ background.

  Truncation experiments:
  Cells expressing truncated Rho4 lacking amino acids 1-61 exhibit morphological defects at 24°C and growth defects at 37°C, confirming the extension's significance for proper function . A less severe truncation (removing amino acids 1-42) had milder effects, suggesting a gradient of functional importance within the extension.

  Regulatory elements:
  The PEST motif (residues 9-40) may regulate protein stability through targeted degradation. Protein level regulation is also affected by Rdi1, which extracts Rho4 from membranes .

  Evolutionary conservation:
  The presence of the N-terminal extension in RHO4 homologs across diverse fungal species suggests functional importance maintained through evolution . This conservation pattern is distinct from other Rho GTPases and may indicate specialized functions.

  Potential mechanisms of action include:

  • Mediating specific protein-protein interactions

  • Regulating subcellular localization

  • Modulating GTPase activity through intramolecular interactions

  • Controlling protein stability and turnover

• What is the relationship between RHO4 and cell polarity maintenance in budding yeast?

  RHO4 plays a specialized role in maintaining cell polarity after bud emergence, with several lines of evidence demonstrating its importance:

  1. Loss-of-function phenotypes:

     • While deletion of RHO4 alone produces no obvious phenotype, the rho3Δ rho4Δ double mutant exhibits lethality at 30°C

     • Cells with depleted RHO3 and RHO4 become rounded and enlarged, with delocalized chitin deposition and actin patches

     • These phenotypes indicate a loss of cell polarity specifically after bud formation has begun

  2. Gain-of-function phenotypes:

     • Overexpression of constitutively active rho4 Q131L or rho4 G81V causes severe growth defects in rdi1Δ strains

     • The affected cells become large, round, and unbudded with depolarized actin

     • This suggests that while RHO4 activity is necessary for polarity maintenance, excessive activity can disrupt polarity establishment

  3. Functional relationship with the CDC42 pathway:

     • High-copy CDC42 can complement rho3 defects

     • Overexpression of RHO3 inhibits growth of mutants defective in the CDC24-CDC42 pathway

     • This suggests RHO4/RHO3 function downstream of or parallel to the CDC42 pathway in polarity control

  4. Genetic interactions:

     • Overexpression of nine genes (SRO genes) can suppress defects in RHO3 function

     • Two of these are CDC42 and BEM1, both involved in bud site assembly

     • This further establishes connections between RHO4/RHO3 and the broader polarity network

  The evidence collectively suggests that while CDC42 is crucial for establishing polarity during bud emergence, RHO4 (with RHO3) is required to maintain this polarity during daughter cell maturation.

• How does RHO4 function differ between Saccharomyces cerevisiae and Schizosaccharomyces pombe?

  Despite being homologous proteins, RHO4 exhibits significant functional differences between budding and fission yeasts:

  In Saccharomyces cerevisiae (budding yeast):

  • RHO4 functions redundantly with RHO3 in maintaining cell polarity after bud formation

  • Deletion of both RHO3 and RHO4 causes lethality and loss of cell polarity at 30°C

  • RHO4 interacts with Bem2, which may function as a GAP

  • The N-terminal extension plays an important role in RHO4 function

  In Schizosaccharomyces pombe (fission yeast):

  • Rho4 is primarily involved in cell separation through regulation of glucanase secretion

  • It controls the localization of glucanases Eng1 and Agn1, which are required for dissolution of the primary septum

  • Rho4 is activated by the GEF Gef3, which interacts with the septin complex during cytokinesis

  • Simultaneous overexpression of Rho4 with Eng1 or Agn1 causes severe cell lysis

  Key experimental evidence of functional divergence:

  • In S. pombe, rho4Δ cells contain multiple septa at 37°C, indicating defects in cell separation

  • In S. cerevisiae, rho3Δ rho4Δ cells lyse at the small-budded stage with depolarized actin

  • Despite these differences, both functions involve aspects of polarized growth and cell wall remodeling

  Aspect	S. cerevisiae	S. pombe
  Primary function	Cell polarity maintenance	Cell separation
  Key interactors	Bem2 (potential GAP)	Gef3 (GEF), Eng1/Agn1 (glucanases)
  Deletion phenotype	No phenotype alone; synthetic lethality with rho3Δ	Cell separation defects; multiple septa
  Temperature sensitivity	Double mutant with rho3Δ more severe at 37°C	Phenotype more pronounced at 37°C

• What advanced methods can be used to measure RHO4 activation in vivo?

  Several sophisticated approaches can be used to measure RHO4 activation in living cells:

  1. Active RHO4 pull-down assays:

     • The Rho-binding domain (RBD) of human Rhotekin preferentially binds to GTP-bound Rho4

     • Cell lysates are incubated with GST-RBD, and the amount of bound RHO4 is quantified by Western blotting

     • This assay has been used to show that gef3Δ cells in S. pombe have approximately half the amount of GTP-Rho4 compared to wild-type cells

     • Controls should include testing with GTP-locked (Q131L) and GDP-locked (T86N) RHO4 mutants

  2. FRET-based biosensors:

     • Involves creating fusion proteins that undergo Förster resonance energy transfer when RHO4 is activated

     • Typically consists of RHO4, an effector binding domain, and two fluorescent proteins

     • Enables real-time visualization of RHO4 activation with subcellular resolution

     • Could be adapted from existing Rho GTPase biosensors used in other systems

  3. Bimolecular fluorescence complementation (BiFC):

     • Can detect interactions between RHO4 and effectors that specifically bind active RHO4

     • Has been used to confirm interaction between RHO4 and Bem2

     • Provides spatial information but lacks temporal resolution due to irreversible complex formation

  4. Genetic reporters of RHO4 activity:

     • Transcriptional reporters controlled by pathways downstream of RHO4

     • Can provide population-level readouts of RHO4 activity

  Key considerations for these assays include:

  • Controlling for expression levels of recombinant proteins

  • Verifying that tags do not interfere with RHO4 function

  • Including appropriate positive and negative controls

  • Performing experiments under physiologically relevant conditions

Methodological and Technical Questions

• How can researchers create and validate temperature-sensitive RHO4 mutants?

  Temperature-sensitive (Ts) alleles of RHO4 are valuable tools for studying its function. The following methodology has been successful:

  Creation of temperature-sensitive rho4 mutants:

  1. Random mutagenesis using error-prone PCR to generate mutations in the RHO4 gene

  2. Co-transformation of the PCR products with linearized plasmid vector containing RHO4 promoter and terminator regions into rho3Δ rho4Δ cells carrying a URA3-marked RHO4 plasmid

  3. Selection on SC-Leu media to allow gap repair between PCR products and the vector

  4. Counter-selection on 5-FOA media to identify cells that have lost the URA3-marked RHO4 plasmid

  5. Replica plating to identify clones that grow at 24°C but not at 37°C

  Validation of temperature-sensitive mutants:

  1. Plasmid recovery and reintroduction into the original strain to confirm the plasmid-based nature of the Ts phenotype

  2. DNA sequencing to identify the mutations responsible for the Ts phenotype

  3. Phenotypic characterization including:

     • Growth assays at permissive (24°C) and restrictive (37°C) temperatures

     • Cell morphology analysis

     • Actin cytoskeleton and chitin staining

     • Cell wall composition analysis

  The four temperature-sensitive rho4-Ts alleles isolated in previous research contained distinct mutation patterns:

  • rho4-2 and rho4-3: mutations in the N-terminal region plus mutations in GTP binding/hydrolysis domains

  • rho4-4: mutations in the G2 box and C-terminal half

  • rho4-1: mutations only in the C-terminal half

  These different alleles provide valuable tools for studying specific aspects of RHO4 function and can reveal domain-specific roles in different cellular processes.

• How can researchers distinguish between direct and indirect effects when studying RHO4 function?

  Distinguishing direct from indirect effects is crucial when studying RHO4 function. The following methodological approaches can help researchers make this distinction:

  1. Use of nucleotide-locked mutants:

     • Constitutively active (Q131L, G81V) or inactive (T86N) RHO4 mutants can distinguish between GTP-dependent and independent functions

     • Comparing phenotypes of these mutants with wild-type or deletion strains helps separate primary from secondary effects

     • If a phenotype appears with both constitutively active and inactive mutants, it likely represents an indirect effect

  2. Biochemical interaction assays:

     • In vitro binding assays with purified proteins can demonstrate direct interactions

     • GST-pulldown experiments that test nucleotide dependence of interactions (as done with Bem2)

     • Surface plasmon resonance to measure binding kinetics between RHO4 and potential effectors

  3. Time-course experiments:

     • Rapid induction/repression systems (like GAL1 promoter) to identify immediate versus delayed responses

     • Tracking the temporal sequence of events after RHO4 activation or inactivation

     • Early effects are more likely to be direct consequences of RHO4 activity

  4. Structure-function analysis:

     • Creating point mutations or truncations in specific domains (like the N-terminal extension)

     • Determining which mutations affect specific interactions or functions

     • Domain-swapping experiments between RHO4 and other Rho GTPases

  5. Genetic interaction mapping:

     • Systematic analysis of genetic interactions (suppressors, synthetic lethality)

     • Epistasis analysis to place RHO4 in relation to other genes in a pathway

     • This approach revealed that RHO4 is in the same pathway as ENG1 in fission yeast

  Example from RHO4 research: In fission yeast, the identical effects of gef3Δ and rho4Δ on Eng1 and Agn1 localization suggested a direct functional relationship, which was confirmed by showing decreased levels of active GTP-bound Rho4 in gef3Δ cells using RBD pulldown assays .