Recombinant Schizosaccharomyces pombe Copper transport protein ctr6 (ctr6)

Basic Research Questions

• What is the function of Ctr6 in Schizosaccharomyces pombe and where is it localized?

  Ctr6 is a vacuolar membrane copper transporter in Schizosaccharomyces pombe that mobilizes stored copper from the vacuole to the cytosol under copper-limiting conditions. Using functional epitope-tagged alleles expressed under native promoter control, researchers have localized Ctr6 specifically to the vacuolar membrane during vegetative growth . Unlike plasma membrane-localized transporters that mediate copper uptake from the external environment, Ctr6 plays a crucial role in utilizing intracellular copper stores when environmental copper becomes scarce .

• How is Ctr6 expression regulated at the transcriptional level?

  The transcription of the ctr6+ gene is induced under copper-limiting conditions through a mechanism mediated by the cis-acting promoter element CuSE (copper-signaling element) and the copper-sensing transcription factor Cuf1 . During meiosis, ctr6+ expression exhibits a broader pattern, being detected throughout the entire meiotic process with increased expression during middle- and late-phase meiosis . Interestingly, while ctr4+ and ctr5+ expression is exclusively dependent on Cuf1, ctr6+ gene expression relies on two distinct regulators: Cuf1 and Mei4 , indicating a more complex regulatory mechanism for this vacuolar transporter.

• What is the structural organization of the Ctr6 protein?

  Ctr6 is an integral membrane protein with the capacity to trimerize . The predicted topology of Ctr6 suggests it possesses three transmembrane domains (TMDs) and conserved motifs characteristic of the Ctr family of copper transporters, including a MX₃M motif in TMD2 and a GX₃G motif in TMD3 . These motifs are believed to be responsible for copper transport function and homotrimer assembly, respectively. Additionally, Ctr6 harbors a putative copper-binding Met-X-His-Cys-X-Met-X-Met motif in its amino terminus, which has been experimentally demonstrated to be essential for its function .

Intermediate Research Questions

• How does Ctr6 differ from other copper transporters in S. pombe?

  S. pombe possesses three members of the copper transporter (Ctr) family: Ctr4, Ctr5, and Ctr6. While Ctr4 and Ctr5 form a heteromeric complex at the plasma membrane to facilitate copper uptake from the extracellular environment, Ctr6 functions at the vacuolar membrane to mobilize stored copper . Their functions are also temporally distinct during meiosis: Ctr4 and Ctr5 proteins co-localize at the plasma membrane shortly after meiotic induction and then progressively disappear after 3 hours, whereas Ctr6 localizes to vacuolar membranes in early meiosis and then undergoes redistribution to reach forespore membranes where it persists until sporulation . This differential localization and temporal expression indicate specialized roles in copper homeostasis throughout the yeast lifecycle.

• What experimental approaches are most effective for studying Ctr6 localization?

  Researchers have successfully employed several complementary techniques to study Ctr6 localization:

  • Epitope tagging: Expression of functional HA₄-tagged Ctr6 under its own promoter allows for tracking without disrupting native function .

  • Indirect immunofluorescence microscopy: This technique has been effectively used to visualize Ctr6-HA₄ in both mitotically growing cells and during meiotic progression .

  • Co-localization studies: Using GFP-Psy1 (an intrinsic component of forespore membrane) alongside Ctr6-HA₄ has confirmed Ctr6's localization to the forespore membrane during late meiosis .

  • Synchronous meiotic induction: Employing temperature-sensitive pat1-114 mutants allows for simultaneous entry into meiosis, facilitating time-course tracking of Ctr6 localization changes .

• What phenotypes are observed in ctr6Δ mutant strains?

  Deletion of the ctr6+ gene results in several distinct phenotypes:

  • Reduced SOD1 activity: A strain bearing a disrupted ctr6Δ allele displays a significant reduction in copper,zinc superoxide dismutase activity .

  • Synergistic effects with other copper transporter deletions: In a ctr4Δ ctr6Δ double mutant, SOD1 activity is completely lost, revealing that both transporters contribute to providing copper to cytosolic copper-dependent enzymes .

  • CAO activity deficiency: Under copper-limiting conditions, ctr6Δ cells exhibit reduced copper amine oxidase (CAO) activity, particularly in early and middle-phase meiosis .

  • Normal Cao1 protein levels: Despite reduced activity, the steady-state protein levels of Cao1 remain similar in wild-type and ctr6Δ mutant strains, indicating that the defect is in copper delivery rather than protein expression .