The Tetrahymena bcd1 mutant implicates endosome trafficking in ciliate, cortical pattern formation

Area of Science:

• Cell Biology
• Genetics
• Developmental Biology

Background:

• Ciliates like Tetrahymena thermophila possess intricate mechanisms for organelle positioning and size control.
• Mutations can disrupt organelle localization and dimensions, impacting cellular functions.
• Cortical organelles, including the oral apparatus, cytoproct, and contractile vacuole pores, are crucial for Tetrahymena's survival.

Purpose of the Study:

• To identify and characterize novel genes involved in regulating cortical organelle assembly and dimensions.
• To elucidate the function of the BCD1 gene and its protein product in Tetrahymena.
• To understand the molecular mechanisms underlying organelle size determination.

Main Methods:

• Genetic screens in Tetrahymena thermophila to identify pattern mutants.
• Gene cloning and sequencing to identify the BCD1 gene.
• Protein localization studies using microscopy.
• Functional assays involving chemical inhibitors of endocytosis and protein kinase A (PKA).

Main Results:

• A novel ciliate pattern gene, BCD1, was identified, encoding a Beige-BEACH domain-containing protein.
• Mutations in BCD1 lead to broadened cortical organelle domains during development.
• Bcd1 protein localizes to ciliary membrane pockets involved in endocytosis.
• Inhibition of clathrin-mediated endocytosis or PKA activity partially phenocopied the bcd1 mutant.

Conclusions:

• The BCD1 gene plays a critical role in regulating the dimensions of cortical organelles in Tetrahymena.
• The Bcd1 protein likely functions in a pathway involving PKA-regulated endocytosis.
• Organelle dimensions are determined by a balance between exocytic delivery and endocytic retrieval of materials.