Coordinated spindle assembly and orientation requires Clb5p-dependent kinase in budding yeast

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• Asymmetric cell division relies on proper mitotic spindle orientation.
• In Saccharomyces cerevisiae, spindle positioning is crucial for segregating cellular components during division.
• The S-phase cyclin Clb5p plays a role in cell cycle regulation and division.

Purpose of the Study:

• To investigate the role of Clb5p in establishing spindle polarity during asymmetric cell division in Saccharomyces cerevisiae.
• To understand the molecular mechanisms underlying spindle positioning defects observed in the absence of Clb5p.

Main Methods:

• Time-lapse digital imaging microscopy was employed.
• Microtubules were visualized using green fluorescent protein (GFP) fusions to tubulin and dynein.
• Mutant yeast strains (cdc28-4 clb5Delta) were analyzed.

Main Results:

• Loss of Clb5p abolished the asymmetric behavior of spindle pole bodies during spindle assembly.
• In mutant cells, both spindle poles exhibited equal interaction probability with the bud cell cortex.
• This led to persistent dynamic interactions and spindle translocation across the bud neck, failing to establish mother-bound spindle pole behavior.

Conclusions:

• Clb5p-associated kinase activity is essential for conferring mother-bound behavior to one spindle pole.
• This ensures correct spindle polarity for asymmetric cell division.
• Other B-type cyclins (Clb3p, Clb4p) are partially redundant for spindle morphogenesis but Clb5p is specifically required for polarity establishment.