High-resolution mapping of the actin fusion focus reveals myosin V-dependent formin transport for aster formation

Area of Science:

• Cell Biology
• Mycology
• Biochemistry

Background:

• Polarized growth and secretion in fungi rely on specific actin networks.
• Cell-cell fusion in fungi necessitates cell wall digestion via secreted lytic enzymes.
• The fission yeast Schizosaccharomyces pombe utilizes an actin fusion focus for cell-cell fusion.

Purpose of the Study:

• To map the spatiotemporal dynamics of the fusion site during S. pombe cell-cell fusion.
• To elucidate the roles of myosin V (Myo52) and formin (Fus1) in actin assembly and vesicle transport at the fusion focus.
• To understand the mechanism underlying actin aster formation and fusion focus compaction.

Main Methods:

• Centroid tracking and averaging to create a high-precision spatiotemporal map of the fusion site.
• Live-cell imaging to observe vesicle and protein dynamics.
• Biochemical assays to determine Myo52-Fus1 interactions and functional contributions.

Main Results:

• A pool of secretory vesicles maintained a constant distance from the membrane during actin structure condensation.
• Myosin V (Myo52) unexpectedly detached from vesicles and colocalized with formin (Fus1) near the membrane.
• Myosin V (Myo52) binds and transports formin (Fus1) along actin filaments, contributing to fusion focus compaction.
• Both Myo52 and Fus1 activities are crucial for actin assembly and focus compaction.

Conclusions:

• Myosin V-driven transport of formin Fus1 along Fus1-nucleated actin filaments creates a positive feedback loop for actin aster formation.
• This mechanism is critical for regulating actin dynamics, vesicle positioning, and ultimately, successful fungal cell-cell fusion.