Unconventional vesicle transport driven by formin-actin-myosin working system during pollen germination.

Directional transport of massive secretory vesicles to the germination site is a key process for pollen germination, but the molecular mechanism remains unclear. Here, we find that Myo11C proteins, Myo11C1 and Myo11C2, act redundantly. This functional redundancy is evidenced by impaired vesicle tethering efficiency and aberrant F-actin organization in myo11c1 myo11c2 double mutant. Myo11C1 unexpectedly colocalized with AtFH5-located secretory vesicles (AtFH5-SVs), rather than with actin filaments. Simulated modeling and truncation mutation analyses show that Myo11C binds actin filaments nucleated from AtFH5-SVs and exhibits plus-end-directed motility, leading to its polarized localization at F-actin plus-ends. Upon dissociating from F-actin, Myo11C directly interacts with AtFH5-SVs to promote their aggregation. E447 is identified as a critical residue for Myo11C1 binding to F-actin, while R1377 and R1452 are conserved residues essential for Myo11C1's interaction with secretory vesicle-located SEC5B. Together, our work uncovers the molecular basis by which formin-actin-myosin working system generates force driving massive secretory vesicles for directional transport during pollen germination.