Exocytosis is the intracellular trafficking step where a secretory vesicle fuses with the plasma membrane to release vesicle content. Actin and microtubules both play a role in exocytosis; however, their interplay is not understood. Here we study the interaction of actin and microtubules during exocytosis in lung alveolar type II (ATII) cells that secrete surfactant from large secretory vesicles. Surfactant extrusion is facilitated by an actin coat that forms on the vesicle shortly after fusion pore opening. Actin coat compression allows hydrophobic surfactant to be released from the vesicle. We show that microtubules are localized close to actin coats and stay close to the coats during their compression. Inhibition of microtubule polymerization by colchicine and nocodazole affected the kinetics of actin coat formation and the extent of actin polymerisation on fused vesicles. In addition, microtubule and actin cross-linking protein IQGAP1 localized to fused secretory vesicles and IQGAP1 silencing influenced actin polymerisation after vesicle fusion. This study demonstrates that microtubules can influence actin coat formation and actin polymerization on secretory vesicles during exocytosis.

The group of intermediate filaments (IFs) comprise numerous different subtypes with keratin filaments (KFs) being characteristic for epithelia. The functions of KFs vary from signaling to coping mechanisms for cell stress 1-3 and their mere abundance and big surface area 4 might facilitate even more, yet, unrecognized roles in the cell. However, KFs' classic role as stretch resistant mechanical stabilizers of the cell 2 remains keratin's most outstanding feature. KFs' tensile resistance is founded on a filament architecture with non-globular subunits providing ample space for overlap between subunits and enabling a rope-like suprastructure. Briefly, a