Pulses of Ca ²⁺ coordinate actin assembly and exocytosis for stepwise cell extension

Abstract

Many eukaryotic cells grow by extending their cell periphery in pulses. The molecular mechanisms underlying this process are not yet fully understood. Here we present a comprehensive model of stepwise cell extension by using the unique tip growth system of filamentous fungi. Live-cell imaging analysis, including superresolution microscopy, revealed that the fungus extends the hyphal tip in an oscillatory manner. The amount of F-actin and secretory vesicles (SV) accumulating at the hyphal tip oscillated with a positive temporal correlation, whereas vesicle amounts were negatively correlated to the growth rate. The intracellular Ca level also pulsed with a positive temporal correlation to the amount of F-actin and SV at the hyphal tip. Two Ca channels, MidA and CchA, were needed for proper tip growth and the oscillations of actin polymerization, exocytosis, and the growth rate. The data indicate a model in which transient Ca pluses cause depolymerization of F-actin at the cortex and promote SV fusion with the plasma membrane, thereby extending the cell tip. Over time, Ca diffuses away and F-actin and SV accumulate again at the hyphal tip. Our data provide evidence that temporally controlled actin polymerization and exocytosis are coordinated by pulsed Ca influx, resulting in stepwise cell extension.

A novel Pezizomycotina‐specific protein with gelsolin domains regulates contractile actin ring assembly and constriction in perforated septum formation

“…The important role of the actin cytoskeleton in polarized hyphal growth of filamentous fungi has been demonstrated by the arrest of colony growth caused by cytochalasin A treatment of A. nidulans (Torralba, Raudaskoski, Pedregosa, & Laborda, ). F‐actin localizes to the hyphal tip (Araujo‐Bazán, Peñalva, & Espeso, ), and tip growth depends on the assembly and disassembly of F‐actin (Takeshita et al, ). Actin cables receive the cargo such as secretary vesicles from microtubule and then deliver it to the region, where exocytosis and hyphal tip extension occur (Peñalva, Zhang, Xiang, & Pantazopoulou, ).…”

Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures

“…In fact, in A. nidulans, the rate of elongation is 10 to 30 times higher than that of bud formation in budding yeast, a rate at which polarity determinants are predicted to become disturbed by incoming vesicles (61, 74,75). The cell extension rate of N. crassa is up to 10 times higher than the one of A. nidulans (76). Still, filamentous fungi are able to maintain cell polarity in the presence of thousands to several tens of thousands of exocytic events predicted to occur at the cell tip (apex) every minute (77,78).…”

“…This strongly suggests that the oscillatory rate of extension of hyphal tips involves a concerted action of Ca 2ϩ , actin, and exocytosis. In fact, live-cell imaging analysis, including superresolution microscopy, revealed that the A. nidulans hyphal tip extends in an oscillatory manner (76,85). The amounts of F-actin and secretory vesicles accumulating at the hyphal tip oscillated with a positive temporal correlation.…”

“…It was shown that the concentration of Ca 2ϩ oscillates during polar hyphal extension and that this oscillation may be responsible for oscillations of vesicle secretion and, thus, the oscillatory growth of hyphae. Because stress responses cause dramatic changes in intracellular Ca 2ϩ concentrations, effects of stress on polar cell extension are likely (76).…”

Developmental regulators FlbE/D orchestrate the polarity site-to-nucleus dynamics of the fungal bZIP FlbB

“…Hyphae are the characteristic cell type of filamentous fungi. These permanently-polarized structures elongate by pulsed extension of the tip apex [9] that is dependent on receiving plasma membrane and cell-wall materials that are transported first on microtubules (MT) and then on actin filaments [10,11]. The fast polar growth of fungal hyphae increases turgor pressure impinged on the substrate, facilitating its efficient colonization.…”