Experimental measurement of endocytosis in fungal hyphae

Abstract

The present study examines the notion that polarized exocytosis in the tips of growing hyphae creates an excess of plasma membrane and thus the need for its removal by endocytosis. To measure endocytosis experimentally, we developed a photobleaching (FRAP) procedure to count endocytic events in hyphae of Neurospora crassa carrying a fluorescent tag on the actin-binding protein fimbrin (FIM-1-GFP). Given 40 nm as the average diameter of endocytic vesicles, we calculated that about 12.5% of the plasma membrane discharged in the apex becomes endocytosed in the subapex. According to our calculations, the GFP-tagged hyphae of N. crassa, measured under the constrained conditions of confocal microscopic examination, needed about 8800 vesicles/min to extend their plasma membrane or about 9800/min, if we include predicted demands for cell wall growth and extracellular secretion. Our findings support the notion that exocytosis and endocytosis operate in tandem with the latter serving as a compensatory process to remove any excess of plasma membrane generated by the intense exocytosis in the hyphal tips. Presumably, this tandem arrangement evolved to support the hallmark features of fungi namely rapid cell extension and abundant secretion of hydrolytic enzymes.

Regulation of Candida albicans Hyphal Morphogenesis by Endogenous Signals

“…The rapid exocytosis of the transported vesicles drives hyphal tip elongation. This exocytosis must be counterbalanced by endocytosis in order to recuperate excess membranes and membrane-anchored enzymes that participate in cell wall biosynthesis [48,49]. A collar of endocytic actin patches surrounds the hypha subapically, suggesting a tight coupling between apical exocytosis and subapical endocytosis [50].…”

Recent advances in understanding Candida albicans hyphal growth

“…Rapid hyphal growth requires active endocytosis to counterbalance exocytosis at the hyphal tip and recycle membrane lipids and proteins 71, 72 . For example, recent work demonstrated that polarization of a chitin synthase to the hyphal apex in A. nidulans occurs by indirect endocytic recycling 73 .…”

Improvement of gibberellin production by a newly isolatedFusarium fujikuroimutant

“…Fungi satisfy the demands for cell wall construction and extracellular enzyme secretion, thus creating the need to recycle the excess membrane via endocytosis (Bartnicki‐Garcia et al . 2018). Endocytosis serves to maintain cellular homeostasis by recovering protein and lipid components inserted into the plasma membrane with ongoing secretory activity (Mellman 1996).…”

“…If cells want to deal with the protein, they will require multiple systems to work, which can explain why we found many mutant genes in different systems, such as the endoplasmic reticulum, bubbles and signal factors (Bartnicki‐Garcia et al . 2018). Once the nutrient substance was absorbed by endocytosis, the MAPK signalling pathway will play a significant role.…”