New study reveals that the yeast eps15-like endocytic protein Pan1p is a key player in endocytosis

TOKYO UNIVERSITY OF SCIENCE

IMAGE: SCIENTISTS REVEAL KEY REGULATORY PROCESSES UNDERLYING THE MOLECULAR MECHANISMS OF ENDOCYTOSIS

CREDIT: SCIENTIFIC ANIMATION

A cell needs to constantly interact with its surrounding environment in a variety of ways to execute a plethora of biological functions. One such way is the uptake of extracellular materials, also known as endocytosis. In this phenomenon, ubiquitous in all eukaryotic cells, a part of the cell membrane invaginates and buds off into the cytoplasm as a vesicle (small fluid-filled sacs enclosed by a membrane) with the engulfed molecule inside. Endocytosis serves several cellular functions such as nutrient intake and the recycling of neurotransmitters.

Clathrin-mediated endocytosis is an important type of endocytosis in which the internalized vesicles are coated with the protein, clathrin. Typically, this process consists of an early phase, where the endocytosis site is selected, and a late phase, where the invagination begins and eventually, separates from the membrane. While previous studies have elucidated the underlying molecular mechanisms, the recruitment processes of the molecular players to the site and the switch between the early and late phases of endocytosis are not well understood.

In this context, the yeast (a good model system for studying endocytosis) protein Pan1p (human EPS15) is pertinent because it is known to anchor several proteins (scaffolding protein) during the later stages of yeast clathrin-mediated endocytosis. For evaluating its regulatory functions, a team of scientists from Austria and Japan, led by Prof. Jiro Toshima (Tokyo University of Science) and Prof. Junko Y. Toshima (Tokyo University of Technology) conducted an interesting set of experiments where the location of Pan1p was altered to peroxisomes (oxidative organelles present across eukaryotic cells) from the cell membrane. The study was published in the Journal of Cell Biology. This paper was made available online on August 19, 2022 and will be published in Volume 221 No. 10 Issue of the journal on of the October 03, 2022.

Indeed, their experiments have revealed a series of fascinating findings. They found that Pan1p could initiate all the processes associated with late-stage clathrin-mediated endocytosis at the peroxisomes when it is ectopically anchored to peroxixomes, suggesting that this molecule plays important regulatory functions in the progression of endocytic process. Apart from recruiting mid-stage (Sla2p, Ent1p, Ent2p) and late-stage (End3p) proteins, Pan1p also plays a crucial role in actin polymerization, a key process mediating the invagination of the cell membrane. The researchers used the fluorescent-tagged markers, Abp1-GFP and GFP-fused actins, for examining actin assembly in cells expressing Pan1p at the peroxisome. Further investigations revealed that Pan1p drives the recruitment of a range of actin nucleation promoting factors, thus confirming that it plays a central role in actin assembly. In endocytosis, actin polymerization is followed by actin disassembly, which eventually leads to the internalization of the clathrin-coated vesicles. It turns out that Pan1p also contributes to the process of actin disassembly.

Taken together, this novel study provides valuable insights into the regulatory functions of Pan1p in endocytosis. Incidentally, endocytosis is also the gateway to cellular pathogen entry, and therefore a detailed understanding of the mechanism is central to disease prevention. “The new mechanism of cellular endocytosis revealed in this study not only elucidates the molecular mechanisms but will also lead to the development of therapeutic methods” hopes Prof. Toshima.