Deciphering a new role of a protein complex aiding the Pac-man job inside cells!

rm-1

Bagging two Nobel Prizes, first in 1974 1 and then recently in 2016 2, work on Autophagy stands at the cutting edge of both fundamental and application based research. The word autophagy with its Greek origin, meaning ‘self-eating’ (auto: self, phagein: to eat) is one among several efficient mechanisms functioning inside a living organism.

The channel through which intracellular materials such as certain byproducts of metabolism, damaged proteins, those that are brewing for degradation are actively ingested by the cell itself is what defines the course and action of autophagy. These intracellular materials that are intended for ultimate degradation are ingested by a structure called autophagosome or the ‘Pacman’ as aptly called by Dr. Ravi Manjithaya, a research scientist at MBGU, JNCASR. These autophagosomes then play the role of a garbage truck where they transport the ingested cargo into another structure called lysosome. Post fusion with the vacuole (lysosome), multiple enzymes help in the ultimate degradation and absorption of intracellular waste. This process also involves sending back useful items like amino acids back to the cytoplasm.

So, you see there is this whole system of collecting the trash, degrading and eventually re-cycling them! What’s more fun and further interesting is digging out the mechanism underlying the efficient functionality of this integral system of autophagy.

rm-2Dr. Ravi Manjithaya with his graduate student Gaurav Barve at the           Autophagy Lab

Dr. Ravi Manjithaya’s (RM) group at MBGU studies autophagy and related elemental processes using yeast, human cells and mouse as model systems. The molecular components of autophagy were first laid out in the yeast Saccharomyces cerevisiae. His group explores through multiple approaches like, the ‘cargo approach’ to identify regulatory mechanisms fundamental to autophagy by examining which of the toughest cargos can be tackled by this process. For this, his group employs both, a ‘chemical biology approach’ and the ‘classical genetics approach’.

One of his group’s latest works has carried out an unbiased screen for autophagy defects in yeast Saccharomyces cerevisiae, and shown the significance of a group of proteins called ‘Septins’ during the early stages of autophagy 3. Septins, first discovered in yeast S. cerevisiae are a group of proteins that are highly conserved in eukaryotes (although absent in plants) and serve as one of the key cytoskeletal elements in cell division process. Functional role of septins in yeast autophagy was unclear before this study. However, septins have been shown to have role in dynamics of cell membrane shapes. In order to determine the importance of septins in autophagy and their potential contribution to the formation of the autophagosomal membrane structure per se, RM’S group carried out well-thought out targeted experiments using budding yeast cells.

By following degradation of cargo (peroxisomes) for autophagic capture and degradation, RM’s group identified several septins whose functional forms were required in this process. To understand their roles better, the lead author and graduate student, Gaurav resorted to fluorescent live cell microscopy. By following these fluorescently tagged (GFP: Green Fluorescent Protein) septins, Gaurav observed the transition of these septins towards the locations inside the cell that are important for autophagosome formation. Interestingly, the septin-GFP proteins were often found in the shape of ring (roughly the size of autophagosomes) surrounding the pre-autophagosomal structures (PAS), which is the birthplace of autophagosomes. The team further went ahead to examine if the septins had physical interaction with autophagy proteins and identified two autophagy proteins, Atg8 and Atg9 as septin interacting partners. Gaurav elucidated how precisely this septin movement is vital  for providing membrane from various cellular locations for building autophagosomes.

rm-3

Overall, RM’s team for the first time exemplified the key role of these groups of proteins called septins in autophagosome maturation, direct physical interaction with autophagosomal membrane proteins (Atg8, Atg9), movement of septins from one location to another and most intriguingly development of septin rings that are similar in dimension to that of the autophagosomes. This study has opened up new questions such as how exactly septins help in autophagosome formation? Since septins interacted with Atg9 vesicles, how do the aid in providing membrane source for autophagosome formation? Does the complex of septins involved in cell division is the same complex that helps in autophagosome formation or are there some additional factors involved in it? And finally, what drives septins off from their usual localization at the bud-neck (region between mother and daughter yeast cell) to the site of autophagosome formation?

This work published in the Journal of Cell Science (JCS), 2018 details the beauty of this entire mechanism at play.

This article is authored by Manaswini Sarangi, Evolutionary Biology Laboratory, EIBU, JNCASR.

References

  1. “Physiology or Medicine 1974 – Press Release”. org.Nobel Media AB (2014). http://www.nobelprize.org/nobel_prizes/medicine/laureates/1974/press.html
  1. “The 2016 Nobel Prize in Physiology or Medicine – Press Release”. org.Nobel Media AB (2014). http://www.nobelprize.org/nobel_prizes/medicine/laureates/2016/press.html
  1. Barve, Gaurav, et al. “Septins are involved at the early stages of macroautophagy in S. cerevisiae.” J Cell Sci(2018): jcs-209098.
  1. Barve, Gaurav. “First person–Gaurav Barve.” (2018).

 

 

Leave a Reply

Your email address will not be published. Required fields are marked *