Prof. Scott Emr, molecular biology, earned the 2021 Shaw Prize for Life Science and Medicine for unpacking the secrets of a set of proteins that are able to detect key molecular signals to transport proteins to their rightful homes in cells.
The Emr lab discovered the function of the endosomal sorting complex required for transport proteins by deleting the genes responsible for the synthesis of the proteins that guide the formation of endosomal vesicles in yeast cells.
“[Genetics allows us to] look into the darkness and ask the organism, tell me, how do you do this,” Emr said.
In their most recent research, Emr and his colleagues described the recognition and coupling of the protein ubiquitin — a molecular switch that functions as a signal for recruiting vacuolar protein sorting genes — cellular United Postal Service and intiating membrane bending and pinching.
The experiment employed the fact that yeast digests its nutrients extracellularly through enzymes like invertase to process the sucrose in the growth medium into glucose and fructose — the energy currency in yeast.
Emr fused invertase to carboxypeptidase, a protein that needs to be transported within the cell to the lysosome. When the wildtype protein sequences — those unaltered by Emr — were expressed together, the hybrid protein was always delivered to the lysosome, a site for recycling materials in the cell such as old receptors, according to Emr.
Through inducing loss-of-function mutations in the ubiquitin binding domains of carboxypeptidase, Emr proved that ubiquitin was necessary for recruiting the ESCRT machinery to traffic the hybrid protein to the lysosome.
Because Emr was able to delete the ubiquitin recognizing moiety in carboxypeptidase the hybrid protein was secreted instead of being sent to the lysosome and allowed for growth in the yeast mutant studied, establishing that ubiquitin signaling recognition was important for protein trafficking.
“We isolated some 40 different genes in yeast that, when mutated, altered the delivery of proteins to the lysosomes,” Emr said.
The ubiquitin binding domains in carboxypeptidase were characterized through Western Blotting — a technique that allows researchers to detect key amino acid sequences in a mixture, using antibodies.
In animal cells, Emr explained that the control of cell growth in mammals is a prime example in which ESCRT protein complexes are frequently used. However, Emr said that ESCRT mutations are not often observed in cancer, which is characterized by uncontrolled cell growth, but is induced by different mutations.
“The [ESCRT proteins] play a critical role in how cells shut off growth factor signaling,” Emr said. “If you get a cut on your hand, cells called platelets enter the wound and release growth factors — triggering growth and repair of the wound, allowing skin cells to multiply, and heal.”
However, in viruses such as HIV where the orientation of the membrane pinching off and exiting animal cells is similar to that of vesicles binding late endosomes, researchers have been able to produce selective drugs inhibiting the ESCRT complexes, according to Emr.
“I think everyone who works in the life sciences is, of course, always amazed by how unexpected events and discoveries cause huge transitions,” Emr said. “It’s been great to be part of this great collaboration among the life scientists.”