Prof. dr. Christian Kaiser

Biophysical Chemistry of Membranes & Proteins

graphic of cellular processes

Proteins are essential for all cellular processes. To function correctly, proteins must promptly and efficiently fold into their native structures. Folding begins co-translationally, while the ribosome still synthesizes the nascent protein. Folding and translation are thus inextricably linked. Co-translational folding is a crucial aspect of protein biogenesis. Like long strands of yarn quickly become tangled if not organized into balls, proteins that fail to adopt their functional structures in a timely fashion tend to misfold and clump together into aggregates. Many misfolded or aggregated proteins are toxic to cells. Neurons are particularly sensitive to this toxicity, giving rise to debilitating neurodegenerative disorders that include Alzheimer’s or Parkinson’s disease, for which there currently is no cure.

My research program addresses fundamental but poorly understood questions about protein biogenesis. How do proteins avoid misfolding and efficiently find their functional structures? How do helper proteins called molecular chaperones assist folding in the cytosol? And why do some proteins robustly fold into their functional shapes whereas others misfold into toxic species? We aim to determine the molecular underpinnings of folding that govern both normal cellular function and neurodegenerative disease states. To this end, we are combining single-molecule biophysics, biochemistry, and live-cell experiments into novel approaches (see below for more details). These studies not only provide fundamental insights into the biology of protein biogenesis, but might also open avenues for combating protein homeostasis diseases.

Single-molecule folding studies

To dissect the folding of large proteins, we are using single-molecule optical tweezers. This tool is ideally suited to untangle complex folding pathways. We have developed approaches for following the co-translational folding of nascent proteins on the ribosome with optical tweezers. Our work revealed that the ribosome itself counters misfolding through finely tuned interactions with the nascent protein. Strikingly, we also found that nascent chain-binding molecular chaperones protect already folded domains against denaturation, a novel function with broad implications for protein folding and structural maintenance. Collectively, our single-molecule studies of co-translational folding and chaperone function are revealing how folding pathways are shaped by the cellular protein biogenesis machinery.

Nascent chain folding in the cell

In the cell, folding is guided by an intricate network of interactions with cellular components that cannot be reproduced in vitro. Live cell experiments are required to capture this complexity. However, measuring folding in cells is challenging. We have developed a high-throughput reporter assay for detecting nascent chain folding in bacterial and cultured mammalian cells. Measuring folding proteome-wide in living cells is revealing general principles of nascent chain folding in the cellular environment. Excitingly, the development of the reporter assay puts within reach the mechanistic investigation of folding in multicellular organisms, such as the nematode Caenorhabditis elegans. Our novel approaches to define folding in vivo enable us to relate detailed in vitro measurements to processes inside living cells, bridging the gap between quantitative single-molecule measurements and biologically relevant cellular studies.