Protein Misfolding and Cellular Signaling: From DNA to Folding to GPCR Pathways

Overview

This lecture links the sequence of DNA to the folding and function of proteins, then explores how cells manage misfolded proteins and maintain proteostasis. It then introduces the three basic steps of cellular signaling and the major receptor types that mediate outside-to-inside communication.

The talk ties together the roles of chaperones, ubiquitin tagging, and the proteasome with a broader discussion of signaling networks, and it uses concrete examples such as autocrine, paracrine, endocrine, and juxtacrine signaling, as well as GPCRs and receptor tyrosine kinases.

Introduction to Protein Folding and Signaling

The video begins by outlining how a protein’s amino-acid sequence, encoded by DNA, determines its final folded structure and function. It emphasizes that the messenger RNA and pre-mRNA (with splicing) influence protein localization and regulation, and notes that post-translational modifications are guided by the underlying DNA sequence.

The presenter then pivots to protein misfolding, explaining that mutations or intrinsic sequence features can slow or misdirect folding. Partially folded proteins can aggregate, especially under overexpression, and cellular quality-control mechanisms are activated to rescue or dispose of them.

Quality Control: Chaperones, Ubiquitin, and the Proteasome

Chaperones help partially folded proteins achieve their proper structure, acting as ‘shielding helpers’ during folding. If folding fails, ubiquitin tags the misfolded protein, signaling delivery to the proteasome, a large protease complex that unfolds and degrades proteins into short peptides. The proteasome’s function is critical for removing misfolded or aggregated proteins and maintaining cellular health. The discussion includes a practical analogy to a paper shredder for misfolded proteins and highlights the role of the 26S proteasome in protein turnover and antigen presentation.

Misfolding and Disease

The talk surveys diseases linked to misfolded proteins, including neurodegenerative disorders such as Alzheimer’s and prion diseases. It touches on how misfolded proteins can seed further misfolding and how aging and genetics can influence susceptibility, underscoring the ongoing research to slow or mitigate these processes with small-molecule inhibitors or interventions.

The Canonical Model of Protein Signaling

The core of the lecture then shifts to signaling, describing a three-component framework: signal reception, signal transduction, and response. It explains that signals are often extracellular, binding to membrane receptors, with signaling flux often initiated outside the cell and transmitted inward, sometimes across the membrane.

The speaker emphasizes the complexity of signaling networks, including cross-talk and integration across multiple pathways, and introduces the concept of systems biology as a way to model flux and regulation in signaling networks.

Signal Forms and Receptor Classes

Signals can be autocrine, paracrine, endocrine, or juxtacrine, depending on their origin and distance to the target cell. The video also discusses intracellular signals if a hydrophobic molecule crosses the membrane. The major receptor classes highlighted are G protein coupled receptors (GPCRs), receptor tyrosine kinases (RTKs), and ion channels, with a nod to nuclear receptors for steroid hormones.

The GPCR example is illustrated with their seven transmembrane helices, showing how extracellular ligand binding can trigger conformational changes that propagate signals inside the cell. The insulin example is used to illustrate endocrine signaling, where a hormone travels through the circulatory system to distant targets.

Implications and Next Steps

The lecture closes with a note on drug discovery considerations, the importance of accounting for network integration, and a preview of next class content that will explore signaling pathways in greater detail with concrete cellular examples.