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For many years, the scientific community held a prevailing belief that TGF-Beta, a crucial signaling protein involved in processes ranging from embryonic development to the progression of cancer, could only function once it was liberated from a restrictive complex known as the latency-associated protein (LAP).
Recent advancements at UC San Francisco have challenged this notion. Utilizing cryogenic electron microscopy (cryo-EM), a sophisticated technique that allows researchers to create three-dimensional models of molecules with atomic precision, scientists have uncovered that TGF-Beta possesses greater complexity than previously understood.
It turns out that TGF-Beta is capable of moving and extending its reach even while still bound within its “straitjacket,” allowing it to engage with a neighboring receptor situated at the cell surface.
The results, detailed in a study published on September 16 in the journal Cell, disrupt long-standing beliefs about the functional dynamics of TGF-Beta. This new understanding could enhance therapies targeting TGF-Beta, particularly emerging cancer treatments known as checkpoint inhibitors, which have achieved mixed results in medical practice.
Additionally, these findings suggest an even more intricate molecular landscape, proposing that influential proteins like TGF-Beta can adopt unexpected configurations to facilitate crucial cellular tasks.
“Historically, the research focus has been on stabilizing these signals for high-resolution imaging, often overlooking the role of flexibility within their function’s framework,” noted Yifan Cheng, Ph.D., a professor of cellular and molecular pharmacology at UCSF and co-senior author of the study. “Our results indicate that this flexibility is essential for TGF-Beta’s activity and may elucidate mechanisms behind other less-understood signals, potentially impacting disease understanding and treatment.”
Signaling While Tethered
Four years prior, Cheng and his co-senior author, Stephen Nishimura, MD, made a surprising discovery: TGF-Beta can initiate signaling pathways to its receptor even when it remains attached to its straitjacket, LAP.
This revelation contradicted the long-held view that TGF-Beta must be fully released from LAP to perform its signaling functions. Previous beliefs suggested that without such a release, fundamental biological processes, including the generation of new cells without tumor formation, would fail to operate correctly.
However, experiments involving the creation of a permanent link between TGF-Beta and its LAP in mice demonstrated that the organisms could still thrive, implying that TGF-Beta retained its signaling capabilities while tethered.
To dive deeper into this phenomenon, Cheng and Nishimura employed their expertise in cryo-EM.
This method entails rapidly freezing protein mixtures, followed by capturing hundreds of thousands of images to analyze structural interactions. Typically, algorithms sort these images to delineate the most common arrangements of proteins, but this has historically overlooked intermediate states that might occur during cellular signaling.
Recognizing that TGF-Beta was signaling while still bound to LAP, UCSF scientists hypothesized that a wider range of intermediate states might exist, which traditional methods would misinterpret and disregard.
“Cryo-EM studies often focus on clear observations, but we realized that the significant insights could be hidden in the less distinct areas of our data,” said Nishimura, a professor of pathology at UCSF. “Our approach targeted these areas.”
Insights into Molecular Dynamics
To better observe the dynamic behavior of TGF-Beta within its LAP confines, the research team meticulously manipulated specific portions of either TGF-Beta or LAP before employing cryo-EM to explore their interactions with TGF-Beta’s receptor.
The observed molecular uncertainties, referred to as entropy, exhibited mobility across different areas of TGF-Beta, indicating its ability to operate, at least partially, despite restriction by LAP.
This molecular motion allowed TGF-Beta to manage a brief but sufficient extension beyond LAP, thereby enabling receptor detection. While the duration of this movement was fleeting, the systematic constraints employed during experiments led to the clearest representations of TGF-Beta’s signaling capabilities yet captured.
Ultimately, this research alters fundamental perceptions of TGF-Beta and potentially other signaling molecules, advocating for a paradigm shift in understanding cellular communication that acknowledges fluid motion instead of solely distinct conformational changes.
“These findings extend beyond TGF-Beta and influence areas such as intercellular communication and disease modeling, prompting a broader reevaluation in how we interpret molecular behavior,” expressed Cheng. “Our investigation indicates a wealth of additional discoveries awaiting exploration through cryo-EM methodologies.”
The research included contributions from UCSF authors Mingliang Jin, Robert I. Seed, Guoqing Cai, Tiffany Shing, Li Wang, Saburo Ito, Anthony Cormier, Stephanie A. Wankowicz, Jillian M. Jespersen, Jody L. Baron, Nicholas D. Carey, Melody G. Campbell, Zanlin Yu, Weihua Wen, Jianlong Lou, and James Marks.
More information:
Mingliang Jin et al, Dynamic allostery drives autocrine and paracrine TGF-β signaling, Cell (2024). DOI: 10.1016/j.cell.2024.08.036
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University of California, San Francisco
Source
phys.org