New study explains how TMEM63B protein regulates cell membranes

An autoinhibitory domain in the C-terminal tail of the TMEM63B, a mechanosensitive lipid scramblase, helps maintain its inactive state under resting conditions, report researchers from Institute of Science Tokyo. Any modifications in this regulatory region, particularly the Leu776 residue, reverses the inhibitory action and causes constitutive lipid scrambling, disrupting normal phospholipid asymmetry. These findings provide new insights into TMEM63B regulation and may improve our understanding of how its dysregulation contributes to neurodegenerative disease pathogenesis.

The plasma membrane is a thin, flexible barrier that surrounds and protects cells. It maintains a carefully controlled distribution of lipids between its inner and outer layers. This asymmetrical distribution of lipids is important for normal cellular function, but under certain conditions, it gets rapidly disrupted by lipid scramblases. Lipid scramblases are proteins that move phospholipids between two sides of the membrane. Cells, therefore, need to regulate when the activity of these proteins.

One such protein is TMEM63B, a mechanosensitive lipid scramblase that responds to changes in the physical properties of the cell membrane. Previous studies have stated that TMEM63B becomes activated when properties such as membrane thickness or curvature are altered. But an important question remains: how does TMEM63B stay inactive when the membrane is at rest?

To fill this gap, a research team led by graduate student(at the time of the study) Megumi Nishimura, Lecturer Yugo Miyata, and Professor Katsumori Segawa, all from the Department of Medical Chemistry, Medical Research Laboratory, Institute of Integrated Research, Institute of Science Tokyo (Science Tokyo), Japan, in collaboration with Associate Professor Norimichi Nomura from Kyoto University, Japan, and Professor Tomohiro Nishizawa from Yokohama City University, Japan, has now identified a novel molecular mechanism in TMEM63B. The study, made available online on June 4, 2026, and published in Volume 302, Issue 7 of the Journal of Biological Chemistry on July 1, 2026, shows that the C-terminal tail of TMEM63B plays a key role. It has as an autoinhibitory region that keeps the scramblase 'switched off' under resting conditions.

The researchers started with the investigation of YN9303-24, an antibody previously shown to promote the open conformation of TMEM63B. To pinpoint where the antibody binds (epitope), the researchers used several approaches: chimeric proteins (that combine regions from different proteins), progressively shortened versions of the C-terminal tail (truncations), and targeted deletions of specific amino acids. These experiments showed that the antibody-binding site lies within the intracellular C-terminal tail of TMEM63B. They identified a short sequence of three amino acids, called the AQV motif, at positions 773–775 within this region, which was recognized by the antibody.

Additionally, they found that the neighboring sequence, LQD at residues 776–778, was specifically important for controlling TMEM63B itself. Eliminating these three amino acids (LQD) resulted in the protein becoming constitutively active, producing lipid scrambling even without membrane stimulation. Further experiments identified Leucine at 776 (Leu776) as the key residue modulating the protein confirmation and activity. Replacing Leu776 with alanine strongly increased exposure of the phospholipid phosphatidylserine (PS) on the cell surface, whereas PS is normally predominant on the inner cytoplasmic side of the plasma membrane under resting conditions. Changes to the neighboring Gln777 or Asp778 didn't result in significant changes. The increased uptake of fluorescent phosphatidylcholine provided additional evidence that lipid scrambling was enhanced in the mutant protein.

The C-terminal tail functions like a molecular brake, keeping TMEM63B inactive under resting conditions until changes in the membrane allow it to become activated."

Professor Katsumori Segawa, Department of Medical Chemistry, Institute of Science Tokyo

 Additional structural analysis helped explain how this brake may work. In the open-state structure, the AQVLQD motif lies next to two intracellular helices, IL2H2 and IL2H3, which form part of a beam-like intracellular domain of TMEM63B. Leu776 is positioned close to several hydrophobic residues in these helices, suggesting that interactions between the C-terminal tail and this intracellular domain could help stabilize the inactive state. When such interactions are disrupted, the restraint on TMEM63B may get released, allowing the lipid translocation pathway to become active.

Moreover, this novel mechanism has implications for neurological diseases. Earlier studies have linked disease associated TMEM63B mutations with neurodevelopmental and neurodegenerative disorders. "Understanding how the C-terminal tail restrains TMEM63B gives us a clearer picture of how inappropriate activation of this scramblase could disrupt membrane function and potentially contribute to neurological disorders," notes Segawa.

Overall, the study reveals a simple yet important principle of TMEM63B regulation: the C-terminal tail acts as a molecular brake. Under resting conditions, this region helps keep the scramblase inactive. Disrupting a critical part of the tail, particularly Leu776, releases this brake and triggers constitutive lipid scrambling. These findings provide a new framework for understanding how TMEM63B is regulated and may guide future studies investigating its role in disease pathogenesis and potential therapeutic strategies.