Yanning team reveals 'transit journey' of cholesterol molecules

Cholesterol Molecular Transport Model Diagram
On June 15, the Princeton University Yanning team and the University of New South Wales Yang Hongyuan teamed up to publish a research paper online in Cell entitled "Structural Basis for The Outward Transport of Low PH-Dependent Lysosome Cholesterol." This is the work of Yan Ning's team, which was once again published in the journal Nature last month, in the top international journals.
this study shows the transport of cholesterol molecules from the lysosome cavity to the lysosome membrane, mediated by NPC1 and NPC2, and reveals for the first time the dependence of NPC1 protein function on pH.cholesterol is an integral part of the cell membrane and regulates the fluidity, thickness and curvature of the cell membrane. It is also a prelude to synthetic bile, vitamin D, and a variety of hormones in mammals.
can be ingested from food or synthesized in liver cells. Because cholesterol is insoluble in water, it is mainly present in the blood in the form of lipoprotein particles, which then enter tissue cells in the form of cellulation. In the lysosome, cholesterol is then released from lipoprotein particles, then mediated by NPC1 and NPC2 proteins, and transported to other parts of the cell to function.
if the NPC1 and NPC2 proteins develop mutations, cholesterol accumulates excessively in lysosomes, affecting the human nervous system and leading to type C Nemanpicosis (NPC). Clinical manifestations include premature death, decreased intelligence, unclear speech, incomplete muscle tone and other symptoms. There is currently no cure and there is a lack of effective means to stop its deterioration.
Yanning's team has been working on structural biology and biochemistry of the cholesterol metabolic regulatory path path, and as early as 2016, it reported on the frozen electromirror structure of the human source NPC1 protein 4.4 E resolution, the first frozen electromirror structure to transfer proteins analyzed by the team. In 2018, the team also analyzed the frozen electroscope structure of THEC1 iso-protein Patched 1 (Ptch1), further revealing the structure of these proteins in transporting steroids.
Qian Hongwu, co-lead author of this paper and a postdoctoral student in the Department of Molecular Biology at Princeton University, told China Science Daily, "On the basis of the fore above, this study further deepens our understanding of NPC1 transshipment capabilities." TheNPC2 is a soluble protein. NPC1 is a membrane protein consisting of 13 trans-membrane helixes and 3 soluble domains (i.e. NTD, domain C, domain I) located on the inside of the cavity. The 3rd-7th transfilm helix forms a conservative steroid-sensing domain (SSD).
in Ptch1, SSD has been shown to bind to cholesterol molecules.
Previous studies have also found that in lysosomes, the first stop on a cholesterol journey released from lipoprotein particles is "meeting" NPC2, where water-insoluble cholesterol is wrapped in NPC2 and free in a hydro-hydro environment. The second stop, NTD in NPC1, is then transferred from NPC1 to the lysosome membrane. However, the synergy between the various parts of this transshipment journey is not clear.
, the researchers analyzed the structure of the NPC1-NPC2 complex. For ease of transport, NPC2 interacts with domain C, facing the cholesterol-binding pockets in NPC2 and NTD. This is the state of the second journey of cholesterol.
the third stage of the journey from NTD to the trans-membrane zone, where a "tunnel" appears - the central channel.
in the structure of NPC1 under pH 8.0 conditions, the authors found that there is a channel connecting the NTD and the trans-membrane region, located in the middle of domain C and domain I, which are tangled by these two domains, namely the central channel. The exit of the central channel across the membrane region points exactly to the point at which SSD binds to cholesterol molecules." has a similar channel in npc1's ymoglobulin Patch1 structure. These findings suggest that cholesterol may have been transported from NTD to SSD across the membrane through the central channel and eventually released to the lysosome membrane. Qian Hongwu said that in pH 5.5 structural analysis, we found a "residual" cholesterol molecule in the central channel, which further verified the results.
through fine data processing, the authors also found that under pH 5.5, NTD has two configuration states (A and B). "In State A, the opening of the cholesterol-binding pocket on the NTD deviates from the middle channel, presenting a state of acceptance of cholesterol from the 'upper home' NPC2, while in State B, the opening of the pocket approaches the entrance to the central channel, presenting a state in which cholesterol is to be passed to the central channel of the 'tunnel'." Qian Hongwu said it is also a re-validation of the cholesterol journey route site.
the above research, the author puts forward the "K-turn" transshipment model. "In the transit of cholesterol molecules, the direction is reversed once, just like a car turning over. This allows it to enter the lysosome membrane. Another co-author of this paper, Dr. Yu Xuelan, a doctoral student in the Department of Chemistry at Princeton University, said.study, the authors compared NPC1's structure under pH 5.5 and pH 8.0 and found that the exit of the central channel connection span sSD was closed and open, respectively.
" suggests that pH may have a regulatory effect on NPC1's transport function. This provides a new direction for the functional study of NPC1. Qian Hongwu said.
, it is understood that about 95% of the mutations directly related to type C Nemanpike disease occur in NPC1 and the remaining 5% in NPC2. In the study, the authors pinpointed 218 mutant amino acid points associated with Type C Nemanpike disease on the high-resolution structure of NPC1, and divided them into structural and functional mutations.
" structural mutations affect protein folding and positioning, functional mutations affect cholesterol transport, which provides clues for future treatment of different factors. Yan Ning told China Science Daily.
"In addition to these interesting observations, new questions have been identified, such as why does NPC1 require an acidic environment?" Are hydrogen ions transported at the same time as cholesterol, or does NPC1 only need acidic environmental activation? How is cholesterol transported to other membrane structures of cells after it enters the lysosome membrane? All these need to be further studied. She said. (Source: Liu Runan, China Science Journal)
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