Topological Mapping of BRIL Reveals a Type II Orientation and Effects of Osteogenesis Imperfecta Mutations on Its Cellular Destination
Bibliographic record
Abstract
BRIL/IFITM5 is a membrane protein present almost exclusively in osteoblasts, which is believed to adopt a type III (N-out/C-out) topology. Mutations in IFITM5 cause OI type V, but the characteristics of the mutant protein and the mechanism involved are still unknown. The purpose of the current study was to re-assess the topology, localization, and biochemical properties of BRIL and compare it to the OI type V mutant in MC3T3 osteoblasts. Immunofluorescence labeling was performed with antibodies directed against BRIL N- or C-terminus. In intact cells, BRIL labeling was conspicuously detected at the plasma membrane only with the anti-C antibody. Detection of BRIL N-terminus was only possible after cell permeabilization, revealing both plasma membrane and Golgi labeling. Trypsinization of live cells expressing BRIL only cleaved off the C-terminus, confirming that it is a type II protein and that its N-terminus is intracellular. A truncated form of BRIL lacking the last 18 residues did not appear to affect localization, whereas mutation of a single leucine to arginine within the transmembrane segment abolished plasma membrane targeting. BRIL is first targeted to the endoplasmic reticulum as the entry point to the secretory pathway and rapidly traffics to the Golgi via a COPII-dependent pathway. BRIL was found to be palmitoylated and two conserved cysteine residues (C52 and C53) were critical for targeting to the plasma membrane. The OI type V mutant BRIL, having a five residue extension (MALEP) at its N-terminus, presented with exactly the same topological and biochemical characteristics as wild type BRIL. In contrast, the S42 > L mutant BRIL was trapped intracellularly in the Golgi. BRIL proteins and transcripts were equally detected in bone from a patient with OI type V, suggesting that the cause of the disease is a gain of function mediated by a faulty intracellular activity of the mutant BRIL.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.003 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".