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. 2023 Nov 27;63(22):7159-7170.
doi: 10.1021/acs.jcim.3c00917. Epub 2023 Nov 8.

Combined Computational-Biochemical Approach Offers an Accelerated Path to Membrane Protein Solubilization

Affiliations

Combined Computational-Biochemical Approach Offers an Accelerated Path to Membrane Protein Solubilization

Mariah R Pierce et al. J Chem Inf Model. .

Abstract

Membrane proteins are difficult to isolate and purify due to their dependence on the surrounding lipid membrane for structural stability. Detergents are often used to solubilize these proteins, with this approach requiring a careful balance between protein solubilization and denaturation. Determining which detergent is most appropriate for a given protein has largely been done empirically through screening, which requires large amounts of membrane protein and associated resources. Here, we describe an alternative to conventional detergent screening using a computational modeling approach to identify the most likely candidate detergents for solubilizing a protein of interest. We demonstrate our approach using ghrelin O-acyltransferase (GOAT), a member of the membrane-bound O-acyltransferase family of integral membrane enzymes that has not been solubilized or purified in active form. A computationally derived GOAT structural model provides the only structural information required for this approach. Using computational analysis of detergent ability to penetrate phospholipid bilayers and stabilize the GOAT structure, a panel of common detergents were rank-ordered for their proposed ability to solubilize GOAT. The simulations were performed at all-atom resolution for a combined simulation time of 24 μs. Independently, we biologically screened these detergents for their solubilization of fluorescently tagged GOAT constructs. We found computational prediction of protein structural stabilization was the better predictor of detergent solubilization ability, but neither approach was effective for predicting detergents that would support GOAT enzymatic function. The current rapid expansion of membrane protein computational models lacking experimental structural information and our computational detergent screening approach can greatly improve the efficiency of membrane protein detergent solubilization, supporting downstream functional and structural studies.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Computational modeling of phospholipid bilayer invasion by detergents. (A) Schematic of detergent invasion of DOPC/DPPC phospholipid (light brown) bilayer by solubilizing detergent (blue). Panel made with Biorender.com. (B) Percent detergent around each lipid, ranked from highest to lowest population. DDM (red, filled triangle), BOG (blue, filled circle), MEGA-9 (green, filled diamond), LMNG (cyan, filled square), FOS-16 (black, open circle), GDN (yellow, open triangle), FOS-12 (brown, open diamond), and CHAPS (pink, open square). (C) Average percentage of detergent around each lipid, with same color scheme as panel B. Error bars represent one standard deviation.
Figure 2
Figure 2
hGOAT structure stabilization by detergents. (A) Root mean square fluctuation (RMSF) of hGOAT residues. (B) Average ΔRMSF for each detergent. Color scheme: DDM (red), BOG (blue), MEGA-9 (green), LMNG (cyan), FOS-16 (black), GDN (yellow), FOS-12 (brown), and CHAPS (pink).
Figure 3
Figure 3
Comparison of hGOAT stabilization and detergent interactions. (A, B) Structural alignment of hGOAT structure (cyan) with hGOAT solubilized by (A) FOS-16 (orange) and (B) MEGA-9 (purple). Black arrows indicate regions of highest deviation between the structures as reflected by RMSF. (C, D) Interactions of hGOAT (gray, cartoon) with hydrophilic head groups (green, sticks) and hydrophobic tail groups (orange, sticks) when solubilized by (A) FOS-16 and (B) MEGA-9 exhibit distinct patterns with more headgroup interactions, with the nonionic MEGA-9 polyol than the FOS-16 phosphocholine zwitterion.
Figure 4
Figure 4
Expression and activity validation of a hGOAT-EGFP construct. To allow fluorescence-based detection, an eGFP tag was appended to the C-terminus of human GOAT (hGOAT). Addition of this tag preserved expression and activity of GOAT. (A) Structural model of the hGOAT-eGFP fusion protein embedded in a phospholipid bilayer. Lipid headgroups are shown as gray spheres, and lipid tails are omitted for clarity. (B) In-gel fluorescence detection of hGOAT-eGFP. The gel was imaged with the Alexa488 filter (samples) and Coomassie Blue (ladder), and these two filter images were overlaid. The presence of the 55 kDa band for hGOAT-eGFP is consistent with a partially denatured protein maintaining the eGFP fold, as described in the text. No fluorescence was observed for the empty vector control and our previously published hGOAT-3xTAG construct. (C) Anti-MBOAT4 immunoblot detects both a 70 and 55 kDa band corresponding to the fully denatured hGOAT-eGFP at 70 kDa and the GOAT denatured by eGFP still fluorescent band at 55 kDa. Our previously published GOAT construct hGOAT-3xTAG (57 kDa; running size 45 kDa) serves as a positive control for the antibody. (D) hGOAT-eGFP construct catalyzed ghrelin acylation. In the presence of enzyme and octanoyl-CoA, the substrate peptide is acylated to yield the more hydrophobic octanoylated product as monitored by reverse-phase HPLC with fluorescence detection.
Figure 5
Figure 5
hGOAT-eGFP solubilization monitored by in-gel fluorescence. (A) Buffer sample negative control and untreated hGOAT-eGFP negative control provide size standard for fluorescent hGOAT-eGFP, with FOS-16 and FOS-12 exhibiting efficient solubilization of hGOAT-eGFP as shown by majority of fluorescence in the supernatant/soluble protein fraction. (B) hGOAT-eGFP solubilization trials exhibited partial solubilization with CHAPS, DDM, and GDN, more effective solubilization with LMNG, and little to no solubilization with MEGA-9 and BOG.
Figure 6
Figure 6
MEGA-9 maintains octanoylation activity in its hGOAT-eGFP supernatant fraction. Each hGOAT-eGFP solubilization supernatant fraction was assessed for ghrelin octanoylation activity. Reaction lacking the acyl donor served as a negative control, with untreated WT hGOAT-eGFP and the pellet from the buffer-treated hGOAT-eGFP serving as positive controls. Only the supernatant fraction from the MEGA-9 solubilizations exhibited significant activity with ∼20% conversion of substrate to octanoylated product. Activity screening reactions were performed in duplicate and analyzed as described in the Methods section.
Figure 7
Figure 7
FSEC analysis indicates polydispersity for hGOAT-eGFP solubilized in DDM, GDN, and LMNG. (A) Optimized conditions increase hGOAT-eGFP solubilization in GDN, LMNG, DDM, and CHAPS. (B) FSEC analysis indicates a single peak for FOS-16 solubilized hGOAT-eGFP, while hGOAT-eGFP in DDM, GDN, and LMNG exhibit multiple peaks indicating enzyme–detergent complex polydispersity. Representative chromatograms reflect solubilizations run in triplicate on different days.

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