Fellow’s research: Cysteine energetically favours hydrophobicity in biological membranes
26 Jun 2019
Dr R Mahalakshmi, Intermediate Fellow
Indian Institute of Science Education and Research, Bhopal
Our recently published work provides a thermodynamic explanation for the infrequent occurrence of the amino acid, cysteine, in proteins present on biological membranes (i.e. membrane proteins).
Proteins present on biological membranes are gatekeepers of the cell and are vital for their structural integrity and function. These membrane proteins are involved in various cellular functions that are important for the survival of any living organism. Since they play an important role in regulating what goes in and out of the cell, membrane proteins are important drug targets.
Membrane proteins are broadly classified in two categories based on their location in the cell membrane – integral proteins and peripheral proteins (see figure 1). Amino acid residues (building block of proteins) in membrane proteins are arranged according to their polarity which determines the placement of these proteins in the biological membrane. The non-polar or hydrophobic residues attach directly to the lipid bilayer in the cell membrane. Whereas, the polar or hydrophilic residues are located facing aqueous solutions on either side of the membrane.
Cysteine, the naturally occurring amino acid, has recently garnered increased attention with its role in redox homeostasis and in determining protein structure and stability. Cysteine residues have a uniquely skewed duality in their physico-chemical attributes, exhibiting both hydrophobic and polar characteristics. As the reactivity of a cysteine depends on its chemical characteristics, it is vital to address whether cysteine presents a hydrophobic or polar nature in proteins, and the outcome of its characteristics on the thermodynamic stability of a folded protein.
Figure 1: Classification of membrane proteins (Credit: BioNinja)
In our recent study, we utilized a cysteine-scanning mutagenesis strategy on a model transmembrane β-barrel protein to measure the magnitude of hydrophobicity presented by cysteine in the lipid membrane. Interestingly, we find (i) that cysteine has a near-universal destabilizing effect when incorporated in a membrane protein, and (ii) a membrane depth-dependent contribution, with maximum destabilization seen at the membrane interface. Our findings provide a thermodynamic basis for why cysteine residues are infrequent in membrane proteins. We also infer that the evolutionary retention of interface cysteines despite its destabilizing contribution, particularly in eukaryotic membrane proteins, has an overwhelming functional significance.
Observations made from this study could prove useful for a variety of applications in the analysis and the ab initio design of membrane proteins to aid in design of therapeutics.
Figure 2: Cartoon representation of the model β-barrel PagP highlighting the positions where cysteines were systematically introduced for measurement of energetics (left). Dependence of the measured change in energetics on the parent residue and membrane depth is compared in the form of a scatter plot (right).
Hydrophobic characteristic is energetically preferred for cysteine in a model membrane protein. Bharat Ramasubramanian Iyer, Radhakrishnan Mahalakshmi. Biophysical Journal, June 2019
Banner Image Credit: Crystal Structure of Outer Membrane Enzyme PagP, DOI: 10.2210/pdb1THQ/pdb, RCSB PDB