title: Helix-helix packing and interfacial pairwise interactions of residues in membrane proteins.
abstract: Helix-helix packing plays a critical role in maintaining the tertiary structures of helical membrane proteins. By examining the overall distribution of voids and pockets in the transmembrane (TM) regions of helical membrane proteins, we found that bacteriorhodopsin and halorhodopsin are the most tightly packed, whereas mechanosensitive channel is the least tightly packed. Large residues F, W, and H have the highest propensity to be in a TM void or a pocket, whereas small residues such as S, G, A, and T are least likely to be found in a void or a pocket. The coordination number for non-bonded interactions for each of the residue types is found to correlate with the size of the residue. To assess specific interhelical interactions between residues, we have developed a new computational method to characterize nearest neighboring atoms that are in physical contact. Using an atom-based probabilistic model, we estimate the membrane helical interfacial pairwise (MHIP) propensity. We found that there are many residue pairs that have high propensity for interhelical interactions, but disulfide bonds are rarely found in the TM regions. The high propensity pairs include residue pairs between an aromatic residue and a basic residue (W-R, W-H, and Y-K). In addition, many residue pairs have high propensity to form interhelical polar-polar atomic contacts, for example, residue pairs between two ionizable residues, between one ionizable residue and one N or Q. Soluble proteins do not share this pattern of diverse polar-polar interhelical interaction. Exploratory analysis by clustering of the MHIP values suggests that residues similar in side-chain branchness, cyclic structures, and size tend to have correlated behavior in participating interhelical interactions. A chi-square test rejects the null hypothesis that membrane protein and soluble protein have the same distribution of interhelical pairwise propensity. This observation may help us to understand the folding mechanism of membrane proteins.
authors.affiliation: Department of Bioengineering, University of Illinois at Chicago, 851 S. Morgan Street RM 218, MC-063, Chicago, IL 60607, USA.
authors.forename: L
authors.initials: L
authors.lastname: Adamian
authors.forename: J
authors.initials: J
authors.lastname: Liang
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D001665:Binding Sites; D016009:Chi-Square Distribution; D019295:Computational Biology / Q000379:methods; D003198:Computer Simulation; D004268:DNA-Binding Proteins; D004220:Disulfides / Q000378:metabolism; D005656:Fungal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006021:Glycophorins / Q000737:chemistry / Q000378:metabolism; D016350:Leucine Zippers; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D011336:Probability; D015202:Protein Engineering; D017510:Protein Folding; D011494:Protein Kinases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D029701:Saccharomyces cerevisiae Proteins; D012995:Solubility
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D004220:Disulfides; D005656:Fungal Proteins; D006021:Glycophorins; D008565:Membrane Proteins; D029701:Saccharomyces cerevisiae Proteins; D011494:Protein Kinases