How Many Protein-Protein Interactions Types Exist in Nature?

“Protein quaternary structure universe” refers to the ensemble of all protein-protein complexes across all organisms in nature. The number of quaternary folds thus corresponds to the number of ways proteins physically interact with other proteins. This study focuses on answering two basic questions: Whether the number of protein-protein interactions is limited and, if yes, how many different quaternary folds exist in nature. By all-to-all sequence and structure comparisons, we grouped the protein complexes in the protein data bank (PDB) into 3,629 families and 1,761 folds. A statistical model was introduced to obtain the quantitative relation between the numbers of quaternary families and quaternary folds in nature. The total number of possible protein-protein interactions was estimated around 4,000, which indicates that the current protein repository contains only 42% of quaternary folds in nature and a full coverage needs approximately a quarter century of experimental effort. The results have important implications to the protein complex structural modeling and the structure genomics of protein-protein interactions

PRUNE and PROBE-two modular web services for protein-protein docking

The protein-protein docking programs typically perform four major tasks: (i) generation of docking poses, (ii) selecting a subset of poses, (iii) their structural refinement and (iv) scoring, ranking for the final assessment of the true quaternary structure. Although the tasks can be integrated or performed in a serial order, they are by nature modular, allowing an opportunity to substitute one algorithm with another. We have implemented two modular web services, (i) PRUNE: to select a subset of docking poses generated during sampling search (http://pallab.serc.iisc.ernet.in/prune) and (ii) PROBE: to refine, score and rank them (http://pallab.serc.iisc.ernet.in/probe). The former uses a new interface area based edge-scoring function to eliminate > 95% of the poses generated during docking search. In contrast to other multi-parameter-based screening functions, this single parameter based elimination reduces the computational time significantly, in addition to increasing the chances of selecting native-like models in the top rank list. The PROBE server performs ranking of pruned poses, after structure refinement and scoring using a regression model for geometric compatibility, and normalized interaction energy. While web-service similar to PROBE is infrequent, no web-service akin to PRUNE has been described before. Both the servers are publicly accessible and free for use

Using Correlated Parameters for Improved Ranking of Protein-Protein Docking Decoys

A successful protein-protein docking study culminates in identification of decoys at top ranks with near-native quaternary structures. However, this task remains enigmatic because no generalized scoring functions exist that effectively infer decoys according to the similarity to near-native quaternary structures. Difficulties arise because of the highly irregular nature of the protein surface and the significant variation of the nonbonding and solvation energies based on the chemical composition of the protein-protein interface. In this work, we describe a novel method combining an interface-size filter, a regression model for geometric compatibility (based on two correlated surface and packing parameters), and normalized interaction energy (calculated from correlated nonbonded and solvation energies), to effectively rank decoys from a set of 10,000 decoys. Tests on 30 unbound binary protein-protein complexes show that in 16 cases we can identify at least one decoy in top three ranks having <= 10 angstrom backbone root mean square deviation from true binding geometry. Comparisons with other state-of-art methods confirm the improved ranking power of our method without the use of any experiment-guided restraints, evolutionary information, statistical propensities, or modified interaction energy equations. Tests on 118 less-difficult bound binary protein-protein complexes with <= 35% sequence redundancy at the interface showed that in 77% cases, at least 1 in 10,000 decoys were identified with <= 5 angstrom backbone root mean square deviation from true geometry at first rank. The work will promote the use of new concepts where correlations among parameters provide more robust scoring models. It will facilitate studies involving molecular interactions, including modeling of large macromolecular assemblies and protein structure prediction. (C) 2010 Wiley Periodicals, Inc. J Comput Chem 32: 787-796, 2011

Ebolavirus interferon antagonists-protein interaction perspectives to combat pathogenesis

Zaire ebolavirus, one of the most pathogenic species of Ebolavirus, is a significant threat to the human community being both highly infectious and lethal. The viral proteins (VPs), specifically VP24 and VP35, antagonize the interferon (IFN) proteins accountable for human immune response. Several efforts have been made to design vaccines and therapeutics drugs. However, the success is not encouraging because of limited knowledge about the binding site information of the VPs. Such limitations stem largely from the highly infectious nature of the virus that requires specialized personnel and biosafety laboratories. As an alternative, computational techniques have also been adopted to improve the success rate of drug discovery. This article elaborates on the interactions between viral and human IFN proteins that lead to IFN antagonism. A computational framework is proposed after evaluating existing computational studies. This protein interaction and protein design-based computational framework identified critical interacting residues of the VP (VP24) responsible for the formation of a stable complex with the human KPNA5 (karyopherin alpha proteins 5). The mutations of those critical residues, as demonstrated in this article, affected the overall stability of the complex because of a sharp decrease in both the number of hydrogen bonds and possible charge-charge interactions. Therefore, we proposed that the framework could be an effective alternative to experimental work for destabilizing interactions between the VPs and human proteins responsible for IFN induction and response

The number of new complex structure entries deposited per year in the PDB.

<p>Data are presented in terms of unique structures (sequence identity <90%), families (mapped with unique Pfam families), and folds (rTM-score <0.5).</p

Number of estimated complex folds for a range of numbers of complex families.

<p>Number of estimated complex folds for a range of numbers of complex families.</p

The estimated number of quaternary folds versus the number of quaternary families in nature.

<p>The solid curve is the fitting from Eq. 13 and dotted line indicates the number of quaternary families following Orengo <i>et al</i>. estimation.</p

Histogram of complex structural clusters versus size of the clusters.

<p>The solid curve is the fitting result from Eq. 12. Inset: the same data drawn in logarithm scale.</p