Preorientation of protein and RNA just before contacting

Protein and RNA molecules interact and form complexes in many biological processes. However, it is still unclear how they can find the correct docking direction before forming complex. In this paper, we study preorientation of RNA and protein separated at a distance of 5–7?Å just before they form contacts and interact with each other only through pure electrostatic interaction when neglecting the influence of other molecules and complicated environment. Since geometric complementary has no meaning at such a distance, this is not a docking problem and so the conventional docking methods, like FTDock, are inapplicable. However, like the usual docking problem, we need to sample all the positions and orientations of RNA surrounding the protein to find the lowest energy orientations between RNA and protein. Therefore, we propose a long-range electrostatic docking-like method using Fast Fourier Transform-based sampling, LEDock, to study this problem. Our results show that the electrostatically induced orientations between RNA and protein at a distance of 5–7?Å are very different from the random ones and are much closer to those in their native complexes. Meanwhile, electrostatic funnels are found around the RNA-binding sites of the proteins in 62 out of 78 bound protein–RNA complexes. We also tried to use LEDock to find RNA-binding residues and it seems to perform slightly better than BindN Server for 23 unbound protein–RNA complexes.