The entangled world of DNA quadruplex folds

DNA quadruplexes take part in many biological functions. It takes up a variety of folds based on the sequence and environment. Here, a meticulous analysis of experimentally determined 392 quadruplex structures (388 PDB IDs) deposited in PDB is carried out. The analysis reveals the modular representation of the quadruplex folds. 48 unique quadruplex motifs (whose diversity arises out of the propeller, bulge, diagonal, and lateral loops that connect the quartets) are identified, leading to simple to complex inter-/intra-molecular quadruplex folds. These structural two-layered motifs are further classified into 33 continuous and 15 discontinuous motifs. The discontinuous motifs cannot further be classified into parallel, antiparallel, or hybrid as one or more guanines of the adjacent quartets are not connected. While the continuous motifs can be extended to a quadruplex fold, the discontinuous motif requires additional loop(s) to complete a fold, as illustrated here with examples. Similarly, the higher-order quadruplex folds can also be represented by continuous or discontinuous motifs or their combinations. Such a modular representation of the quadruplex folds may assist in custom engineering of quadruplexes, designing motif-based drugs, and the prediction of quadruplex structure. Further, it could facilitate understanding the role of quadruplexes in biological functions and diseases.