Knotting data

The crucial information about a topology of a given structure is presented under “Knotting data”. This screen presents the “knotting fingerprint of aprotein” that encodes information about locations of the “knot core”, “knot tails”, “slipknot loops”, and “slipknot tails”, whose definitions are given below. The same information is also shown in a graphical (in JSmol) and sequential representation. The topological fingerprint was introduced in [1] and motivated by work [2]; for various applications see also [3].

Knotting fingerprint

A knotting fingerprint takes form of a matrix diagram, constructed as follows. For a given chain of length N, all its subchains spanned between aminoacids k and l (for 1≤k<l≤N) are analyzed. If a subchain from k to l is knotted (as determined by the analysis described in section Knot detection), then a point with coordinates (k,l) is denoted in relevant color in a plot. For example, point representing knots 31, 41, and 51 are respectively green, red and orange. The intensity of a color represents a probability of detecting a given knot (knots are detected only with some probability, which is a consequence of a choice of its closure to a closed loop, as explained in section Knot detection). The figure below presents an example of a knotting fingerprint, which in this case contains areas representing a knot and two types of slipknots.

Example of a knotting fingerprint with S314131 topological notation. View protein 2wsw in the KnotProt.

In a given knot or slipknot several geometric elements can be distinguished. These elements are denoted in different colors along diagonals of matrix diagrams in database, and also shown schematically in figure below:

Topological notations of knotted proteins

Frequently knotting fingerprints (matrices encoding the knotting types of all subchains) show the presence of more than one knot type or the same knot type appears in disjoint territories of the fingerprint. This feature was used to define a topological notation for knotted proteins. In this notation we list distinct knotted territories, ordering them according to the size of the subchain forming a given knot, starting with the largest one. If the entire protein is knotted its topological notation starts with K; if it is unknotted but contains a slipknot its topological notation starts with S. Among the complete protein structures deposited in PDB we found proteins with the following topological notations:

Interestingly, the topological notation is strongly conserved among ortologous proteins even if their structure highly diverged during hundreds of million of years from their evolutionarily separation. Therefore topological notation can be used to identify proteins with the same or similar function, as a template to model new proteins (e.g. to impose topological constraints on threading), or to identify new members of a given family, etc.

In the KnotProt we also list (as “putative notations”) a few cases where the notation is associated to protein chains with undetermined fragments and where the “missing” portion was replaced by a line segment. In such cases the line segment can pierce through the existing portion of the chain and introduce spatial structure that is an artifact. In particular, we found cases with topological notations K5131, S715131. At present we do not think that these notations reflect the notation of the entire protein structure of the respective proteins. Once complete structural information about these proteins is provided (e.g. by new crystallization results, or proper reconstruction), the KnotProt analysis will be repeated and the results will be updated.

[1] Sułkowska, J.I., Rawdon, E.J., Millett, K.C., Onuchic, J.N. and Stasiak, A. (2012) Conservation of complex knotting and slipknotting patterns in proteins. Proc. Natl. Acad. Sci. U.S.A. 109, E1715–E1723
[2] King NP, Yeates EO, Yeates TO. (2007). Identification of rare slipknots in proteins and their implications for stability and folding. J. Mol. Biol. Oct 2007. 373(1):153-66.
[3] E.J. Rawdon, K.C. Millett, J.I. Sulkowska and A. Stasiak, Knot localization in proteins, Biochem. Soc. Trans., 41, 538-541, (2013).

KnotProt | Interdisciplinary Laboratory of Biological Systems Modelling