9/14/22, 6:53 AM L5: Frequent Motifs and Their Function: Network Science – CS-7280-O01
L5: Frequent Mo fs and Their Func on
Most real-world networks are either designed by humans (such as electronic circuits or communication networks) or they evolve naturally (such as biological or social networks) to perform certain functions.
Consequently, the frequent presence of a network motif in a network suggests that that specific connectivity pattern has a functional role in the network. For example, the frequent presence of the Feedback Loop motif (see FBL motif in previous pages) suggests that the network includes control loops that regulate the function of a three-node network path with feedback from the output node back to the input node.
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Similarly, the absence of a network motif from a network suggests that that connectivity pattern is functionally ¡°not allowed¡± in that kind of network. For example, a hierarchical network that shows who is reporting to whom at a company should not contain any motifs that include a directed cycle.
The research paper that we refer to in this page has analyzed a large variety of networks (gene regulatory networks, neuronal networks, food webs in ecology,
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9/14/22, 6:53 AM L5: Frequent Motifs and Their Function: Network Science – CS-7280-O01
electronic circuits, the WWW, etc) and identified the network motifs that are most frequently seen in each type of network. Their study did not consider only 3-node motifs but also larger motifs. For each type of network, the authors identify a few common network motifs and associated a plausible function for that motif.
For instance, in gene regulatory networks the FeedForward Loop (FFL) motif is a prevailing structure.
In that context, an FFL instance consists of three genes (X,Y,Z): two input transcription factors (X and Y), one of which regulates the other ( ), both jointly regulating a target gene Z. The FFL has eight possible structural types, because each of the three interactions in the FFL can be activating or repressing. Four of the FFL types, termed incoherent FFLs, act as sign-sensitive accelerators: they speed up the response time of the target gene expression following stimulus steps in one direction (e.g., off to on) but not in the other direction (on to off). The other four types, coherent FFLs, act as sign-sensitive delays. For additional information about the biological function of the FFL motif, please see the following research paper:
Structure and function of the feed-forward loop network motif, S. Mangan and U. Alon, PNAS October 14, 2003 100 (21) 11980-11985 (https://doi.org/10.1073/pnas.2133841100)
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9/14/22, 6:53 AM L5: Frequent Motifs and Their Function: Network Science – CS-7280-O01
In food webs, on the other hand, we rarely see the FFL motif. The reason is that if a carnivore species X eats a herbivore species Y, and Y eats a plant species Z, we rarely see that X also eats Z.
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