(155a) MAPK Substrates Control the Level of MAPK Phosphorylation

One of the key concepts of molecular systems biology is the existence of a module, defined as a network with a function that is largely independent of its context. Based on the modular decomposition of large networks it may be possible to predict their dynamics and function from properties of constituent modules. Integral to such an approach is the ability to define individual modules and distinguish their core components from the rest of the network. While conceptually simple, this task can be highly nontrivial in reality.

As an example, we consider the Mitogen Activated Protein Kinase (MAPK) pathway, an essential regulator of cellular processes in all eukaryotes. The MAPK pathway is a three-tiered cascade of phosphorylation-dephosphorylation cycles. An input to the pathway can be provided by a cell surface receptor; its output is the activation level of MAPK, a kinase at the bottom of the cascade. MAPK is activated when it is phosphorylated by a MAPK kinase; this process is reversed by a MAPK phosphatase. Active MAPK controls cellular processes by phosphorylating its substrates.

According to the current mathematical and conceptual models, the dynamics of the MAPK cascade can be understood independently of MAPK substrates [1]. This unidirectional view of MAPK signaling is consistent with a large body of experimental work, but the reality is more complex: The enzymes that regulate MAPK and the substrates phosphorylated by MAPK have the potential to interact with the same domains on the MAPK protein [2]. Thus, MAPK substrates can compete with each other, as well as with the MAPK regulators for binding to MAPK. This creates a potential where MAPK phosphorylation and signaling can be controlled by MAPK substrates.

Using the early Drosophila embryo as an experimental system, we explored substrate mediated control of MAPK signaling in vivo. We first characterized the signal transduction in the MAPK signaling by quantifying the spatial distribution of Capicua (Cic), a target of MAPK that undergoes nuclear export upon phosphorylation by MAPK. In addition, we showed that the level of MAPK substrates can influence the activation status of this enzyme and its ability to distribute its activity among multiple substrates. These results are summarized in a chemical kinetics model that can recapitulate the observed effect of substrate on MAPK.

Our results demonstrate how the levels of MAPK phosphorylation and its downstream signaling can be controlled by the level of Cic. This effect is more general and applies to other MAPK substrates as well: we established that Bcd, an anteriorly localized MAPK substrate, has a similar effect [3]. Based on this we propose that substrate mediated control of MAPK signaling plays a key role in the operation of the terminal patterning system.

References

[1] Huang, C. and Ferrell, J.J. (1996). Ultrasensitivity in the mitogen-activated protein kinase cascade. Proc. Nat. Acad. Sci. 93, 10078-83.

[2] Bardwell, A.J., Abdollahi, M., and Bardwell, L. (2003). Docking sites on mitogen-activated protein kinase (MAPK) kinases, MAPK phosphatases and the Elk-1 transcription factor compete for MAPK binding and are crucial for enzymic activity. Biochem J 370, 1077-1085.

[3] Kim, Y., Coppey, M., Grossman, R., Ajuria L, Jimenez, G., Paroush, Z., and Shvartsman, SY. (2010). MAPK substrate competition integrates patterning signals in the Drosophila embryo. Curr. Biol. 20, 446-451.