MAPK cascades in the choanoflagellate, Monosiga brevicollis

MAP Kinase (MAPK) cascades are an almost universal eukaryotic signaling pathway, of critical importance to growth and stress responses. Humans have several such parallel pathways, which emerged from a single common ancestor. Now, using the genome of Monosiga brevicollis, one of the closest unicellular relatives of the animal kingdom, we see some of the early steps in the evolution of multiple MAPK cascades, including some surprises.

MAPK signaling

The typical MAPK cascade consists of a MAPK kinase such as Erk, Jnk or p38, which is activated by phosphorylation from an upstream kinase called MAP2K, MEK or Ste7. This in turn is phosphorylated and activated by a MAP3K (MEKK, MKK, Ste11), and that sometimes by a MAP4K (Ste20 family). Erk MAPKs largely transduce growth signals downstream from receptor tyrosine kinases and other extracellular signals, while Jnk and p38 MAPKs respond to various stresses.

Most animals have p38 and Jnk pathways, and several variants of the Erk pathway, and there is some cross-talk between the different pathway genes. In fungi, there is one main Erk-like pathway, and some poorly classified related genes, so we looked at the Monosiga genome to see if we could find an intermediate state, where maybe some of these pathways had emerged, but not all of them. This was done in collaboration with Matthew Good from the Lim lab at UCSF, and coordinated by Nicole King of UC Berkeley. The results were published as part of the Monosiga genome paper in 2008.

The genome of the choanoflagellate Monosiga brevicollis and the origins of metazoan multicellularity.
King, N, Westbrook, MJ, Young, SL, Kuo, A, Abedin, M, Chapman, J, Fairclough, S, Hellsten, U, Isogai, Y, Letunic, I, Marr, M, Pincus, D, Putman, N, Rokas, A, Wright, KJ, Zuzow, R, Dirks, W, Good, M, Goodstein, D, Lemons, D, Li, W, Lyos, J, Morris, A, Nichols, S, Richter, DJ, Salamov, A, JGI Sequencing, Bork, P, Lim, WA, Manning, G, Miller, WT, McGinnis, W, Shapiro, H, Tijan, R, Grigoriev, IV, Rokhsar, D. Nature 451, 783-788 (Medline, PDF)

What we found

Monosiga does indeed have an intermediate MAPK system between that of fungi and metazoans. A clear Erk pathway is found (MKK1 -> MEK1 -> Erk), and, surprisingly, a complete Erk5 cascade is also present (MEKK2 -> MKK4 -> Erk5). Up to recently, this was thought to be vertebrate-specific, since these genes are absent from Drosophila and C. elegans, but now are found in the sea urchin and the sea anemone, Nematostella, to which Monosiga now extends it further. This cascade is not well studies, but in mammals it is primarily activated by stress stimuli, and can also be activated by traditional Erk stimuli such as nerve growth factor (NGF) and Erk5 can also be directly activated by PI3 Kinase downstream of the insulin receptor.

Initial traces are also seen of the stress-induced pathways. While yeast has a functionally p38-like kinase (HOG1), it does not have the characteristic sequence similarity of known p38 kinases. Monosiga has at least 3 clear p38 genes, all of which have the canonical TxY activation set. However, no trace of the upstream Mkk3/4 kinases is seen. This suggests that TOPK -> p38 was the original pathway, which was later expanded to include activation by MAP2K and MAP3K genes. Monosiga also contains MAP3K kinases that are homologous to p38 activators, but not found in fungi, including apoptosis specific kinase (Ask1), Tao2 and multiple members of the mixed-lineage kinase (MLK) family. Whether these act through TOPK, or even activate p38 in Monosiga is not yet known.

Finally, the choanoflagellate and Nematostella genome data reinforce the metazoan-specificity of Jnk signaling. No members of the Jnk MAPK family can be found in fungi or choanoflagellates, and the Jnk activators, Mkk4 and Mkk7, are also missing. Interestingly, many of the MAPKKKs that activate the p38 pathway and the Jnk pathway in mammals are present in M. brevicollis. Since Jnk MAPK is most closely related to p38, one hypothesis is that Jnk evolved from a duplication event of p38, and co-opted the upstream components already in place for p38 signaling.

Outside of the Jnk pathway, the MAPKs Erk3 and NMO, and the Erk activators Raf and Mos also appear to be exclusive to metazoans.

In summary, Monosiga shows that the complete Erk5 cascade evolved by the time of the metazoan-choanoflagellate common ancestor, and that parts of the p38 pathway had also emerged. The lack of Jnk signaling suggests that it later emerged as a partial duplication of the p38 pathway, and other refinements such as the presence of raf genes that couple to ras activation, are not found in Monosiga.

Table S9 from the genome paper (PDF) maps the occurance of various MAPK genes across different genomes.

Back to the Monosiga kinome page.