Alternative Splicing of MAPKs in the Regulation of Signaling Specificity

Abstract

The mitogen-activated protein kinase (MAPK) cascades transmit signals from extracellular stimuli to a variety of distinct cellular processes. The MAPKKs in each cascade specifically phosphorylate and activate their cognate MAPKs, indicating that this step funnels various signals into a seemingly linear pathway. Still, the effects of these cascades vary significantly, depending on the identity of the extracellular signals, which gives rise to proper outcomes. Therefore, it is clear that the specificity of the signals transmitted through the cascades is tightly regulated in order to secure the desired cell fate. Indeed, many regulatory components or processes that extend the specificity of the cascades have been identified. Here, we focus on a less discussed mechanism, that is, the role of distinct components in each tier of the cascade in extending the signaling specificity. We cover the role of distinct genes, and the alternatively spliced isoforms of MAPKKs and MAPKs, in the signaling specificity. The alternatively spliced MEK1b and ERK1c, which form an independent signaling route, are used as the main example. Unlike MEK1/2 and ERK1/2, this route's functions are limited, including mainly the regulation of mitotic Golgi fragmentation. The unique roles of the alternatively spliced isoforms indicate that these components play an essential role in determining the proper cell fate in response to distinct stimulations.

Keywords: ERK; ERK1c; JNK; MAPK; alternative splicing; p38.

Figure 1

Signaling specificity and multiple isoforms of the MAPK signaling pathways. The MAPKs operate within signaling cascades composed of three to five layers (tiers) of protein kinases. The signals from the cascades are transmitted via sequential phosphorylation and activation of the components in each layer. The four cascades, which have been identified are shown: the human ERK1/2 cascade with MEK1/2/1b and ERK1/1c/2 at the MAPKK and MAPK layers. The p38MAPK cascade with MKK3/3b/6/6b, and p38α/Exip/Mxi2/p38β/p38γ/p38δ; the JNK cascade with MKK4/4δ/7γ1/γ2/β1/β2/α1/α2 and JNK1α1/1α2/1β1/1β2/2α1/2α2/2β1/2β2/3α1(L)/3α2(L)/3α2(S); and the ERK5 cascade with MEK5, and ERK5/5-T. It should be noted that the isoforms presented are those whose expression has been confirmed. Other alternatively spliced transcripts whose protein expression is not confirmed or may exist in other organisms are not presented here as well. The main proteins in each layer of each cascade (except for JNK) appear on top. As for JNK, it seems that all components may be substantially expressed, at least in some cells. Each of the kinases is composed of a kinase domain (central region) as well as N and C terminus represented by a line on the left (N terminus) and right (C terminus) of all kinase domains. The patterns in some of the proteins (e.g., MEK1b) represent low expression levels (less than 10% of the main gene product). Different colors and length in the N or C terminus represent distinct sequences and number of AA compared to the main isoform (e.g., ERK1c). In order to make changes more visible, the scale is not always accurate. * insertion, ** β2 without insertion, γ2 with insertion, *** α1/α2 have different length C termini, # alternative exon 6 between the α and β isoforms that result in a change of 5–7 amino acids in this region. ## deletion. More information on the structure appears in Table 1.

Figure 2

Schematic presentation of ERK1c function in the Golgi. An illustration demonstrating the unique role of ERK1c isoform in the Golgi fragmentation process at different stages of the cell cycle. Prophase: (A) CDK1 becomes active, and phosphorylates ERK1c on Ser343 in its unique C-terminus. (B) Phosphorylated ERK1c interacts with a shuttling complex of PI4KIIIβ and 14-3-3γ, which mediates its Golgi translocation. Prometaphase: (C) In the Golgi, ERK1c is phosphorylated by MEK1b, becomes fully active and induces mitotic Golgi fragmentation. The Golgi is organized in stacks, while GM130 is found in between these stacks. The microtubules (MTs) are polymerized from the microtubules originating center (MTOC) and stabilize the Golgi structure. Both HOOK3 and CLASP2 interact together with the MTs and Golgi. (D) Once entering mitosis, HOOK3 is phosphorylated by both ERK1c and AurA, while CLASP2 might be phosphorylated by AurA. At this time the Golgi stacks starts to break into ribbons. Metaphase: (E) The phosphorylation of HOOK3 and CLASP2 allow the complete fragmentation of the Golgi into a haze. At this time, phosphorylated CLASP2 maintains its interaction with the MTs that originate from the centromeres, while phosphorylated HOOK3 interacts with GM130.