The C-terminal domain of Mnk1a plays a dual role in tightly regulating its activity

Abstract

The human family of MAP kinase signal-integrating kinases (Mnks) comprises four related proteins derived from two genes by alternative splicing. The Mnk1 gene gives rise to two proteins, Mnk1a and Mnk1b, which possess distinct C-termini and properties.Despite lacking the C-terminal MAP kinase-binding site, Mnk1b shows higher basal activity than Mnk1a. In contrast, the activity of Mnk1a is tightly regulated by signalling through ERK and p38 MAP kinase.We show that the short C-terminus of Mnk1b confers on it a 'default' behaviour of substantial, but unregulated, activity. In contrast, the longer C-terminus of Mnk1a represses the basal activity and T (activation)-loop phosphorylation of this isozyme while allowing both properties to be stimulated by upstream MAP kinase signalling.Two features of the C-terminus of Mnk1a appear to account for this behaviour: the known MAP kinase-binding site and a region (predicted to be -helical) which occludes access to the catalytic domain and the T-loop. The activation of Mnk1a results in a marked conformational change leading to a more 'open' structure. We also identify a conserved phenylalanine in an Mnk-specific insert as playing a key role in governing the ease with which Mnk1a can be phosphorylated.These studies help to identify the features that give rise to the diverse properties of human Mnk isoforms.Keywords: protein kinase; Mnk; ERK; p38 MAP kinase; eIF4E Abbreviations used: eIF, eukaryotic initiation factor; ERK, extracellular ligand-regulated kinase; HEK, human embryonic kidney; hMnk, human Mnk; MAP kinase, mitogen-activated protein kinase; mMnk, mouse Mnk; Mnk, MAP kinase signal-integrating kinase (or MAP kinase-interacting kinase); NES, nuclear export sequence; NLS, nuclear localisation sequence. A c c e p t e d M a n u s c r i p t Licenced copy. Copying is not permitted, except with prior permission and as allowed by law. © 2009 The Authors Journal compilation © 2009 Portland Press Limited IntroductionThe MAP kinase signal-integrating kinases (also termed 'MAP kinase-interacting' kinases; Mnks) were first discovered by virtue of their abilities to be phosphorylated by [1] or to bind to [2] ERK and/or p38 MAP kinase. There are two Mnk genes in mice and humans, Mnk1 and Mnk2. The Mnk proteins that were originally reported (here termed Mnk1a and Mnk2a) each contain, within their C-terminal regions, a motif for binding to MAP kinases (Fig. 1A). Mnk1a binds well to both ERK and p38 MAP kinases ( / ) while Mnk2a binds better to ERK and only weakly to p38 MAP kinases / [2;3]. ERK and p38 MAP kinases phosphorylate two threonine residues in Mnk1/2 which are located in the activation or 'T'-loop. In many protein kinases phosphorylation in this region leads to their activation.Subsequently it was shown that the human Mnk1 and Mnk2 genes each give rise to two mRNAs, and thus to two distinct polypeptides, as a consequence of alternative mRNA splicing [4][5][6][7] (Fig. 1A). In each case, the second form of each polypeptide (Mnk1b or Mnk2b) contains a distinct C-termina...

Contrasting Roles of Mitogen-Activated Protein Kinases in Cellular Entry and Replication of Hepatitis C Virus: MKNK1 Facilitates Cell Entry

“…The MKNKs were first discovered as ERK substrates (58,59) and comprise four proteins, with two isotypes produced from each of two genes (MKNK1 and MKNK2) (48,60). These differ in their cellular localization and degree of regulation by ERK, with MKNK1a being predominantly cytoplasmic and highly regulated by ERK and p38 MAP kinases.…”

“…These differ in their cellular localization and degree of regulation by ERK, with MKNK1a being predominantly cytoplasmic and highly regulated by ERK and p38 MAP kinases. MKNK1b differs from MKNK1a in lacking a C-terminal MAPK-binding domain and nuclear export signal, and it is at least partly nuclear in its distribution and minimally regulated by ERK/p38 kinases (48,60). MKNK1 is involved in stress responses, cytokine expression, and control of translation.…”

Mnk kinases in cytokine signaling and regulation of cytokine responses

“…Unlike Mnk1a, Mnk1b is also localized in the nuclear compartment, where it may regulate the phosphorylation of eIF4E and possibly other nuclear proteins [134]. Mnk1b exhibits higher basal activity as compared to Mnk1a possibly due to the lack of a C-terminal end containing negative regulatory elements [6]. Interestingly, Mnk1b lacks a MAPK domain and therefore cannot be activated by either p38 MAPK or Erk [6] suggesting regulation by other unknown mechanisms.…”

“…Mnk1b exhibits higher basal activity as compared to Mnk1a possibly due to the lack of a C-terminal end containing negative regulatory elements [6]. Interestingly, Mnk1b lacks a MAPK domain and therefore cannot be activated by either p38 MAPK or Erk [6] suggesting regulation by other unknown mechanisms. As in the case of Mnk1, Mnk2 is also known to alternatively spliced into two isoforms Mnk2a and Mnk2b.…”

“…Mnk1 and Mnk2 are both alternatively spliced in humans (a and b isoforms). It should be noted that the b isoforms lack a MAPK binding domain and are therefore resistant to regulation by Erk or p38 MAPK [5,6]. …”