Regulation of mammalian Ste20 (Mst) kinases

Abstract

Initially identified as mammalian homologs to yeast Ste20 kinases, the mammalian sterile twenty-like (Mst) 1/2 kinases have been widely investigated subsequent to their rediscovery as key components of the Hippo tumor suppressor pathway in flies. To date, our understanding of Mst substrates and downstream signaling outstrips our knowledge of how these enzymes are controlled by upstream signals. While much remains to be discovered regarding the mechanisms of Mst regulation, it is clear that Mst1 kinase activity is governed at least in part by its state of dimerization, including self-association and also heterodimerization with various other signaling partners. Here we review the basic architecture of Mst signaling and function and discuss recent advances in our understanding of how these important kinases are regulated.

Keywords: dimerization; serine/threonine protein kinases; signal transduction; tumor suppressor.

Fig. 1. Upstream links to the Hippo pathway in flies and mammals

(A) In Drosophila, Ex, Mer and Kibra upon activation by Fat, activate Hpo-Sav complex. Activated Hpo-Sav complex phosphorylates and activates Wts-Mts complex. Activated Wt-Mts phosphorylates Yki, leading to its cytoplasmic retention and inactivation. In the absence of Hpo signaling, Yki binds transcription factor Sd and promotes cell proliferation and inhibition of apoptosis. (B) In mammals, Merlin interaction with Lats leads to recruitment of Lats to the plasma membrane. At the plasma membrane Lats is phosphorylated and activated by Mst. Activated Lats phosphorylates Yap leading to its inactivation and cytoplasmic retention. Pointed arrowheads indicate activation and blunted ends indicate inhibition. Solid lines indicate direct interaction while dashed lines indicate unknown mechanism.

Fig. 2. Hippo-pathway independent Mst signaling

In lymphocytes, Mst1/2 is activated by chemokines and T-cell receptor (TCR) ligation. These signals are propagated by activation of the small GTPase Rap1, which binds to the adaptor protein Rassf5. Rassf5 contains a SARAH domain, which mediates interaction with Mst1/2. Mst has been shown to phosphorylate the guanine-nucleotide exchange factor (GEF) DENND1C, leading to activation of the small GTPase Rab13, and subsequent clustering of the integrin LFA-1. Activated Mst1 also phosphorylates the Rac/Rho GEF Dock8, promoting cell migration.

Fig. 3. Schematic representation of the domain structure of Mst1/2

Full-length Mst1/2 is composed of N-terminal kinase domain (green), a C-terminal SARAH domain (yellow), and an auto-inhibitory domain between the kinase and the SARAH domain (blue). Mst1 is regulated by phosphorylation at T120, T183, T387 and Y433, and caspase cleavage at D326 and D349 (equivalent phosphorylation and caspase cleavage sites are present in Mst2).

Fig. 4. Mst1/2 regulation by Rassf1

Mechanistic models of Mst1/2 activation by Rassf1. A. Rassf1 can activate Mst1 and Mst2 is by preventing dephosphorylation by PP2A. B. Rassf1 can activate Mst2 by releasing it from its inhibitory complex with Raf-1. C. Rassf1 can activate Mst2 by preventing its degradation. D. Rassf1 can activate Mst2 by promoting its interaction with its substrate, Lats. Pointed arrowheads indicate activation while blunted ends indicate inhibition.

Fig. 5. Regulation of Mst2 by Rassf5

Rassf5 can act as an activator or inhibitor of Mst2, depending on whether it binds Mst2 before or after activation-loop phosphorylation. When Rassf5 binds Mst2, before activation-loop phosphorylation, it blocks Mst2 homodimerization and hence activation, but when Rassf5 binds Mst2 after activation-loop phosphorylation, it promotes Mst2 activation and function.