Mapping of MST1 Kinase Sites of Phosphorylation

Abstract

MST1 is a member of theIn mammalian cells, Sterile-20 (Ste20)-related kinases participate in the regulation of the cytoskeleton that controls cell morphology and motility, and in the regulation of apoptosis (1). These kinases share a conserved catalytic (kinase) domain at the amino terminus and a C-terminal regulatory region of great structural diversity, which interacts with signaling molecules that regulate the cytoskeleton. Currently, four closely related MST kinases have been described (5-9). Most Ste20 group kinases activate mitogen-activated protein kinase (MAPK) 1 cascades in the signaling pathways between the cellular membrane and the nuclear compartment (2). In yeast, the mating pheromone receptor Ste20p phosphorylates and activates a MAPK kinase kinase, Ste11p, raising the possibility that mammalian homologs of Ste20p (e.g. MST1 kinase) also function as MAPK kinase kinase kinases (3, 4). MST1 was shown to act upstream of MAPK kinases that regulate p38 and JNK activities, probably acting via the MAPK kinase kinase MEKK1 (10).There is substantial evidence that MST1 promotes apoptosis, although its role in this process may vary in different cell types. Overexpression of MST1 can induce apoptosis and nuclear condensation in BJAB, 293T and COS-1 cells (4, 14, 15), whereas MST1 promotes nuclear condensation without apparent chromosomal cleavage in HeLa cells (13).A consistent feature of MST1 in all tested cell types is its proteolytic cleavage by caspase, in response to apoptotic stimuli, to a 34 -36-kDa product (hereafter referred to as 36-kDa MST1). Cleavage increases MST1 kinase activity severalfold (4, 16 -19) and influences its subcellular localization (13, 15). Recently, a second caspase cleavage site was identified in the human MST1 sequence that is absent in mouse MST1 and in MST2 from several species (10). Mutation of this site has no impact on accumulation of the 36-kDa species.Expression of a kinase-dead MST1 mutant (K59R) can partially or fully suppress apoptosis or chromatin condensation in HL-60 and 293T cells treated with chemical apoptotic stimuli (15,18). This protective effect is associated with suppression of both JNK activity and DNA fragmentation. However, the K59R mutant fails to suppress apoptosis in BJAB and HeLa cells treated with Fas ligand or TNF-␣ (4, 16). The basis for the cell type-and stimulus-specific differences in MST function has not yet been elucidated.Interestingly, like PAK proteins (11, 12), MST1 (full-length or cleaved) can have a profound effect on cell shape (cell rounding and detachment) independent of caspase activation and prior to nuclear condensation (13). This action and the high basal activity of MST1 kinase suggest a possible function unrelated to apoptosis.Stress-inducing agents such as staurosporine and sodium arsenite (9) can increase MST1 kinase activity; however, no physiological activator of MST1 has been identified, and little is known about the endogenous activation of MST1. Recently, it was proposed that in addition to caspase cleavage, MST1 phosphor...

Germinal center kinases in immune regulation

“…In addition, MST1 may be differentially targeted by different caspases to activate distinct MAPK pathways. [18][19][20][21][22][23] MST1 has been implicated in T-cell biology, as mice lacking MST1 exhibit a variety of T-cell abnormalities. 24 Cross-talk between MST1 and Akt appears able to antagonize Akt1 activity, 25 which is involved in T-cell costimulation and the regulation of NF-kB-dependent gene transcription.…”

Prognostic significance of mammalian sterile20-like kinase 1 in colorectal cancer

“…7 In fact, Mst1 has also shown to be activated by phosphorylation of Thr 183 and Thr 187 in its activation loop which is promoted by dimerization. Mst1 phosphorylation results in caspase activation and apoptosis 7,8 indicating that Mst1 might function both upstream and downstream of caspases. Recent findings that most Ste20 group kinases activate mitogen-activated protein kinase (MAPK) and more specifically stressed-induced p38 MAPK and JNK (c-Jun N-terminal kinase) support this model ( Figure 1).…”

“…Recent findings that most Ste20 group kinases activate mitogen-activated protein kinase (MAPK) and more specifically stressed-induced p38 MAPK and JNK (c-Jun N-terminal kinase) support this model ( Figure 1). 1,[8][9][10] Cellular localization of Mst1 is also regulated by caspase-mediated cleavage as well as threonine phosphorylation. 7,8,11 Under resting conditions, Mst1 is localized predominantly in the cytoplasm but cleavage of Mst1 by caspases or phosphorylation in its activation loop lead to Mst1 translocation to the nucleus.…”

“…1,[8][9][10] Cellular localization of Mst1 is also regulated by caspase-mediated cleavage as well as threonine phosphorylation. 7,8,11 Under resting conditions, Mst1 is localized predominantly in the cytoplasm but cleavage of Mst1 by caspases or phosphorylation in its activation loop lead to Mst1 translocation to the nucleus. 7,8,11 Nuclear retention of Mst1 results in chromatin condensation and DNA fragmentation indicating that nuclear entry of Mst1 may be important in its pro-apoptotic function.…”

“…7,8,11 Under resting conditions, Mst1 is localized predominantly in the cytoplasm but cleavage of Mst1 by caspases or phosphorylation in its activation loop lead to Mst1 translocation to the nucleus. 7,8,11 Nuclear retention of Mst1 results in chromatin condensation and DNA fragmentation indicating that nuclear entry of Mst1 may be important in its pro-apoptotic function. 11,12 Extensive studies to identify potential Mst1 substrates in the nucleus have led to identification of Histone H2B as a physiologic substrate for the catalytic domain of Mst1, the phosphorylation of which leads to chromatin condensation and apoptosis.…”