The mammalian STE20-like kinase 1 (MST1) is a substrate for the apoptosis inhibiting protein kinase CK2.

Apoptosis and the response to cell stress are evolutionary highly conserved mechanisms. Both processes require strict regulation, which is often performed by protein kinases. The mammalian Sterile 20-like kinase 1 (MST1) is a pro-apoptotic protein kinase, which is activated and cleaved by caspases upon the induction of cell stress. Being a phosphoprotein itself, the activity of MST1 is regulated by phosphorylation. Protein kinase CK2 is an anti-apoptotic protein kinase which seems to be involved in the regulation of many different cellular processes including apoptosis. There is increasing evidence that the cleavage of many substrates by caspases is regulated by phosphorylation in the close vicinity of the caspase cleavage sites. One of these kinases, implicated in the phosphorylation of caspase substrates, is protein kinase CK2. Here, we report that serine 320 of the MST1 protein is a novel phosphorylation site for the anti-apoptotic protein kinase CK2. Although serine 320 is in close vicinity to the caspase 3 cleavage site, caspase 3 cleavage of MST1 is not affected by CK2 phosphorylation. Using biochemical approaches, we were able to show that MST1 co-localizes with the CK2 subunits in the pancreatic ß-cell line INS-1 and that full-length MST1 and the activated N-terminal fragment of MST1 both interacted with the CK2 subunits in vitro and in vivo. MST1 is a basophilic kinase whereas CK2 is an acidophilic kinase. Thus, binding of these two kinases in the cytosol and in the nucleus opens the door to the phosphorylation of a variety of new substrates.

The Sec63p J-domain is required for ERAD of soluble proteins in yeast.

How misfolded proteins are exported from the ER to the cytosol for degradation (ER-associated Degradation, ERAD) and which proteins are participating in this process is not understood. Several studies using a single, leaky mutant indicated that Sec63p might be involved in ERAD. More recently, Sec63p was also found strongly associated with proteasomes attached to the protein-conducting channel in the ER membrane which presumably form part of the export machinery. These observations prompted us to reinvestigate the role of Sec63p in ERAD by generating new mutants which were selected in a screen monitoring the intracellular accumulation of the ERAD substrate CPY*. We show that a mutation in the DnaJ-domain of Sec63p causes a defect in ERAD, whereas mutations in the Brl, acidic, and transmembrane domains only affect protein import into the ER. Unexpectedly, mutations in the acidic domain which mediates interaction of Sec63p with Sec62p also caused defects in cotranslational import. In contrast to mammalian cells where SEC63 expression levels affect steady-state levels of multi-spanning transmembrane proteins, the sec63 J-domain mutant was only defective in ERAD of soluble substrates.

Glucose regulates protein kinase CK2 in pancreatic ß-cells and its interaction with PDX-1.

The pancreatic duodenal homeodomain transcription factor PDX-1 plays a pivotal role in the development of the pancreas and the maintenance of glucose homeostasis by pancreatic ß-cells. Recently, we found that the highly conserved, ubiquitously expressed tetrameric Ser/Thr protein kinase CK2, which is formed by two catalytic subunits (α and/or α') and two non-catalytic subunits (ß), phosphorylates PDX-1. So far, only little is known about CK2 in pancreatic ß-cells and how this enzyme is regulated in these cells. In the present study, we found that (i) CK2 binds to PDX-1, (ii) the binding between CK2 and PDX-1 is regulated by glucose, (iii) glucose modulates the subcellular localization of PDX-1 and CK2 and (iv) the kinase activity is also regulated by glucose. Our novel data indicate that CK2 is a co-factor of PDX-1 in response to glucose in pancreatic ß-cells.