User:Hannes Röst/Article1

Sch9 is the Saccharomyces cerevisiae homolog of mammalian Akt/protein kinase B. It is a regulator of cell size control and transcription in response to nutrient availability. Like other molecules involved in nutrient signaling, it is also a determinant of cellular aging. Recently, its role in osmotic stress adaptation has been defined.

Role as a regulator of cell size
Cell size homeostasis is crucial to maintain growth. This requires the coordination of cell growth and cell division. An important feature in this regulation is time of commitment to cell cycle (start) in late G1 phase. The cell needs to grow to a sufficient size before dividing.

This needs regulators that activate cell growth (i.e. protein and lipid biosynthesis) as well as repress progression through start. In yeast, one such regulator has been identified: Sch9

Sch9 is a protein kinase involved in cell size homeostasis. It acts as an activator of the ribosomal protein (RP) and ribosome biogenesis (Ribi) regulons, the transcriptional programs that dictate ribosome synthesis rate.

Sch9 modulates ribosomal protein transcription, perhaps by phosphorylating Rap1.

It also acts a potent negative regulator for start. The combination of this makes Sch9 a protein that drives cell growth and can keep the cell from dividing until it has reached the appropriate size.

Role in cellular aging
There is a general link between stress resistance and longevity in higher eukaryotes. In order to find long-surviving mutants a screen-study for resistance to multiple stresses (superoxide and heat shock) was performed, whereby sch9Δ mutants were shown to survive three times longer than wild-type cells.

By additional experiments it was shown, that kinase-inactivating mutation (see the structural part) is actually the one responsible for life-span extension. Because of conservation of yeast Sch9 and PKA and mammalian Akt/PKB in glucose metabolism it was proposed that the regulation of aging may be likewise conserved from yeast to humans.

Role in osmotic stress adaptation
Besides the regulation of cell size and aging, Sch9 also plays a role in the activation of osmostress inducible genes. Loss-of-function mutants sch9 are sensitive to hyperosmotic stress and show an impaired transcriptional response upon osmotic shock of several defense genes (GRE2, CTT1 and STL1), which are regulated by Sko1, a transcription factor, which is directly targeted by the Hog1 MAP kinase.

Coprecipitaion assays show that Sch9 interacts with both Sko1 and Hog1. Additionally, Sch9 specifically phosphorylates Sko1 in vitro (32P-assay). ChIP-studies reveal interdependent Sch9 and Hog1 recruitment to GRE2 and CTT1 genes in vivo.

Regulation
Action of Sch9 is influenced by nutrient level and stress conditions. It is a substrate for TOR which confers its dependence on external stimuli. Together with Sfp1, Sch9 act as component of the TOR and the Ras/PKA pathway. There are multiple redundancies in this pathway (Ras/PKA, Sfp1 and Sch9 all act on the RP regulon) which suggests that more than one modification to the RB promoter network have to be made for transcription to be activated therefore multiple inputs are integrated .

Structure and Mechanism
The structure of Sch9 has not yet been resolved. However since it shares a 49% sequence homology with Akt/PKB, structure and molecular mechanism of regulation could be similar. Strictly speaking, the C-terminal region of Sch9 is highly homologous to Akt/PKB, whereas the N-terminal region contains a C2 phospholipid and calcium-binding motif. Therefore it makes sense to discuss already solved structure of Akt-2 as the possibly related structure of C-terminal region of Sch9.

The overall shape of Akt-2 can be described by two lobes with a deep cleft in-between. The smaller N-terminal lobe contains 5-standed antiparallel β sheet, whereas the C-terminal lobe is mostly α-helical. The ATP binding site of Akt-2 is positioned inside the cleft. Interestingly the cleft region allows certain conformational flexibility to the whole kinase domain noticeable by different relative orientation of the two lobes depending on substrate bound.