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Osmoadaptive responses

Figure 4.1 The Hog1 pathway is composed of a central core of MAP kinases and two osmosensing branches: Sln1 and Sho1-Msb2. The signal converges at the MAPKK Pbs2. Once Hog1 is active, different osmoadaptive responses are unleashed.

Ssk1 binds to the N-terminal regulatory domain of Ssk2, leading to Ssk2 activation (Posas and Saito, 1998).

A second mechanism for Pbs2 activation involves several proteins: the transmembrane protein Sho1, the mucin-like protein Msb2 (Albertyn et al., 1994), the MAPKKK Ste11, the Ste11-binding protein Ste50, the Ste20 p21-activated kinase, and the small GTPase Cdc42 (Posas et al., 1998), but the complete mechanism used by this osmosensor is still unknown.

In fission yeast, the Sty1 SAPK pathway is linked to osmostress adaptation. The central elements of this pathway are the MAPK Sty1 (also known as Spc1), the MAPKK Wis1, and the two MAPKKK Wak1 and Win1 (Wilkinson and Millar, 1998). Cells deleted for styl or wisl are highly elongated as a consequence of a delay in the timing of mitotic initiation, which is exacerbated in response to stress. The pathway is activated by a sensor composed of three histidine kinases (Phk1/Mak2, Phk2/Mak3, and Phk3/Mak1)(Aoyama et al., 2001) and two response regulators (Mcs4 and Prr1) (Ohmiya et al., 1999; Shieh et al., 1997). This pathway is also inactivated by protein phosphatases. A number of effectors of the Sty1 SAP kinase have been identified, including the Atf1, Pcr1, and Pap1 transcription factors, which are homologues of mammalian ATF-2 and c-Jun, respectively (Shiozaki and Russell, 1996; Wilkinson In addition, Sty1 binds and phosphorylates two downstream kinases, Cmk2 and Srk1 (Sty1-regulated kinase), which are related to the mammalian calmodulin-dependent and MAPKAP kinases.

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