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Fig. 1 The RTK/Ras/MAPK signaling pathway in vulval induction in C. elegans. The ligand, its receptor, and core signal
transducers are indicated. LIN-3 encodes the worm epidermal growth factor (EGF) ligand and is secreted from the gonadal
anchor cell. It binds to its receptor LET-23 on the vulval precursor cells which then dimerizes and undergoes autophosphoryla-
tion ( Aroian et al., 1990; Hill and Sternberg, 1992 ). SEM-5, an adaptor protein, binds phosphorylated LET-23 ( Clark et al., 1992 )
and recruits SOS-1 ( Chang et al., 2000 ), a guanine nucleotide exchange factor (GEF) that activates LET-60, a Ras GTPase. KSR-
1, a putative scaffold protein, is required for robust activation of LET-60 downstream signaling ( Kornfeld et al., 1995b;
Sundaram and Han, 1995 ). LET-60 activates a mitogen-activated protein kinase (MAPK) cascade, including LIN-45 (Raf),
MEK-2 (MAPK kinase), and MPK-1 (MAPK) ( Chong et al., 2003; Han et al., 1993; Kornfeld et al., 1995a; Lackner et al., 1994;
Sternberg et al., 1993; Wu et al., 1995; Wu and Han, 1994 ). Activated MPK-1 inhibits LIN-1, a transcription factor forming a
complex with LIN-31. Upon LIN-1 inhibition, LIN-31 is released to promote the acquisition of vulval cell fates ( Tan et al.,
1998 ). MPK-1 also indirectly activates SUR-2, a mediator protein that positively regulates vulval cell fate ( Singh and Han,
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