The
induction of sexual development and virulence in the smut fungus Ustilago maydis depends on Crk1,
a novel MAPK protein.
Garrido
E, Voss U, Muller P, Castillo-Lluva S, Kahmann
R, Perez-Martin J.
Departamento de Biotecnologia Microbiana,
Centro Nacional de Biotecnologia
CSIC, Campus
MAP kinases (mitogen-activated
protein kinases) are activated by dual phosphorylation on specific threonine
and specific tyrosine residues that are separated by a single residue, and the
TXY activation motif is a hallmark of MAP kinases. In
the fungus Ustilago maydis,
which causes corn smut disease, the Crk1 protein, a kinase
previously described to have roles in morphogenesis, carries a TXY motif that
aligns with the TXY of MAP kinases. In this work, we
demonstrate that Crk1 is activated through a mechanism that requires the phosphorylation of this motif. Our data show that Fuz7, a
MAPK kinase involved in mating and pathogenesis in U.
maydis,
is required to activate Crk1, most likely through phosphorylation
of the TXY motif. Consistently, we found that Crk1 is also required for mating
and virulence. We investigated the reasons for sterility and avirulence of crk1-deficient cells, and we found that Crk1
is required for transcription of prf1, a central regulator of mating and pathogenicity in U. maydis. Crk1
belongs to a wide conserved protein group, whose members have not been
previously defined as MAP kinases, although they
carry TXY motifs. On the basis of our data, we propose that all of these
proteins constitute a new family of MAP kinases.
PMID: 15601825 [PubMed - indexed for MEDLINE
A major question in cell
biology is how molecular specificity is achieved by different growth factor
receptors that activate apparently identical signaling
events. For the neurotrophin family, a distinguishing
feature is the ability to maintain a prolonged duration of signal transduction.
However, the mechanisms by which neurotrophin
receptors assemble such a sustained signaling complex are not understood. Here
we report that an unusual ankyrin-rich transmembrane protein (ARMS+kidins220) is closely
associated with Trk receptor tyrosine kinases, and not the EGF receptor. This association
requires interactions between transmembrane domains
of Trk and ARMS. ARMS is rapidly tyrosine phosphorylated after binding of neurotrophins
to Trk receptors and provides a docking site for the
CrkL-C3G complex, resulting in Rap1-dependent sustained ERK activation.
Accordingly, disruption of Trk-ARMS or the ARMS-CrkL interaction with dominant-negative ARMS mutants, or
treatment with small interference RNA against ARMS substantially reduce neurotrophin-elicited signaling to ERK, but without any
effect upon Ras or Akt
activation. These findings suggest that ARMS acts as a major and
neuronal-specific platform for prolonged MAP kinase
signaling by neurotrophins
Phospholipase Cepsilon
(PLCepsilon) is a novel PLC that has a CDC25 guanine
nucleotide exchange factor (GEF) domain and two Ras
association (RA) domains of which the second (RA2) is critical for Ras activation of the enzyme. In the present studies, we
examined hormonal stimulation to elucidate receptor-mediated pathways that
functionally regulate PLCepsilon. We demonstrate that
epidermal growth factor (EGF), a receptor tyrosine kinase
(RTK) agonist, and lysophosphatidic acid (LPA),
sphingosine-1-phosphate (S1P), and thrombin, G protein-coupled receptor
agonists, stimulate PLCepsilon overexpressed
in COS-7 cells. EGF stimulated PLCepsilon in an
RA2-dependent manner through Ras and Rap. In
contrast, LPA, S1P and thrombin stimulated PLCepsilon
by both RA2-independent and dependent mechanisms. To determine the G proteins
that mediate the effects of these GPCR agonists, we coexpressed
constitutively active G proteins with PLCepsilon and
found that Galpha12, Galpha13,