PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 71

cDNA, which are unable to produce ROS when treated with cryptogein, the clathrin-
coated pits stimulation was inhibited. These results indicate that the very early endo-
cytic process induced by cryptogein in tobacco is due to clathrin-mediated endocytosis
and is dependent on ROS production (Leborgne-Castel et al. 2008 ).

2.16.3 Ubiquitin-Proteasome System May Be Involved

in PRR Endocytosis

Ubiquitin- and proteasome-mediated degradation of proteins plays an important
role in endocytosis of PRRs (Robatzek et al. 2006 ; Aker and de Vries 2008 ). The
main function of the proteasome is to degrade unneeded or damaged proteins by
proteolysis. Proteasomes regulate the concentration of particular proteins and
degrade misfolded proteins. Proteins are tagged for degradation by a small protein
called ubiquitin. Proteins are targeted for degradation by the proteasome (Dreher
and Callis 2007; Goritschnig et al. 2007). Proteasome inhibitors, such as MG132,
which could deplete the cell's pool of freely available ubiquitin moieties, prevented
fl g22-induced internalization of FLS2 (Robatzek et al. 2006 ). It suggests that
ubiquitin-proteasome system may be involved in endocytosis of the PRR.

2.16.4 Phosphorylation of PRR May Be Involved

in PRR Endocytosis

PAMP-induced PRR-mediated endocytosis has been shown to be dependent on
phosphorylation of the PRR. Only those fl g22 peptides that could activate the FLS2
receptor were able to target FLS2 for endocytosis (Robatzek et al. 2006 ). Mutation
of a conserved potentially phosphorylated residue within the juxtamembrane region
abolished FLS2 internalization. Addition of the protein kinase inhibitor K252a also
abolished FLS2 internalization (Robatzek et al. 2006 ). The kinase-associated pro-
tein phosphatase interacts with FLS2 in vitro (Gómez-Gómez et al. 2001 ) and the
protein phosphatase 2A inhibitor cantharidin affected FLS2 subcellular traffi cking
(Serrano et al. 2007 ). These results suggest that phosphorylation of PRR is involved
in PRR endocytosis.

2.16.5 Involvement of EHD in Endocytosis

Study of mammalian systems revealed that endocytosis depends on a large number
of protein-protein interactions mediated by specifi c modules. One such module is
the EH (Esp15 Homology) domain. The EHD (EHDomain) structure generally
consists of two EF-hands and a helix-loop-helix structure that binds calcium,