Lessons on signalling in plant self-incompatibility systems - Intercellular Communication in Plants - page 264

similar manner to that proposed for CLV1 and FLS2 (Williams et al. , 1997; Stone
et al. , 1998; Gomez-Gomez et al. , 2001). There is also evidence that additional
phosphatases may be involved in the regulation of SRK. SI in Brassica has been
shown to breakdown on application of the phosphatase inhibitors okadeic acid and
mycrostatin (Rundle et al. , 1993; Scutt et al. , 1993). These inhibitors are expected
to act on type 1 and type 2A phosphatases, but not type 2C phosphatases, such as
KAPP.
An additional physical regulation of SRK may also be present. An interaction
between a sorting nexin ( Brassica SNX1) and SRK has recently been demonstrated
(Vanoosthuyse et al. , 2003). Sorting nexins have been implicated in the down-
regulation of animal receptor kinases by endocytosis (Kurten et al. , 1996; Haft et al. ,
1998; Chin et al. , 2001; Phillips et al. , 2001). Over-expression of sorting nexins
increases the rate of internalisation and degradation of the epidermal growth factor
receptor and the platelet-derived growth factor in mammalian cells (Kurten et al. ,
1996; Phillips et al. , 2001). The SNX1 homologue (Vps5p/Grd2p) in yeast is part of
a retromer complex that is essential for retrograde transport from the endosome to
the Golgi apparatus (Seaman et al. , 1998). While such a protein trafficking pathway
is yet to be established in plants, the Arabidopsis genome is predicted to encode
orthologues of the retromer complex proteins involved, and hence, plants potentially
possess all of the components of a sorting-nexin-based vesicle trafficking system
(Vanoosthuyse et al. , 2003). The role of Brassica SNX1 seems likely to be in
mediating endocytotic removal of SRK from the plasma membrane and in so doing
to play an important role in SRK regulation.
10.4.5 SRK substrates
Binding of SP11/SCR by SRK is envisaged to lead to autophosphorylation of SRK
and subsequent phosphorylation of intracellular substrate(s). The signal transduc-
tion cascade downstream of SRK has been partially characterised. Several sub-
strates have been identified through yeast two-hybrid interaction screening and one
has been confirmed to be involved in the mechanism of action of SI. ARC1 (Ar-
madillo repeat containing protein 1) is a gene that was identified as interacting
with the kinase domain of SRK in a yeast two-hybrid library screen. ARC shows
a phosphorylation-dependent interaction with the cytosolic domain of SRK in vitro
(Gu et al. , 1998), and importantly, transgenic plants in which ARC1 production in
the stigma was suppressed, exhibited a substantial breakdown of SI (Stone et al. ,
1999). The function of ARC1 is not completely understood; however, in addition to
the ARM (Armadillo) repeats, which are involved in protein-protein interactions,
ARC1 also contains the recently identified U-box motif (Azevedo et al. , 2001). The
U-box is involved in ubiquitination, specifically the transfer of ubiquitin to other
proteins, in so doing targeting them for proteasome degradation (Koegl et al. , 1999).
Hence, it is likely that ARC1 targets specific proteins for degradation; these targets
are unknown but would presumably be either inhibitors of SI or stimulators of pollen
growth. It should be noted, however, that ubiquitination can have roles other than