AtPIP2;1 facilitated ion transport is influenced by activity of
endogenous oocyte kinases
Stimulation or inhibition of the endogenous kinase activity of X.
laevis oocytes expressing wild-type (WT) AtPIP2;1 was manipulated by
treatment with cyclic nucleotide monophostphates (cNMPs) or a kinase
inhibitor (H7). These treatments resulted in changes in the magnitude of
AtPIP2;1-associated ionic conductance when compared to untreated
AtPIP2;1 or either treated or untreated water injected control oocytes
(Figure 1). Treatments with cAMP and cGMP can stimulate oocyte
endogenous kinase activity, and this can alter protein phosphorylation
(Glass and Krebs, 1980; Kuwahara et al., 1995). Different kinases
respond to cAMP verses cGMP signalling, and the kinases responding to
these different signals have different target sites (Conti et
al., 2012). Endogenous oocyte kinases have been previously shown to
alter plant aquaporin phosphorylation state and influence their water
channel activity, and this was demonstrated by applying treatments with
kinase stimulators and inhibitors (Maurel et al., 1995; Van
Wilder et al., 2008). Treatment of AtPIP2;1 expressing oocytes
with cAMP or cGMP increased and decreased AtPIP2;1-associated ionic
conductance, respectively (Figure 1). Treatment of AtPIP2;1 expressing
oocytes with the kinase inhibitor H7 decreased AtPIP2;1-associated ionic
conductance (Figure 1b). H7 treatment similarly reduced
aquaporin-associated ionic conductance for HsAQP1 expressing oocytes by
inhibiting the influence of cAMP on endogenous kinases (Yool et
al., 1996). The different responses to the different cNMP treatments
indicate that multiple phosphorylation sites, such as candidate sites in
the CTD may be involved in regulating AtPIP2;1-facilitated ionic
conductance and that transport function can be altered by the activity
of endogenous oocyte kinases. However, a direct effect of cNMPs on
AtPIP2;1 may also be possible; HsAQP1 ion channel function is activated
by direct cGMP binding to its loop D in a phosphorylation-dependent
manner (Anthony et al., 2000; Campbell et al., 2012)
although an equivalent site is not present in AtPIP2;1.