The Pinoid Kinase Orienting Polar Auxin Transport

The first indication that PID has a role in polar auxin transport came from the phenotypic similarities between the pinoid and the pin1 loss-of-function mutants, as both mutants show cotyledon defects and develop pin-like inflorescences (Bennett et al. 1995). Subsequent experiments showed that overexpression of PID results in phenotypes such as agravitropic growth and collapse of the main root meristem. The root meristem collapse could be rescued by the PAT inhibitor NPA, suggesting that the PID protein kinase is a regulator of PAT (Benjamins et al. 2001). More recently, it was shown that PID determines the apico-basal subcellular polarity of PIN proteins. PID overexpression in the root tip causes basally localized PINs (PIN1,2 and 4) to be re-localized to the apical cell side. This results in apicalized auxin transport that deprives the root meristem of auxin, and leads to meristem collapse. Conversely, pid loss-of-function causes apically localized PIN1 to be re-localized to the basal side of epidermis cells in the inflorescence apex, preventing the formation of auxin maxima needed for organ initiation, thus resulting in pin-like inflorescences

(Friml et al. 2004). Thus, cellular PID levels modulate the direction of PAT by determining the subcellular targeting of PIN proteins.

Several lines of evidence point to the fact that phosphorylation of PINs is important in the regulation of their subcellular localization. Firstly, modification of a serine residue in one of the short cytoplasmic loops of PIN2 to a glycine resulted in failure of PIN2 deployment to the plasma membrane (Petrasek et al. 2006). Moreover, large-scale identification of phosphorylation sites in plasma membrane proteins by mass spectrometry showed that the large central hydrophilic loop of several Arabidopsis thaliana PIN proteins is phosphorylated at multiple positions in vivo (Nuhse et al. 2004; Benschop et al. 2007). Recently, we have obtained evidence that PID may be (one of) the kinase(s) responsible for phosphorylating the PIN proteins in their large hydrophilic loop (Michniewicz et al. 2007).

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