Activation of Self-Incompatibility Signaling in Transgenic Is Independent of AP2-Based Clathrin-Mediated Endocytosis.

Internalization of plasma membrane (PM)-localized ligand-activated receptor kinases and their trafficking to sorting endosomes have traditionally been viewed as functioning primarily in the downregulation of receptor signaling, but are now considered to be also essential for signaling by some receptors. A major mechanism for internalization of PM proteins is clathrin-mediated endocytosis (CME). CME is mediated by the Adaptor Protein Complex 2 (AP2), which is involved in interaction of the AP2 mu-adaptin subunit with a tyrosine-based Yxx phi motif located in the cytoplasmic domain of the cargo protein. In this study, we investigated the role of AP2-mediated CME for signaling by the S-locus receptor kinase (SRK), a protein localized in the PM of stigma epidermal cells, which, together with its pollen coat-localized S-locus cysteine-rich (SCR) ligand, functions in the self-incompatibility (SI) response of the Brassicaceae. Using Arabidopsis thaliana plants that were made self-incompatible by transformation with an A. lyrataderived SRK/SCR gene pair, we tested the effect on SI of site-directed mutations in each of the two Yxx phi motifs in SRK and of a CRISPR/Cas9-induced null mutation in the AP2 mu-adaptin gene AP2M. Both in vitro SRK kinase activity and the in planta SI response were abolished by substitution of tyrosine in one of the two Yxx phi motifs, but were unaffected by elimination of either the second Yxx phi motif or AP2M function. Thus, AP2-mediated CME is considered to be unnecessary for SRK signaling in the SI response.