Functional analysis of MLPK, a membrane-anchored cytoplasmic protein kinase involved in Brassica self-incompatibility.

Abstract

In Brassica self-incompatibility, self/non-self recognition is controlled by S-haplotype-specific interaction between the pollen-borne ligand SP11 and its stigmatic receptor-kinase, SRK. The downstream signaling event leading to self-pollen rejection, however, still remains largely unknown. Our recent analyses on the self-compatible B. rapa variant Yellow salson suggested the involvement of MLPK (M-locus protein kinae), a member of receptor like cytoplasmic kinase, in this signaling, but its precise role is still unclear.
Here we report that MLPK is transcribed into two alternative isoforms, MLPKf1 and MLPKf2, by using different transcription initiation sites. Transient expression analyses of Venus fusion proteins in tobacco protoplasts revealed that both MLPKf1 and MLPKf2 is localized to plasma membrane by distinct mechanisms. The expression of mutant form of MLPKf1 that lack myristoylation motif failed to restore the self-incompatibility response in mlpk/mlpk stigmatic cells, suggesting that plasma membrane localization is essential for MLPK to function in self-incompatibility signaling.