Minimal requirements and evolution of polar membrane targeting in the land plant SOSEKI protein family

Accurate spatial distribution of polar proteins is essential for cell function, but mechanisms underlying polar protein targeting in plants are poorly understood. Here, we dissect the molecular mechanisms underlying the polar targeting of the SOSEKI1 (SOK1) protein in Arabidopsis thaliana. Using extensive mutagenesis and truncation analyses of the polarity domain, we mapped the minimal polarity fragment necessary and sufficient for stable polar targeting to a 25-amino-acid region. This region likely forms an amphipathic helix, and mutations reveal that both its polar and apolar faces are essential for membrane association and polarity. Using phylogenetic analysis, we reconstruct the evolutionary history of the minimal polar targeting region across land plants. Using cross-species expression and protein localization, we infer the ancestral polar SOK localization pattern and show how it remained remarkably conserved, only to diversify upon gene duplication. This work identifies a polar targeting peptide in plants, reveals a mechanism for polar membrane targeting of a plant protein family, and demonstrates both conservation and diversification within this family.