SOSEKI proteins (SOK1-5) locally interpret global polarity cues and can influence cell division orientation to coordinate cell polarization relative to body axes, probably by guiding ANGUSTIFOLIA (AN) polarized localization. Homodimer (PubMed:30737509). Forms long polymer filaments with other SOKs proteins polymers (e.g. SOK1, SOK2, SOK3 and SOK4) crucial for polar localization and biological activity (PubMed:32004461). Binds to ANGUSTIFOLIA (AN) (PubMed:32004461). SOSEKI proteins (SOK1-5) integrate apical-basal and radial organismal axes to localize to polar cell edges in roots, pointing towards the endodermis, mainly to inner apical edges. Expressed during embryogenesis and in roots. During embryogenesis, first observed in the apical side of lower tier inner cells at the early globular stage (PubMed:30737509). At transition to heart stage and later in the post-embryonic root and lateral roots, accumulates at the outer apical corner or outer lateral side of young vascular cells, including the pericycle, and outer corners of the hypophysis (PubMed:30737509). Also present in the columella root cap of the primary root (PubMed:30737509). Highly unstable; disappears during cell division but is afterwards quickly re-established in lateral root primordia and primary root (at the protein level). The DIX-like oligomerization domain is required for polymerization, edge localization and biological activity. 'Soseki' means cornerstone in Japanese. Belongs to the SOSEKI family.