Regulates membrane-cell wall junctions and localized cell wall deposition. Required for establishment of the Casparian strip membrane domain (CSD) and the subsequent formation of Casparian strips, a cell wall modification of the root endodermis that determines an apoplastic barrier between the intraorganismal apoplasm and the extraorganismal apoplasm and prevents lateral diffusion. Homodimer and heterodimers with other CASP proteins. Interacts with CASP1, CASP3 and CASP4. Very restricted localization following a belt shape within the plasma membrane which coincides with the position of the Casparian strip membrane domain. Belongs to the Casparian strip membrane proteins (CASP) family.