This domain is found mainly in plant proteins known as Casparian strip membrane proteins (CASPs) and CASP-like proteins (CASPLs). CASPs are four-membrane-span proteins that mediate the deposition of Casparian strips in the endodermis by recruiting the lignin polymerization machinery. Interestingly, the CASP first extracellular loop was found conserved in euphyllophytes but absent in plants lacking Casparian strips [1,2].

1: A novel protein family mediates Casparian strip formation in the endodermis. Roppolo D, De Rybel B, Denervaud Tendon V, Pfister A, Alassimone J, Vermeer JE, Yamazaki M, Stierhof YD, Beeckman T, Geldner N; Nature. 2011;473:380-383. PMID:21593871

2: Functional and Evolutionary Analysis of the CASPARIAN STRIP MEMBRANE DOMAIN PROTEIN Family. Roppolo D, Boeckmann B, Pfister A, Boutet E, Rubio MC, Denervaud-Tendon V, Vermeer JE, Gheyselinck J, Xenarios I, Geldner N; Plant Physiol. 2014;165:1709-1722. PMID:24920445