Functional and Evolutionary Analysis of the CASPARIAN STRIP MEMBRANE DOMAIN PROTEIN Family    

Abstract

CASPARIAN STRIP MEMBRANE DOMAIN PROTEINS (CASPs) are four-membrane-span proteins that mediate the deposition of Casparian strips in the endodermis by recruiting the lignin polymerization machinery. CASPs show high stability in their membrane domain, which presents all the hallmarks of a membrane scaffold. Here, we characterized the large family of CASP-like (CASPL) proteins. CASPLs were found in all major divisions of land plants as well as in green algae; homologs outside of the plant kingdom were identified as members of the MARVEL protein family. When ectopically expressed in the endodermis, most CASPLs were able to integrate the CASP membrane domain, which suggests that CASPLs share with CASPs the propensity to form transmembrane scaffolds. Extracellular loops are not necessary for generating the scaffold, since CASP1 was still able to localize correctly when either one of the extracellular loops was deleted. The CASP first extracellular loop was found conserved in euphyllophytes but absent in plants lacking Casparian strips, an observation that may contribute to the study of Casparian strip and root evolution. In Arabidopsis (Arabidopsis thaliana), CASPL showed specific expression in a variety of cell types, such as trichomes, abscission zone cells, peripheral root cap cells, and xylem pole pericycle cells.

Across the great divide: the plant cell surface continuum

“…Formation of a localized Casparian strip membrane domain (CSD) precedes formation of the Casparian strip and shows no lateral mobility [44 ]. CASP protein localization forms a barrier in the PM which blocks movement of lipid tracer dyes such as FM4-64 [45]. Subsequently, it has been demonstrated that the NADPH oxidase Curve-fitting FRAP data illustrates the range of mobilities in plasma membrane proteins.…”

“…We speculate that this method of membrane scaffold production for targeted alteration of the CW is a model for how CW modification occurs in other conditions. In addition to the CASP proteins, CASP-like (CASPL) family members are present in all land plants and have recently been shown to form membrane scaffolds [45]. Given that moss species such as Physcomitrella patens possess these proteins but do not produce lignin, it is possibly that the CASP and CASPL membrane scaffold method of CW modification can affect other CW components.…”

Role of Reactive Oxygen Species during Cell Expansion in Leaves

“…Still, DUOX proteins are absent in plants, indicating a necessity for a flexible protein scaffold. The identified scaffold proteins belong to the CASPARIAN STRIP MEMBRANE DOMAIN PROTEIN (CASP) family, which, in Arabidopsis, has 20 members (Roppolo et al, 2014). Therefore, it would be interesting to determine which CASP-LIKE proteins might regulate ROS channeling from NADPH oxidases during leaf growth.…”

Cytoskeletal Components Define Protein Location to Membrane Microdomains*

“…More recently, it became obvious that membrane microdomains within a single cell are highly diverse and of different compositions (9). Generally, in the plant model, organisms' plasma membrane microdomains turned out to be important in plant defense (10,11), cell polarity (12,13), and general signaling properties of the plasma membrane (14,15).…”