UniProt: C5YAP3 (CASP1_SORBI) — "Casparian strip membrane protein 1" / SbCASP1
Ordered locus: Sb06g033470 / SORBI_3006G273300 ; Chromosome 6 ; 229 aa
PANTHER: PTHR36488:SF11 ("CASP-LIKE PROTEIN" — the bona fide CASP subfamily, with CASP1-5/CASP1-6 type members)
Pfam: PF04535 (CASP_dom) ; InterPro: IPR006702 (CASP_dom), IPR006459 (CASP/CASPL)
SbCASP1 is a sorghum member of the CASP (CAsparian Strip membrane domain Protein)
family. CASPs are four-transmembrane-span MARVEL-domain plasma-membrane proteins. They
self-assemble into the Casparian Strip Membrane Domain (CSD), a stable, immobile
plasma-membrane scaffold in root endodermal cells. The CSD recruits and positions the
lignin-polymerization machinery (peroxidase PER64, the NADPH oxidase RBOHF, the
dirigent-like protein ESB1) that locally deposits the lignified Casparian strip — a
ring-like cell wall modification forming the root apoplastic diffusion barrier.
Direct experimental characterization (CASP1-CASP5) is in Arabidopsis; the rice ortholog
OsCASP1 is also characterized. SbCASP1 function is inferred by orthology (UniProt
"By similarity"), but the inference is strong: family-wide conservation of the four-TM
topology, the membrane-scaffold behaviour, and the Casparian-strip role in grasses.
PMID:21593871 — Roppolo et al.
2011 Nature identified the CASP family (CASP1-5 in Arabidopsis) as the first molecular
factors establishing a plasma-membrane and extracellular diffusion barrier in plants.
PMID:21593871 — CASP1 behaves as a membrane
scaffold: forms homo/hetero-complexes, becomes immobile, sediments as a large polymer.
PMID:21593871 — Genetic
loss of function: casp1 casp3 double mutants have disorganized Casparian strips. This is
the key evidence that CASPs organize a specific cell wall structure.
PMID:24920445 — Roppolo et al. 2014 Plant Physiol. CASPs are
4-TM proteins; they mediate Casparian strip deposition by recruiting the lignin
polymerization machinery.
PMID:24920445 — The CASP membrane domain is a stable scaffold.
PMID:24920445 — The broader CASP-like (CASPL) family is pan-plant and even in
green algae; outside plants the homologs are MARVEL-domain proteins. This places CASP in
the MARVEL tetraspan superfamily.
PMID:24920445 — The first extracellular loop is
conserved specifically in euphyllophytes (which have Casparian strips). This is an
evolutionary correlate of Casparian-strip-forming capacity, relevant to the bona-fide
CASP subfamily (SF11) that SbCASP1 belongs to.
PMID:23940370 — Hosmani et al. 2013
PNAS: the Casparian strip is lignin; CASPs position the lignin-biosynthetic machinery
(NADPH oxidase + peroxidases). ESB1 (dirigent-like) is part of this machinery and is
localized to the Casparian strip in a CASP-dependent manner.
PMID:26124109 — Kamiya et al. 2015 PNAS: explicit list
of recruited components (PER64 peroxidase, RBOHF NADPH oxidase, ESB1 dirigent). The
transcription factor MYB36 directly regulates CASP1, PER64 and ESB1.
PMID:21735349 — Alassimone et al. 2011 review: the Casparian strip is the defining cell wall
modification of the differentiated endodermis.
PMID:21735349 — CASPs
localize to a plasma-membrane domain directly underneath the Casparian strip; the
endodermis blocks apoplastic diffusion.
PMID:21593871 — Casparian strips are
belts/rings of specialized cell wall material forming the apoplastic diffusion barrier.
The Casparian strip GO cellular-component term is GO:0048226 "Casparian strip"
(a cellular anatomical structure). The biological-process term GO:0160073
"Casparian strip assembly" exists ("The aggregation, arrangement and bonding together of
a set of components to form a Casparian strip") — this is the precise process term for
what CASP proteins do.
The PANTHER family PTHR36488 has 6602 members across 1073 taxa, all land plants/algae.
The bona-fide CASP subfamily SF11 ("CASP-LIKE PROTEIN") includes named Casparian strip
membrane proteins from rice, grape, sorghum, etc. SbCASP1 (C5YAP3, Sb06g033470) is in
SF11. Sorghum has multiple SF11 members: SbCASP1 (C5YAP3) and SbCASP2 (C5YLC9,
Sb07g000300), plus CASP4 (C5Y9U6) in the broader CASP-like SF12, mirroring the
Arabidopsis CASP1-5 / rice OsCASP1-2 expansion.
[PMID:24920445 GENE FAMILY / NOMENCLATURE — cited in the SbCASP1 UniProt entry as the
reference for the CASP family and SbCASP1 nomenclature] — Roppolo et al. 2014 is the
phylogenetic/nomenclature study that named CASP-family members across plant genomes,
including the sorghum SbCASP genes.
PMID:36600916 —
Yang et al. 2022: the CASP scaffold-and-recruit mechanism holds in rice (a grass /
Poaceae, like sorghum).
PMID:36600916 — OsCASP1 loss-of-function delays Casparian strip formation
and causes uneven lignin deposition; functional CASP role is conserved in grasses.
PMID:36600916 — Grass CASP1
contributes to nutrient homeostasis via the endodermal barrier.
PMID:37653341 — Song et al. 2023 Nat Plants: in rice, OsCASP1
is a core component of the CS-CSD junctional complex; a new GAPLESS protein family
tethers the plasma membrane to the Casparian strip via OsCASP1. Confirms OsCASP1 is the
grass CASP scaffold protein and that the CSD/Casparian-strip tight-junction biology is
conserved in Poaceae.
The retired SwissProt-keyword-derived annotation maps the UniProt keyword
"Cell wall biogenesis/degradation" (KW-0961, present in the SbCASP1 entry) to
GO:0071555 "cell wall organization".
This is biologically correct and informative: CASP proteins organize the formation of
a specific, localized cell wall structure (the lignified Casparian strip). The UniProt
FUNCTION text for SbCASP1 itself states it "Regulates membrane-cell wall junctions and
localized cell wall deposition" and is "Required for ... the subsequent formation of
Casparian strips, a cell wall modification of the root endodermis". Experimentally,
Arabidopsis casp double mutants have "disorganized Casparian strips" PMID:21593871 —
i.e. loss of CASP disrupts cell wall organization. So GO:0071555 captures genuine biology
and GOA's removal of this SPKW annotation is a loss of correct information.
However, GO:0071555 is broad. A more specific and more accurate term is available:
- GO:0160073 "Casparian strip assembly" — exact process; precise MODIFY target.
- GO:0009664 "plant-type cell wall organization" — also more specific (cellulose/pectin
plant cell wall) and a child of GO:0071555.
Recommended action for the retired SPKW annotation: MODIFY to GO:0160073 (Casparian
strip assembly), retaining retired: true. GOA's removal was not the ideal outcome —
the keyword carried real, plant-specific biology — but the original broad term GO:0071555
was suboptimal; the correct response is to replace it with the specific term, not delete
the biology. This gene exemplifies the "unique-but-correct, informative, plant-specific"
category of SPKW annotations.
UniProt SUBCELLULAR LOCATION: "Cell membrane; Multi-pass membrane protein. Note=Very
restricted localization following a belt shape within the plasma membrane which coincides
with the position of the Casparian strip membrane domain in the root endodermis." The
four predicted TRANSMEM helices (FT 68-88, 116-136, 158-178, 206-226) confirm a tetraspan
plasma-membrane protein. CASP1 in Arabidopsis localizes to the plasma membrane (the CSD)
PMID:21593871. GO:0005886 "plasma membrane" is correct but generic; a more precise term
is GO:1990877 "Casparian strip" / the CSD-specific component — see below.
OLS check: GO:0048226 "Casparian strip" is a cellular_component (cellular anatomical
structure). There is no separate GO CC term for the "Casparian strip membrane domain"
distinct from the plasma membrane; the CSD is a plasma-membrane subdomain. GO:0005886
plasma membrane is therefore the appropriate (if generic) CC; ACCEPT.