CRPPA (previously ISPD) synthesizes CDP-ribitol from CTP and ribitol phosphate. FKTN and FKRP subsequently transfer ribitol phosphate to the alpha-dystroglycan glycan. CRPPA therefore performs an upstream chemical step in glycan biosynthesis; it does not itself transfer the ribitol phosphate onto the protein. CDP-ribitol supplementation can rescue glycosylation in deficient cells PMID:26923585. The original human-enzyme study identifies the product by chromatography and mass spectrometry, with substrate controls and matched mutant protein amounts.
The principal physiological synthesis is represented by D-ribitol-5-phosphate cytidylyltransferase activity, protein O-linked glycosylation via mannose, and cytosol. The process definition includes elongation of the mannose-initiated glycan. This is donor synthesis rather than a claim that CRPPA catalyzes the first mannose-transfer reaction. Human patient fibroblasts show wild-type complementation of functional glycosylation PMID:22522420. The early POMT-activity interpretation in that study is qualified by later experiments showing retained POMT activity in other deficient-cell contexts. The positive patient-cell rescue is preserved. The 2012 original includes the actual constructs and POMT assay Methods.
Human CRPPA also acts on ribose and ribulose phosphates in vitro PMID:26687144. Accordingly, the broader existing cytidylyltransferase assertions retain measured substrate breadth. The human biochemical and structural original uses full-length 1–451 protein expressed in bacteria and a distinct 43–451 construct produced in insect cells. It reports low basal MEP turnover; this is not an absence-of-catalysis claim. Two HAP1 knockout clones nevertheless retained the measured dolichol species and levels while losing functional dystroglycan glycosylation. Together with the identified nucleotide-sugar products, these results distinguish CRPPA from the bacterial isoprenoid-pathway role assigned through the conserved IspD site.
Constructs differ across studies. The human 43–451 protein in PMID:27130732(https://pmc.ncbi.nlm.nih.gov/articles/PMC4924997/) was expressed in HEK293F with an N-terminal His8/TEV tag. The purified enzyme in PMID:27194101(https://pmc.ncbi.nlm.nih.gov/articles/PMC4873967/) was mouse Ispd expressed in Sf9 cells; human knockout/rescue cells and rat-muscle metabolites are separate experiments. Bacterial TarI complementation supports the importance of the nucleotide donor rather than an obligatory additional mammalian activity. Differences in low-level alternative-substrate turnover remain dependent on species and assay conditions.
The human crystal construct forms a homodimer; the same study reports consistent solution gel filtration without displaying those data. This supports the existing homodimerization assertion but does not establish endogenous cellular stoichiometry. Tagged full-length human protein partitions to the cytosolic fraction with organelle controls PMID:26687144. The Reactome reaction independently models the A4D126 dimer in cytosol.
The official displayed product has 451 residues. The second listed product has 401 residues and lacks residues 179–228, including the region containing the structural dimer arm. Its sequence is reconstructed from the recorded VSP edit and matches the official product FASTA. Its catalytic activity and oligomerization have not been established here. Neither the engineered 43–451 construct nor the patient exon-3 deletion is automatically assigned to this numbered splice product.
Human recessive variants and zebrafish knockdown support the same glycan pathway, but retain their experimental distinctions. Two splice-blocking zebrafish morpholinos give concordant structural and sarcolemma defects; the direct glycosylated-dystroglycan measurement uses MO1 PMID:22522421. The original main article and its linked five-page supplementary Methods document the human cohort, fish splice controls, morpholino controls and muscle assays. Figure 4 was inspected directly. The phenotype is not converted into an additional autonomous muscle-development function.
The three PAINT assertions are traced to literal ancestral IBDs on the human path in PTHR43015. Cytosol derives from PTN000440569; the glycan process and specific ribitol-phosphate reaction derive from PTN001452245. Human self-evidence is valid experimental grounding. The zebrafish process donor is A0JPF9/ZDB-GENE-061110-16, whose actual experimental evidence is retained. The exact human MSA matches the displayed product, and no target-path loss annotation was found. The current ARBA00027560 branch matches the target FunFam and Euteleostomi conditions and supports the broader glycoprotein-biosynthesis assertion. Biological support is evaluated separately from rule membership.
The two available interaction records are contextual screen observations, not new molecular-function assertions. The CCT2 record reports crosslinking/physical association, and the viral-protein record reports co-immunoprecipitation. Their literal records are available in the evidence index; their primary experiments have not been independently read here. No new binding or regulatory function is inferred.
The evidence index records exact source objects, product sequences, public original-source locators and hashes, ontology definitions, ancestral-node and donor records, rule conditions, and the Reactome target join. Selected publication caches for PMID:26687144 and PMID:26923585 contain abstracts, while authentic external original PDFs provided the stated Methods/Results access. The other four selected sources contain full-text material, with the separately recovered clinical supplementary Methods identified explicitly. The index is authored provenance, not independent biological evidence.
The broad glycoprotein-biosynthesis annotation is refined to protein O-linked glycosylation via mannose. The current ontology places this process beneath glycoprotein biosynthesis and includes elaboration of the mannose-initiated glycan. Human enzyme assays and donor rescue support CRPPA doing the upstream chemistry that supplies CDP-ribitol for that pathway. The original ARBA output and its matching branch 26 remain positive source evidence; they are not rewritten as a more specific rule output. This refinement supersedes the earlier choice to retain the broad process alongside the specific assertion (GO:0035269; PMID:26923585(https://da.lib.kobe-u.ac.jp/da/kernel/90003339/90003339.pdf), human enzyme assays and donor rescue).
Homodimerization remains experimentally supported, but is retained as a non-core structural property. The human 43–451 crystal and reported gel filtration establish association with their stated limits; they do not establish an additional primary activity or endogenous stoichiometry for every splice product. This refines the earlier core-status choice without rejecting the positive association (PMID:26687144(https://hdl.handle.net/2066/152310), printed page1645, structural Results and gel-filtration statement).
Product interpretation belongs with these biological evidence limits. The following descriptions are preserved from the preceding assessment, while the listed product identifiers, names and sequence note remain the original machine-derived fields:
A4D126-1: The displayed 451-residue product contains the cytidylyltransferase region and C-terminal domain. Human full-length protein and distinct engineered 43–451 constructs support the biochemical activity; construct and expression-host differences remain source-specific.
A4D126-2: The official 401-residue product lacks displayed-sequence residues 179–228 (VSP_044044), including the region containing the structural dimer arm. This sequence change does not by itself establish its catalytic activity, dimerization or physiological contribution. The tested 43–451 truncation is a different construct. The patient exon-3 deletion in PMID:22522420 is not equated with this numbered product without exact sequence correspondence.
CRPPA itself catalyzes donor formation, so a donor-biosynthesis process is a meaningful question rather than a substrate-necessity inference. The current nucleotide-sugar definition specifies a glycosidic nucleotide–carbohydrate linkage; its application to the exact CDP-ribitol chemistry merits explicit clarification. A bounded current comparison found no GO:0009226 or descendant annotations for human CRPPA, GALT or UGP2. This limited absence does not establish a universal convention or prove a missed annotation. No additional process assertion is made here (GO:0009226).