| Feature category | Component / subfamily | Structural characteristics | Distinguishing notes / likely functional implication | Evidence |
|---|---|---|---|---|
| Core domain architecture | N-terminal signal peptide | Typically an N-terminal hydrophobic signal peptide of about 20 amino acids; many BURP proteins are predicted to enter the secretory pathway, though some family members lack a clear signal peptide | Supports secretion or targeting to extracellular/periplasmic, vacuolar, or endomembrane compartments; consistent with roles in cell wall, apoplast, or storage-associated processes | (pqac-00000000, pqac-00000003, pqac-00000009) |
| Core domain architecture | Variable internal region | Between the signal peptide and BURP domain lies a short conserved region and/or a variable segment often containing repeated sequence units; this region differs markedly among subfamilies | Major source of family diversification; repeat content is especially characteristic of some RD22-like members, while BNM2 proteins may lack the repeat segment | (pqac-00000000) |
| Core domain architecture | C-terminal BURP domain | Conserved C-terminal BURP domain of about 230 amino acids | Defines the family; strong C-terminal conservation contrasts with highly variable N-terminal/intervening regions | (pqac-00000000) |
| Conserved motif | Canonical BURP CH motif | Conserved cysteine/histidine-rich BURP-domain signature: CH-X10-CH-X25-27-CH-X25-26-CH-X8-W | This motif is a major diagnostic feature of BURP proteins and is widely used in family identification/classification | (pqac-00000000, pqac-00000009) |
| Conserved residues | N- and C-terminal BURP-domain features | Two conserved phenylalanines (FF) near the N-terminal side of the BURP structural region; highly conserved V, D, T, P, G residues toward the C-terminal portion of the BURP domain | Indicates strong structural conservation within the BURP domain despite wide divergence elsewhere in the proteins | (pqac-00000000) |
| Structural comparison note | PG catalytic motifs vs BURP motifs | The motifs NTD, DD, GHG, and RIK are hallmark catalytic-site features of polygalacturonases (PGs), not of the BURP domain itself | Relevant chiefly for interpreting PG1β-like BURP proteins as polygalacturonase-associated proteins; these motifs define catalytic PG enzymes rather than the BURP scaffold | (pqac-00000007) |
| Subfamily classification | BNM2-like | Often lacks the repeat-containing variable segment seen in some other BURP proteins; part of the original defining BURP set | Associated with pollen grain embryogenesis / reproductive development in plants | (pqac-00000000) |
| Subfamily classification | USP-like | BURP proteins related to unknown seed proteins; generally secretory-pathway-associated proteins with conserved C-terminal BURP domain | Frequently linked to seed development and storage-related compartments | (pqac-00000000, pqac-00000003) |
| Subfamily classification | RD22-like | Distinguished by a variable region that may contain repeated units; only RD22 proteins were noted to have ~20-amino-acid repeats between the signal peptide and BURP domain | Often ABA- and drought-responsive; commonly implicated in abiotic-stress biology | (pqac-00000000) |
| Subfamily classification | PG1β-like | Characterized by a distinctive 14-amino-acid region containing FTNYGXXGNGGXXX; includes rice OsBURP14/OsBURP16-type proteins | Associated with the β-subunit of polygalacturonase isozyme 1 and thus with pectin/cell-wall remodeling rather than independent glycosidase catalysis | (pqac-00000000, pqac-00000001, pqac-00000005) |
| Subfamily classification | BURP V | Rice BURP-family clade identified in genome-wide classification; structurally within the BURP superfamily but less functionally characterized in the cited sources | Likely lineage-expanded monocot/rice-associated BURP branch; specific biochemical role remains unclear from available evidence | (pqac-00000001, pqac-00000009) |
| Subfamily classification | BURP VI | Rice-enriched/monocot-dominated BURP clade identified by phylogenetic analysis | Suggests diversification of BURP functions in monocots, but direct biochemical features are not detailed in the cited sources | (pqac-00000009) |
| Subfamily classification | BURP VII | Rice-enriched/monocot-dominated BURP clade identified by phylogenetic analysis | As with BURP VI, indicates lineage-specific expansion; precise structural distinctions beyond phylogenetic placement are not resolved here | (pqac-00000009) |
| Family-level functional interpretation | Secreted structural/regulatory proteins | Many BURP proteins are not catalytic enzymes themselves; instead they are implicated in extracellular or endomembrane processes such as stress adaptation, seed development, or modulation of PG activity/cell-wall properties | Important for annotation of poorly characterized rice BURP proteins such as B8BAB0: function is often inferred from subfamily/domain context rather than direct enzymology | (pqac-00000000, pqac-00000001, pqac-00000005) |


*Table: This table summarizes the conserved architecture, signature motifs, and major subfamilies of plant BURP-domain proteins, highlighting which features are universal and which are subfamily-specific. It is useful for inferring the likely properties of poorly characterized rice BURP proteins such as B8BAB0.*