| Domain | Approx. structural description | InterPro / UniProt domain mapping | Key structural features | Functional role in BBSome assembly/interactions | Evidence |
|---|---|---|---|---|---|
| N-terminal β-propeller / WD40 | N-terminal propeller domain; one of five conserved domains shared with BBS1/2/7 | WD40_repeat_dom_sf (IPR036322); PHTB1_N_dom (IPR028073) | β-propeller scaffold mediating extensive protein-protein contacts; in BBS family proteins this domain contributes to head/body architecture and interaction specificity | Contributes to BBS-9’s hub-like connectivity within the BBSome; by homology with BBS1-family architecture, the propeller helps organize subunit contacts and overall complex topology (pqac-00000001, pqac-00000005) | (pqac-00000001, pqac-00000005) |
| Heterodimerization α-helix | Short α-helical segment linking propeller to downstream domains | No specific InterPro listed in provided UniProt summary | Conserved helical module in the five-domain BBS1/2/7/9 architecture; supports packing between adjacent domains | Part of the conserved structural core enabling correct domain arrangement and partner matching during BBSome assembly (pqac-00000001) | (pqac-00000001) |
| GAE domain | Immunoglobulin-like GAE/adaptin-related domain in central region | PTHB1 (IPR026511) | BBS9 GAE heterodimerizes with BBS1 GAE; contains a strand insertion between β3 and β4 that helps enforce structural specificity and prevent incorrect pairing | Critical for subunit recognition and core-body organization; forms a BBS1–BBS9 GAE heterodimer in the BBSome body and supports correct assembly specificity (pqac-00000000, pqac-00000002) | (pqac-00000000, pqac-00000002) |
| Platform (pf) domain | Mixed α/β platform domain associated with the GAE module | PTHB1_pf_dom (IPR055362) | Together with the GAE domain forms a GAE-pf module structurally analogous to clathrin/adaptin and coatomer modules; makes extensive hydrophobic contacts with the GAE domain | Provides structural body framework and supports the evolutionary coat-complex-like architecture of the BBSome; likely helps position interaction surfaces for IFT/cargo-related functions (pqac-00000000, pqac-00000002) | (pqac-00000000, pqac-00000002) |
| C-terminal coiled-coil | Distal α-helical coiled-coil / neck-forming region | PTHB1_hp_dom (IPR055363) | Coiled-coil associates directly with the BBS2 coiled-coil to form the neck connecting head and body lobes | Essential for higher-order BBSome architecture; helps connect head and body lobes and stabilizes assembly of the core complex (pqac-00000001, pqac-00000002) | (pqac-00000001, pqac-00000002) |
| Integrated architecture summary | Full-length BBS-9/PTHB1 comprises a five-domain scaffold conserved with BBS1/2/7 | Combined mapping from UniProt: IPR028073, IPR026511, IPR055362, IPR055363, IPR036322 | BBS-9 directly contacts all other BBSome subunits and acts as a central structural hub | Central organizer of the BBSome core and a major determinant of complex assembly, stability, and connectivity to ciliary trafficking machinery (pqac-00000001, pqac-00000003, pqac-00000005) | (pqac-00000001, pqac-00000003, pqac-00000005) |


*Table: This table summarizes the five-domain architecture of C. elegans BBS-9/PTHB1 inferred from BBSome structural studies and UniProt/InterPro annotations. It highlights how each domain contributes to BBSome assembly, structural organization, and protein-protein interactions.*