Coiled coil region of thrombospondin in protostomes
This family contains the N-terminal coiled coil region of thrombospondin (TSP) in some protostomes, which suggest ancient functions that include bridging activities in cell-cell and cell-ECM interactions. It appears that most protostomes and inferred basal metazoa encode a single TSP with the general domain organization of subgroup B TSPs and with a pentamerizing coiled coil. This region has heparin-binding activity and is a component of extracellular matrix (ECM), showing that the pentameric TSPs are of earlier origin and that the trimeric TSP subfamily A form is associated with higher chordates. The left-handed coiled coil pentamer forms a channel that is a unique carrier for lipophilic compounds, and is stabilized by inter-subunit disulfide bonds formed between cysteine residues adjacent to the C-terminal end of the coiled coil region. Several heparan sulphate (HS) proteoglycans are known in D. melanogaster, including both transmembrane and matrix forms, which could contribute to its retention in pericellular matrix.
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