The extracellular matrix (ECM) of Volvox contains insoluble fibrous layers that surround individual cells at a distance to form contiguous cellular compartments. SSG 185 is the monomeric precursor of this substructure (C3Z structure). Polymer. Intersubunit cross-links are formed between saccharide chains rather than between polypeptide chains. Hydroxylated on proline residues in the Pro-rich central domain. Glycosylated; contains sulfate-substituted glycans.