A novel extensin that may organize extracellular matrix biogenesis in Volvox carteri.

ISG is a sulphated, extracellular glycoprotein synthesized for only a few minutes in inverting Volvox embryos and inverting sperm cell packets. This control operates at the level of transcription. ISG has been characterized by studies of protein chemistry and electron microscopy. The primary structure of ISG has been derived from genomic DNA and cDNA. ISG is composed of a globular and a rod-shaped domain. The rod-shaped domain represents a member of the extensin family with numerous repeats of Ser-(Hyp)4-6 motifs. A synthetic decapeptide matching the C-terminal sequence is able to disaggregate the organism into individual cells. Immunofluorescence microscopy localizes ISG within the boundary zone of the ECM.