The alga Volvox carteri is one of the simplest multicellular organisms, yet it has a surprisingly complex extracellular matrix (ECM), making Volvox suitable as a model system in which to study ECM self-assembly. Here, we analyze the primary structures and post-translational modifications of two main ECM components synthesized in response to sexual induction as well as wounding. These proteins are members of the pherophorin family with as yet unknown properties. They contain polyhydroxyproline spacers as long as 500 and 2750 residues. Even the highly purified proteins retain the capacity to self-assemble and cross-link, producing an insoluble fibrous network in an apparently autocatalytic reaction. This pherophorin-based network is located within the deep zone of the ECM. A molecular genetic search for additional members of the pherophorin family indicates that at least nine different pherophorin species can be expected to serve as precursors for ECM substructures. Therefore, the highly diversified members of the pherophorin family represent region-specific morphological building blocks for ECM assembly and cross-linking.

The extracellular matrix (ECM) of Volvox is modified during development or in response to external stimuli, like the sex-inducing pheromone. It has recently been demonstrated that a number of genes triggered by the sex-inducing pheromone are also inducible by wounding. By differential screening of a cDNA library, a novel gene was identified that is transcribed in response to the pheromone. Its gene product was characterized as an ECM glycoprotein with a striking feature: it exhibits a hydroxyproline content of 68% and therefore is an extreme member of the family of hydroxyproline-rich glycoproteins (HRGPs). HRGPs are known as constituents of higher plant ECMs and seem to function as structural barriers in defense responses. The Volvox HRGP is also found to be inducible by wounding. This indicates that the wound response scenarios of higher plants and multicellular green algae may be evolutionary related.