Transcriptional activation by the sexual pheromone and wounding: a new gene family from Volvox encoding modular proteins with (hydroxy)proline-rich and metalloproteinase homology domains

The green alga Volvox represents the simplest kind of multicellular organis m: it is composed of only two cell types, somatic and reproductive, making it suitable as a model system. The sexual development of males and females of Volvox carteri is triggered by a sex-inducing pheromone at a concentrati on of < 10(-16) M. Early biochemical responses to the pheromone involve str uctural modifications within the extracellular matrix (ECM). By differentia l screenings of cDNA libraries made from mRNAs of pheromone-treated Volvox, four novel genes were identified that encode four closely related Volvox m etalloproteinases that we use to define a new protein family, the VMPs. The existence of several features common to matrix glycoproteins, such as sign al peptides, a (hydroxy)proline content of 12-25%, and Ser(Pro)(2-4) repeat s, suggest an extracellular localization of the VMPs within the ECM. Synthe sis of VMP cDNAs is triggered not only by the sex-inducing pheromone, but a lso by wounding, and is restricted to the somatic cell type. Sequence compa risons suggest that the VMPs are members of the MB clan of zinc-dependent m atrix metalloproteinases, although the putative zinc binding site of all VM Ps is QEXXHXXGXXH rather than HEXXHXXGXXH. The presence of glutamine instea d of histidine in the zinc binding motif suggests a novel family, or even c lan, of peptidases. Like the matrixin family of human collagenases, Volvox VMPs exhibit a modular structure: they possess a metalloproteinase homology domain and a (hydroxy)proline-rich domain, and one of them, VMP4, also has two additional domains. Metalloproteinases seem to be crucial for biochemi cal modifications of the ECM during development or after wounding in the lo wer eukaryote Volvox with only two cell types, just as in higher organisms.