Enzymes in the extracellular matrix of Volvox: an inducible, calcium-dependent phosphatase with a modular composition.
The volvocine algae provide the unique opportunity for exploring development of an extracellular matrix. Volvox is the most advanced member of this family and represents the simplest multicellular organism, with differentiated cells, a complete division of labor, and a complex extracellular matrix, which serves structural and enzymatic functions. In Volvox carteri a glycosylated extracellular phosphatase was identified, which is partially released from the extracellular matrix into the growth medium. The phosphatase is synthesized in response to inorganic phosphate starvation and is strictly calcium-dependent. The metalloenzyme has been purified to homogeneity and characterized. Its gene and cDNA have been cloned. Comparisons of genomic and cDNA sequences revealed an extremely intron-rich gene (32 introns). With an apparent molecular mass of 160 kDa the Volvox extracellular phosphatase is the largest phosphatase cloned, with no sequence similarity to any other phosphatase. This enzyme exhibits a modular composition. There are two large domains and a small one. The large domains are highly homologous to each other and therefore most likely originated from gene duplication and fusion. At least one EF-hand motif for calcium binding was identified in this extracellular protein. Volvox extracellular phosphatase is the first calcium-dependent extracellular phosphatase to be cloned.