A SPOROPHYTE CELL-WALL PROTEIN OF THE RED ALGA PORPHYRA-PURPUREA (RHODOPHYTA) IS A NOVEL MEMBER OF THE CHYMOTRYPSIN FAMILY OF SERINE PROTEASES

A complementary DNA (cDNA) clone from a Porphyra purpurea(Roth) C. Aga rdh sporophyte-specific subtracted cDNA library was found to encode a protein similar to serine proteases of the chymotrypsin class. The enc oded protein contains a typical signal peptide and is particularly sim ilar to chymotrypsins in the regions surrounding the active site resid ues and the activation site where cleavage of the propeptide occurs. I n addition, the six cysteine residues characteristic of chymotrypsins are conserved. However, two of the three residues of the active site H is / Asp / Ser charge relay triad have been replaced, indicating that the protein is unlikely to have peptidase activity. Northern hybridiza tion confirmed that this cDNA is derived from an abundant, sporophyte- specific messenger RNA (mRNA). The presence of signal peptide on the e ncoded protein and the abundance of its mRNA suggested that this prote in might be localized in the cell wall. Consequently, sporophyte cell walls were isolated and a major protein having a molecular weight simi lar to that estimated for the encoded protein was purified. N-terminal sequence analysis indicated that this cell wall protein is identical to that encoded by the cDNA with the amino terminus of the mature prot ein beginning at the activation site. This cell wall structural protei n appears to have evolved from a chymotrypsin-like progenitor but has been adapted to bind cell wall proteins and / or polysaccharides rathe r than to cleave proteins.