Qy. Liu et M. Reith, A SPOROPHYTE CELL-WALL PROTEIN OF THE RED ALGA PORPHYRA-PURPUREA (RHODOPHYTA) IS A NOVEL MEMBER OF THE CHYMOTRYPSIN FAMILY OF SERINE PROTEASES, Journal of phycology, 32(6), 1996, pp. 1003-1009
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.