CELL‐ASSOCIATED PROTEOLYTIC ENZYMES FROM MARINE PHYTOPLANKTON 1

Despite their central importance in cell metabolism, little is known about proteases in marine phytoplankton. We surveyed caseinolytic and leucine aminopeptidase (LAP) activities in log‐phase cultures of the chlorophyte Dunaliella tertiolecta Butcher, the diatom Thalassiosira weissflogii (Gru.) Fryxell et Hasle, the chrysophyte Isochrysis galbana Parke, the coccolithophorid Emiliania huxleyi (Lohm.) Hay et Mohler, and the cyanobacterium Synechococcus sp. (WH 5701). LAP activity was very low at pH < 6 and peaked between pH 7.5 and 8.5 in all species, whereas caseinolytic activity in most species showed only minor peaks in the pH 4–5 range and broad maxima above pH 8. Thus, acidic vacuolar proteases apparently represented only a small fraction of total protease activity. Attempts to classify proteases using selective inhibitors were inconclusive. Neither the serine/cysteine protease inhibitor leupeptin nor the aspartic protease inhibitor pepstatin. A inhibited caseinolytic or LAP activity in any species. The metalloprotease inhibitor EDTA was only effective against LAP activity in some species, causing average decreases of 30–50%, whereas the cysteine/serine protease inhibitor phenyl methyl sulfonylfluoride achieved at best a 30–60% decrease in caseinolytic activity. Caseinolytic activities were remarkably stable. At pH 7.5 and 25°C, extracts of D. tertiolecta, E. huxleyi , and Synechococcus showed no changes in activity after 24 h, whereas activity declined by less than 50% in the other species. Incubation of cell extracts for 1 h at 25°C in pH 7.5 buffer did not alter patterns of cell proteins, suggesting that endogenous proteases did not effectively degrade endogenous proteins. Casein zymograms were used to identify >200‐and <20‐kDa proteases in homogenates of log‐phase T. weissflogii; only the smaller protease was found in D. tertiolecta. Antibodies to the ATPase subunit (C) of the conserved, chloroplastic Clp protease from Pisum cross‐reacted with proteins in Synechococcus, D. tertiolecta , and I. galbana , but no cross‐reactions were found for any species with antibodies against the ClpP subunit from either E. coli or Nicotiana. Our results show that phytoplankton contain a diverse complement of proteases with novel characteristics.