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Abstract
Western blotting experiments performed with an anti-ubiquitin antibody showed that Skeletonema costatum cells contained free ubiquitin and high molecular weight ubiquitin conjugates. An estimate of free ubiquitin and ubiquitin conjugates by a solid phase immunoassay (Haas & Bright 1985) provided values of 0.31 ± 0.04 pmol/107 cells for free ubiquitin and 0.46 ± 0.04 pmol/107 cells for conjugated ubiquitin.
S. costatum cell free extracts were able to conjugate 125I-ubiquitin to endogenous proteins in an ATP-dependent manner. Furthermore, by ion exchange and affinity chromatography on immobilized ubiquitin, five polypeptides (Mr of approximately 96, 48, 34, 31 and 26 kDa) able to form thiol esters with 125I-ubiquitin were isolated. These proteins most likely correspond to the ubiquitin-conjugating enzymes (Ei and E2s) present in all eukaryotic cells. S. costatum cell extracts are able to degrade 125I-labelled S. costatum proteins and 125I-labelled bovine serum albumin (BSA). This degradation is stimulated by addition of exogenous ATP and an ATP-generating system.
Thus, S. costatum contains ubiquitin and an active ubiquitin conjugating system that could be involved in the easy adaptability of S. costatum to different environmental conditions.