Extensive accumulation of an extracellular l‐amino‐acid oxidase during gametogenesis of Chlamydomonas reinhardtii

Abstract

In a previous study [Bulté, L. & Wollman, F.‐A. (1992) Eur. J. Biochem. 204, 327–336], we identified a novel gamete‐specific polypeptide of Chlamydomonas reinhardtii, Mα. This 66‐kDa polypeptide reacts with antibodies to cytochrome f and accumulates in gametes only in conditions that promote destabilisation of the cytochrome b6lf complex. Here, we show that Mα is not a modification product of cytochrome f, but is part of protein M, a high‐molecular‐mass l‐amino‐acid oxidase located in the periplasm. It catalyzes oxidation of all l‐amino acids tested, except cysteine. Using phenylalanine as a substrate, saturation of the enzymatic rate is reached at 2 μM. These characteristics suggest that protein M may operate in vivo as an efficient scavanger of ammonium from extracellular amino acids. The enzyme contains non‐covalently bound FAD. It exists in two forms with essentially similar enzymatic properties, of 1.2–1.3 MDa and 0.9–1.0 MDa, respectively. The lighter form is an oligomer of Mα, while the heavier form contains, in addition to Mα, a second polypeptide of 135 kDa, Mβ, in a molar ratio of 3–4 Mα/Mβ. Both polypeptides are glycosylated. Copyright © 1993, Wiley Blackwell. All rights reserved