Algae. 1992; 7(1): 121-129.

Biochemical and Physiological Properties of a Gametic Protoplast-Release-Inducing Protein in Closterium

Hiroyuki Sekimoto, Shinobu Satoh, Tadashi Fujii

Institute of Biological Sciences, University of Tsukuba

When mating-type minus (mt super(-)) and plus (mt super(+)) cells of the Closterium peracerosum-strigosum-littorale complex were mixed together in nitrogen-deficient mating medium, cells of both types released protoplasts, this release being the first step in the process of conjugation. Release of protoplasts by mt super(-) cells also proceeded without pairing in a medium in which mt super(-) and mt super(+) cells had previously been cultured together. The protoplast-release-inducing activity in medium was completely lost by exposure to a high temperature (50℃) for 10 min, and precipitated in 80% saturated ammonium sulfate solution. A protein with the ability to induce the release of protoplasts was purified from this medium by sequential column-chromatographic steps, and named PR-IP (protoplast-release-inducing protein). The PR-IP had an apparent molecular mass (Mr) of 95,000 on gel filtration and could seseparated into several isoforms by anion-exchange chromatography. Each isoform consisted of two glycopolypeptides of Mr 42,000 and 19,0, while the deglycosylated polypeptides had Mr values of 34,000 and 18,0, respedtively. From an analysis of does-response curves, the concentration required for 50% of the maximum response (ED sub(50)) as 4.1 × 10 super(-9) M. The protoplast-release could be induced by only 4 hour incubation with PR-IP, while actual protoplast-release by mt super(-) cells occurred after 13 h incubation with PR-IP. We suggest that the PR-IP is a biologically active glycoprotein which induces the release of gametic protoplasts from mt super(-) cells of this Closterium complex.

Keywords : Closterium, conjugation, glycoprotein, protoplast release in conjugation Abbreviations: MI = mating medium; Mr = apparent molecular mass; PI = protoplast-release-inducing; PR-IP = protoplast-release-inducing protein