Biochemical and physiological properties of a protein inducing protoplast release during conjugation in theClosterium peracerosum-strigosum-littorale complex.

When mating-type minus (mt(-)) and plus (mt(+)) cells of theClosterium peracerosum-strigosum-littorale complex were mixed together in a 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(-) cells also proceeded without pairing in a medium in which mt(-) and mt(+) cells had previously been cultured together. 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 95000 on gel filtration and could be separated into several isoforms by anion-exchange chromatography. Each isoform consisted of two glycopolypeptides of Mrs 42000 and 19000, while the deglycosylated polypeptides had Mrs of 34000 and 18000, respectively. From an analysis of dose-response curves, the numbers of PR-IP molecules required for the release of a protoplast by a single cell was calculated as 1.5·10(9) and the concentration required for 50% of the maximum response (ED50) as 4.1·10(-9)M. We suggest that the PR-IP is a biologically active glycoprotein which induces the release of gametic protoplasts from mt(-) cells of thisClosterium complex.