Chlamydomonas reinhardtii produces a profilin with unusual biochemical properties.

We report the characterization of a profilin orthologue from Chlamydomonas reinhardtii. CrPRF, probably the only profilin isoform, is present in both the cell body and flagella. Examination of vegetative and gametic cells by immunofluorescence microscopy using multiple fixation procedures also revealed enrichment of CrPRF at the anterior of the cell near the base of flagella and near the base of the fertilization tubule in mating type plus gametes. Purified, recombinant CrPRF binds to actin with a Kd value approximately 10(-7) and displaces nuclei in a live cell 'nuclear displacement' assay, consistent with profilin's ability to bind G-actin in vivo. However, when compared with other profilin isoforms, CrPRF has a relatively low affinity for poly-L-proline and for phosphatidylinositol (4,5) bisphosphate micelles. Furthermore, and surprisingly, CrPRF inhibits exchange of adenine nucleotide on G-actin in a manner similar to human ADF or DNase I. Thus, we postulate that a primary role for CrPRF is to sequester actin in Chlamydomonas. The unusual biochemical properties of CrPRF offer a new opportunity to distinguish specific functions for profilin isoforms.