Cytoskeleton-dependent polarized secretion of arylsulfatase in the unicellular green alga, Chlamydomonas reinhardtii

Abstract

In the green alga Chlamydomonas reinhardtii, protein secretion has not been well investigated at the molecular level although the process plays important roles in cell wall biogenesis, the mating process, and stress response. Here, we studied the cytoskeleton dependence of Chlamydomonas protein secretion by examining the effects of cytoskeleton-disrupting reagents. Protein transport activity was monitored by the secretion of arylsulfatase (ARS), a glycoprotein which is synthesized in response to sulfur deprivation and transported to the periplasmic space. Microtubule-disrupting reagents, oryzalin and nocodazole, inhibited ARS secretion in a dose-dependent manner, while actin-disrupting reagents, cytochalasins B and D, induced up to a 1.4-fold increase in ARS secretion. The effect of oryzalin on the intracellular ARS distribution was different from that of brefeldin A (BFA), which disrupts the ER to Golgi protein transport. Immunofluorescence analysis using an anti-ARS polyclonal antibody revealed specific punctate staining on the anterior side of the cell surface of cells cultured in sulfur-depleted medium. This staining was not observed when the cells were treated with oryzalin. These results suggest that Chlamydomonas protein secretion is dependent on microtubules and inhibited by actin-related cytoskeletons. This cytoskeleton dependence is similar to that found in mammalian cells rather than that found in plant cells.