Ubiquitin in Chlamydomonas reinhardii

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Abstract
Ubiquitin, a highly conserved 76-amino-acid protein, is involved in the response of many types of eukarotic cells to stress but little is known abut its role in lower plants. In the present study we have investigated the distribution of ubiquitin in the unicellular alga Chlamydomonas reinhardii as well as the effect of heat and light stress on its conjugation to cellular proteins. Immunoelectron microscopy shows that ubiquitin is located in the chloroplast, nucleus, cytoplasm, pyrenoid and on the plasma membrane. The location of ubiquitin within chloroplasts has not been observed previously. In immunoblots of whole cell extracts with an antibody to ubiquitin a prominent conjugate band with an apparent molecular mass of 29 kDa and a broad region of high-molecular-mass conjugates (apparent moleculear mass > 45 kDa) were observed. Exposure of cells to a 41.5.degree.C heat shock in both the dark and light caused the disappearance of the 29-kDa conjugate and an increase in the high-molecular-mass conjugates. After step down to 25.degree.C the 29-kDa conjugate reappeared while the levels of high-molecular-mass conjugates decreased. In light, the recovery of the 29-kDa band was more rapid than in the dark. Photoinhibition alters the ubiquitin conjugation pattern similarly to heat shock, but to a lesser degree. These observations imply than, in Chlamydomonas, ubiquitin has a role in the chloroplast and in the response to heat and light stress.