Abstract
Oleosin, the most abundant protein in oil bodies of all examined angiosperm seeds, has been demonstrated to serve as a structural protein to maintain the integrity of these lipid storage organelles. Caleosin, a minor protein in oil bodies of angiosperm seeds, is assumed to anchor to the organelles in a manner similar to oleosin, i.e., via its central hydrophobic domain with a unique proline-knot motif presumably responsible for the targeting to nascent oil bodies. Recently, we found that stable oil bodies in cycad seeds were mainly sheltered by caleosin in the absence of oleosin. This finding suggests that caleosin is competent as the major structural protein, just like oleosin, on the surface of seed oil bodies. A search of literature shows that genes putatively encoding for caleosin-, but not oleosin-like proteins, are present in more primitive species, such as algae and fungi. We hypothesize that oleosin may have been derived from caleosin after a long term of divergent evolution.