Two distinct steroleosins are present in seed oil bodies

In addition to oleosin isoforms, three minor proteins, Sop1, 2 and 3 are present in sesame oil bodies. Genes encoding Sop1 and Sop2, named caleosin and steroleosin for their calcium and sterol-binding capacity, respectively, have been cloned recently. Blast sequence analysis of the first 32 N-terminal residues revealed that Sop3 was presumably a steroleosin-like protein homologous to Sop2. A putative cDNA clone of Sop3 was obtained by PCR, and subsequently confirmed by immunological recognition with antibodies against its over-expressed protein in Escherichia coli. Although Sop2 and Sop3, tentatively named steroleosin-A and -B, were found homologous, they could not be cross-recognized immunologically. Sequence comparison showed that these two steroleosins possessed a conserved NADP+ binding subdomain but a diverse sterol-binding subdomain of different size. Both steroleosins were progressively accumulated in maturing seeds but with different cumulating patterns. Dehydrogenase activity detected in their expressed proteins indicated that steroleosin-B might comparably possess a broader sterol selectivity and higher NADP+ specificity than steroleosin-A. Immunological cross-recognition implies that steroleosin-B is present in seed oil bodies of diverse species. A structural model of an oil-body was drawn with all its known essential constituents, and secondary structure organizations of the three classes of oil-body proteins were compared.