The Brassica napus calcium-binding protein, caleosin, has distinct endoplasmic reticulum- and lipid body-associated isoforms

Caleosins are a recently described class of plant and fungal EF-hand, calcium-binding proteins that have been reported variously as binding exclusively to lipid bodies or binding to both lipid bodies and the endoplasmic reticulum (ER). Separations on high-resolution tricine/SDS-PAGE gels show that in developing Brassica napus embryos there are two major isoforms of caleosin with respective relative masses of 25 and 27 kDa. Differential and sucrose density gradient centrifugation show that the 25-kDa isoform is exclusively localised on lipid bodies while the 27-kDa isoform is ER-bound. Proteinase K digestions indicate that the 25-kDa caleosin isoform is deeply embedded in the lipid-body core with only about 1 kDa of the N-terminal region accessible to proteolytic attack. The 25-kDa lipid-body isoform is only found in tissues, such as developing embryos and germinating cotyledons, that contain storage lipid bodies. In contrast, the 27-kDa ER-bound isoform is present in a variety of tissues including roots, stems and leaves. Genomic analysis of Arabidopsis thaliana, a closely related species, shows seven caleosin genes, two of which correspond respectively to the highly expressed 25- and 27-kDa Brassica proteins. The relative abundance and dual localisation of caleosins in the ER and lipid bodies are discussed in the context of their possible role in membrane/lipid trafficking.