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, calci um-binding proteins that have been reported variously as binding exclusivel y to lipid bodies or binding to both lipid bodies and the endoplasmic retic ulum (ER). Separations on high-resolution tricine/SDS-PAGE gels show that i n developing Brassica napus embryos there are two major isoforms of caleosi n with respective relative masses of 25 and 27 kDa. Differential and sucros e density gradient centrifugation show that the 25-kDa isoform is exclusive ly localised on lipid bodies while the 27-kDa isoform is ER-bound. Proteina se K digestions indicate that the 25-kDa caleosin isoform is deeply embedde d in the lipid-body core with only about I kDa of the N-terminal region acc essible to proteolytic attack. The 25-kDa lipid-body isoform is only found in tissues, such as developing embryos and germinating cotyledons, that con tain storage lipid bodies. In contrast, the 27-kDa ER-bound isoform is pres ent in a variety of tissues including roots, stems and leaves. Genomic anal ysis of Arabidopsis thaliana, a closely related species, shows seven caleos in 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 the ir possible role in membrane/lipid trafficking. (C) 2001 Editions scientifi ques et medicales Elsevier SAS.