BURP domain proteins comprise a novel, strictly plant specific protein family. All these proteins exhibit a similar modular structure consisting of a signal peptide, a protein specific, more or less expanded, partially repeated middle part and a highly conserved Cterminal BURP domain with a characteristic cystein-histidin pattern. This study aims to the functional characterisation of the genes AtUSPL1 and AtRD22. The products of both genes co-localise with storage proteins in the protein storage vacuoles of cotyledons. The ectopic expression of AtUSPL1 leads to a shrunken seed phenotype and distortion in germination as well as to characteristic changes in the ultrastructure of lipid vesicles and protein bodies, which corresponds well with changes in storage fatty acid composition and the reduction of storage protein content. To study loss of function conditions, two T-DNA insertion alleles of the AtUSPL1 gene, a T-DNA-insertion line of the AtRD22 gene as well as a corresponding double mutant have been isolated. In contrast to the deleterious effects of AtUSPL1 overexpression, these gene destructions do not result in obvious seed phenotypes, although seeds show precocious germination and slight changes in seed protein content. To facilitate the cellular localisation of the AtUSPL1 and AtRD22 gene products, specific antibodies have been generated and GFP fusion constructs have been transformed into Arabidospsis. The AtUSPL1-GFP fusion protein was localised in storage protein vacuoles of seed cotyledon cells. The analysis of the GFP fusions revealed that both primary translation products become posttranslationally processed. Taken together, the results suggest a function of the AtUSPL1 and AtRD22 gene product and specifically of the BURP domain related to storage compound synthesis, transport and deposition.