SPARC (secreted protein, acidic, rich in cysteine, also called BM40 and osteonectin) is a multifunctional calcium-binding glycoprotein whose modular organization has been highly conserved between invertebrates and vertebrates, indicating a conservation of function during metazoan evolution. Genome analysis has revealed a single copy of the DrosophilaSPARC ( dSPARC) gene. As a first step towards investigating the function of SPARC in Drosophila, we examined its spatiotemporal distribution during development. During embryogenesis, dSPARC mRNA transcripts are restricted to mesoderm derivatives, hemocytes, and the fat body. Immunostaining with anti- DrosophilaSPARC antibodies indicates that dSPARC secreted by the hemocytes and fat body cells is concentrated in basal laminae surrounding internal organs. During oogenesis, dSPARC transcripts are restricted to the somatic cells of the germarium and follicles. Consistent with embryonic development, the resultant protein is concentrated in basal laminae. Mutations in type IV collagen are associated with a dramatic decrease in dSPARC protein immunostaining in hemocytes. The data suggest that the production and assembly of dSPARC in the basal lamina is dependent on type IV collagen, and raise the possibility that dSPARC and type IV collagen interactions are a prerequisite to the assembly and structural integrity of basal laminae in Drosophila.

Opening of the nucleosome structure is essential for accessing genomic DNA. To study the mechanism of this process, we monitor the distance between various fluorescently labeled positions on mononucleosomes by single-molecule F€ orster resonance energy transfer (FRET). Here, we compare nucleosomes reconstituted from recombinant mouse, Xenopus, and yeast histones. As DNA sequences we compared, the effect of 5S rDNA, MMTV-B sequence, and Widom 601 DNA. The stability, as measured by the salt concentration at the opening transition midpoint, is lowest for yeast, followed by Xenopus and mouse. The 601 DNA sequence builds much more stable nucleosomes and the distribution of FRET efficiencies is narrower than for those reconstituted on 5S rDNA or MMTV-B sequences. The opening pathway through an intermediate state, as found for Xenopus histones, could be verified for the mouse and yeast systems and for the different DNA sequences, suggesting a general mechanism for accessing nucleosomal DNA.