The lamin B receptor of Drosophila melanogaster.

The lamin B receptor (LBR) is an integral membrane protein of the inner nuclear membrane that has so far been characterized only in vertebrates. Here, we describe the Drosophila melanogaster protein encoded by the annotated gene CG17952 that is the putative ortholog to the vertebrate LBR. The Drosophila lamin B receptor (dLBR) has the following properties in common with the vertebrate LBR. First, structure predictions indicate that the 741 amino acid dLBR protein possesses a highly charged N-terminal domain of 307 amino acids followed by eight transmembrane segments in the C-terminal domain of the molecule. Second, immunolocalization and cell fractionation reveal that the dLBR is an integral membrane protein of the inner nuclear membrane. Third, dLBR can be shown by co-immunoprecipitations and in vitro binding assays to bind to the Drosophila B-type lamin Dm0. Fourth, the N-terminal domain of dLBR is sufficient for in vitro binding to sperm chromatin and lamin Dm0. In contrast to the human LBR, dLBR does not possess sterol C14 reductase activity when it is expressed in the Saccharomyces cerevisiae erg24 mutant, which lacks sterol C14 reductase activity. Our data raise the possibility that, during evolution, the enzymatic activity of this insect protein had been lost. To determine whether the dLBR is an essential protein, we depleted it by RNA interference in Drosophila embryos and in cultured S2 and Kc167 cells. There is no obvious effect on the nuclear architecture or viability of treated cells and embryos, whereas the depletion of Drosophila lamin Dm0 in cultured cells and embryos caused morphological alterations of nuclei, nuclear fragility and the arrest of embryonic development. We conclude that dLBR is not a limiting component of the nuclear architecture in Drosophila cells during the first 2 days of development.