9.B.115 The Steroid 5α-reductase/Lamin B Receptor (LBR) Family

The proteins of the DUF1295 Superfamily, present in bacteria and eukaryotes, appear to be of unknown function. However, several entries in NCBI are annotated as various enzymes. The most common annotation names several of these homologues as steroid 5α-reductases. These homologs are integral membrane proteins, most with 8 - 10 TMSs.

The lamin B receptor of fruit flies anchors the lamina and the heterochromatin to the inner nuclear membrane. It has the following properties in common with the vertebrate LBR. First, structural predictions indicate that the 741 amino acid dLBR protein possesses a highly charged N-terminal domain of 307 amino acids followed by eight (to ten) transmembrane segments in the C-terminal domain of the molecule. Second, the dLBR is an integral membrane protein of the inner nuclear membrane. Third, dLBR binds to the Drosophila B-type lamin Dm0. Fourth, the N-terminal domain 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. Thus, during evolution, the enzymatic activity of this insect protein had been lost. Deletion gave rise to 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. (Wagner et al. 2004). One such homolog is found under TC# 1.I.1.1.5 as part of the nuclear pore complex.