3D3/lyric: a novel transmembrane protein of the endoplasmic reticulum and nuclear envelope, which is also present in the nucleolus.

We previously performed a gene-trap screen in mouse cells with particular focus on clones in which the trapped protein-reporter fusions localise to compartments of the nucleus. Here we describe one such gene-trap line in which the fusion protein showed a unique, patchy distribution at the nuclear periphery. We have cloned the endogenous mouse and human cDNAs encoding the protein trapped in the F9/3D3 cell line. The predicted proteins (64 kDa) encoded by this novel gene are highly conserved and similar to an unpublished rat protein in sequence databases called p80 or lyric. The amino acid sequence of 3D3/lyric indicates that it may be a type-1b membrane protein with a single transmembrane domain (TMD). Antibodies against the endogenous protein recognise multiple isoforms, consistent with multiple 3D3/lyric mRNAs detected by Northern blot analysis. Subcellular fractionation and immunostaining show that 3D3/lyric is located not only principally in the endoplasmic reticulum (ER), but also in the nuclear envelope (NE), which is contiguous with this compartment. Furthermore, 3D3/lyric is also found in the nucleolus and is therefore a rare example of a protein that suggests a possible connection between this compartment and the endoplasmic reticulum.