The danger of "multi-tasking": LBR out of control.

The nuclear envelope (NE) is a barrier that separates nuclear from cytoplasmic processes. It is composed of an inner and outer nuclear membrane (INM, ONM), separated by the perinuclear space (PNS). The ONM is contiguous with the endoplasmic reticulum (ER), and thus, the lumen of the NE and that of the ER constitute one compartment. The lamin B receptor (LBR) is a NE protein that has a central structural role as a linker of the INM, the lamina and chromatin, and a less well characterized functional role as a sterol reductase. In a recent study, we reported that the forced expression of mutant variants of LBR in some cell types induces a separation of the INM from the outer nuclear envelope concomitantly with a separation of ER membranes, whereas in other cells no separation is observed. In this extra view, we speculate about the mechanism that leads to this fundamental disruption of NE and ER structure. Our observations furthermore raise the question to what extent LBR contributes to the establishment or maintenance of the ER and PNS luminal compartment, and how a single mutant protein can so drastically interfere with its regular organization.