The making and breaking of the endoplasmic reticulum.

The endoplasmic reticulum (ER) is a dynamic organelle central to many essential cellular functions. It is an important calcium store, which functions in cellular signal transduction cascades. It is also the site of entry for secreted proteins into the secretory pathway. Lumenal enzymes will fold and glycosylate these proteins, and if a protein is destined to be secreted, it will be packaged into membrane vesicles that bud off from the ER. The ER is also the site where most cellular lipids are synthesized. It is contiguous with the nuclear envelope, which serves as a diffusion barrier to control entry into and out of the nucleus. In the life cycle of a cell, the ER is in a constant flux of membrane traffic. What maintains the ER in the shape of an intact reticulum among this constant flux of material? We discuss the mechanisms that contribute to the biogenesis of the ER, the maintenance of the organelle, as well as processes that give the ER its characteristic shape and pattern of inheritance.