Highway to the inner nuclear membrane: rules for the road

Transport of soluble proteins into the nucleus depends either on binding a protein-transport complex or on being small enough to diffuse in. Recent studies indicate that the delivery of integral membrane proteins into the inner nuclear membrane is governed by the same rules. To enter the nucleus a protein must be chaperoned by a transport factor through the nuclear pore complex or it must be small enough to pass through by diffusion. Although these principles have long described the nuclear import of soluble proteins, recent evidence indicates that they also apply to the import of integral inner nuclear membrane proteins. Here we develop a set of rules that might govern the transport of proteins to the inner nuclear membrane.