Traffic to the inner membrane of the nuclear envelope

Past research has yielded valuable insight into the mechanisms that regulate the nuclear transport of soluble molecules like transcription factors and mRNA. Much less is known about the mechanisms responsible for the transportation of membrane proteins to the inner membrane of the nuclear envelope. The key question is: does the facilitated transport of integral inner membrane proteins exist in the same way as it does for soluble proteins and, if so, what is it used for? Herein, we provide an overview of the current knowledge on traffic to the inner nuclear membrane, and make a case that: (a) known sorting signals and molecular mechanisms in membrane protein biogenesis, membrane protein traffic and nuclear transport are also relevant with respect to INM traffic; and (b) the interplay of the effects of these signals and molecular mechanisms is what determines the rates of traffic to the INM

Active Nuclear Import of Membrane Proteins Revisited

It is poorly understood how membrane proteins destined for the inner nuclear membrane pass the crowded environment of the Nuclear Pore Complex (NPC). For the Saccharomyces cerevisiae proteins Src1/Heh1 and Heh2, a transport mechanism was proposed where the transmembrane domains diffuse through the membrane while the extralumenal domains encoding a nuclear localization signal (NLS) and intrinsically disordered linker (L) are accompanied by transport factors and travel through the NPC. Here, we validate the proposed mechanism and explore and discuss alternative interpretations of the data. First, to disprove an interpretation where the membrane proteins become membrane embedded only after nuclear import, we present biochemical and localization data to support that the previously used, as well as newly designed reporter proteins are membrane-embedded irrespective of the presence of the sorting signals, the specific transmembrane domain (multipass or tail anchored), independent of GET, and also under conditions that the proteins are trapped in the NPC. Second, using the recently established size limit for passive diffusion of membrane proteins in yeast, and using an improved assay, we confirm active import of polytopic membrane protein with extralumenal soluble domains larger than those that can pass by diffusion on similar timescales. This reinforces that NLS-L dependent active transport is distinct from passive diffusion. Thirdly, we revisit the proposed route through the center of the NPC and conclude that the previously used trapping assay is, unfortunately, poorly suited to address the route through the NPC, and the route thus remains unresolved. Apart from the uncertainty about the route through the NPC, the data confirm active, transport factor dependent, nuclear transport of membrane-embedded mono- and polytopic membrane proteins in baker's yeast

Traffic to the inner membrane of the nuclear envelope

Past research has yielded valuable insight into the mechanisms that regulate the nuclear transport of soluble molecules like transcription factors and mRNA. Much less is known about the mechanisms responsible for the transportation of membrane proteins to the inner membrane of the nuclear envelope. The key question is: does the facilitated transport of integral inner membrane proteins exist in the same way as it does for soluble proteins and, if so, what is it used for? Herein, we provide an overview of the current knowledge on traffic to the inner nuclear membrane, and make a case that: (a) known sorting signals and molecular mechanisms in membrane protein biogenesis, membrane protein traffic and nuclear transport are also relevant with respect to INM traffic; and (b) the interplay of the effects of these signals and molecular mechanisms is what determines the rates of traffic to the INM

Long Unfolded Linkers Facilitate Membrane Protein Import Through the Nuclear Pore Complex

Active nuclear import of soluble cargo involves transport factors that shuttle cargo through the nuclear pore complex (NPC) by binding to phenylalanine-glycine (FG) domains. How nuclear membrane proteins cross through the NPC to reach the inner membrane is presently unclear. We found that at least a 120-residue-long intrinsically disordered linker was required for the import of membrane proteins carrying a nuclear localization signal for the transport factor karyopherin-α. We propose an import mechanism for membrane proteins in which an unfolded linker slices through the NPC scaffold to enable binding between the transport factor and the FG domains in the center of the NPC.