Sorting of Nuclear Envelope - Targeted Proteins at the ER Membrane
(Dr. Sharon Braunagel and Suraj Saksena)



Current Research Projects:

Suraj Saksena's research: The current dogma for protein trafficking to the Inner Nuclear Membrane (INM) states that the driving force for protein movement to the INM is simple lateral diffusion from the ER to the INM.  In the absence of regulatory protein-protein interactions, this conceptually simplistic model fails to explain how cells sort proteins destined for the INM away from resident ER or plasma membrane proteins, all of which integrate into the membrane of the ER.   In an attempt to identify any potential regulatory and/or sorting signals involved in the trafficking of proteins to the INM, several baculovirus envelope proteins (e.g., ODV-E66, ODV-E25) were examined to ascertain the molecular basis for the targeting and integration of viral envelope proteins into the ER membrane and their interaction with translocon components during integration.  The data shows that ODV-E66 targets to the ER membrane in a signal recognition particle-dependent manner and integrates into the bilayer through the translocon, as does ODV-E25.  When compared to two host INM proteins, Nurim and LBR, photocrosslinking experiments revealed that the viral and host INM proteins interact with the translocon in the same way.  In fact, we have identified a translocon photocrosslinking pattern that is unique for proteins that traffic to the INM, which suggests that the translocon is involved in the recognition and sorting of these substrates.  While the current dogma for protein trafficking to the INM rules out the existence of protein-protein interactions, chemical crosslinking studies with ER membranes isolated from virus-infected host cells have shown that following integration, ODV-E66 associates with two viral proteins FP25K and E26, required for transport into the INM.  Thus, in contrast to current dogma, co-translational integration and sorting of the viral envelope protein ODV-E66 at the ER membrane is a protein-facilitated and protein–regulated multistep process.