Structural Basis of Vesicle Formation at the Inner Nuclear Membrane

Summary

Vesicular nucleo-cytoplasmic transport is becoming recognized as a general cellular mechanism for translocation of large cargoes across the nuclear envelope. Cargo is recruited, enveloped at the inner nuclear membrane (INM), and delivered by membrane fusion at the outer nuclear membrane. To understand the structural underpinning for this trafficking, we investigated nuclear egress of progeny herpesvirus capsids where capsid envelopment is mediated by two viral proteins, forming the nuclear egress complex (NEC). Using a multi-modal imaging approach, we visualized the NEC in situ forming coated vesicles of defined size. Cellular electron cryo-tomography revealed a protein layer showing two distinct hexagonal lattices at its membrane-proximal and membrane-distant faces, respectively. NEC coat architecture was determined by combining this information with integrative modeling using small-angle X-ray scattering data. The molecular arrangement of the NEC establishes the basic mechanism for budding and scission of tailored vesicles at the INM.

Graphical Abstract

Highlights

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  Multimodal imaging reveals mechanism of vesicle formation at inner nuclear membrane

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  Nucleo-cytoplasmic cargo vesicle coat in situ comprises two distinct lattices

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  Lattices are formed by hexameric building blocks made of the nuclear egress complex

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  Induction of membrane curvature based solely on heterodimeric interactions

Abstract

A multi-modal live-cell and cryo-imaging approach reveals how vesicles assemble at the inner nuclear membrane for transport to the cytoplasm during herpesvirus maturation. This also suggests a functional model for counterparts in uninfected cells that mediate nuclear egress of large cargo like ribonucleoprotein particles.