Nuclear pore transport puzzle solved by super resolution microscopy

a – c Transport routes of LBR with N-terminal tag (LBR-N), LBR with C-terminal tag (LBR-C) and Lap2β (Lap2β-N) with N tag respectively. (i) Schematic cartoon representation of each INM protein (N-terminal orientation to the left and C-terminal orientation to the right). The represented membrane represents a single NE bilayer. (ii) 2D (projected) spatial locations of each INM protein candidate. In the X dimension of the Cartesian graph, 0 represents the axial center of the APN, and in the Y dimension, +20 represents the ONM and −20 represents the INM. (iii) A normalized 2D probability density map showing the regions where the projected single molecule locations had the highest and lowest density. (iv) 3D probability density map generated using the 2D to 3D52 transformation algorithm, with regions from highest to lowest density (each color bar shows the color change of the gradient from high to low density). d Side-by-side comparison of the 3D probability density map of the term Lap2β N and C with the term LBR N and C. The colored bars indicate the regions of highest and lowest density. Credit: Nature’s Communications (2020). DOI: 10.1038 / s41467-020-16033-x

A study has resolved a long-standing debate on the transport of essential proteins, implicated in many human diseases, through one of the cell’s most complex and sophisticated structures.

The results reveal that most nuclear envelope transmembrane (NET) proteins travel to the cell nucleus through small tunnels, known as peripheral channels, the existence of which has been debated by scientists.

Errors in the location or function of NET proteins are linked to many human diseases such as muscular dystrophies, cardiomyopathy, blood and bone disorders, and cancers.

Most soluble proteins travel through nuclear pore complexes (NPCs), vast tunnel-like structures that span the nuclear envelope, the protective membrane that surrounds the nucleus, the cell’s command center.

“NPCs are one of the most important molecular machines in cells, they control the transport of proteins and RNA inside and outside the nucleus, but their size and complexity has led to the emergence of many different theories of NET protein transport.” says one lead author of the study, Professor Eric Schirmer of the University of Edinburgh.

The team, including researchers from the University of Edinburgh and Temple University (Philadelphia, USA), used a new type of super-resolution microscopy, called single-point edge excitation subdifraction (SPEED) that tracks the movement of individual protein molecules in living cells.

The study is the first to find conclusive functional evidence for the existence and importance of smaller peripheral channels, at the edge of the central NPC channels, for the transport of NET proteins.

The discovery could explain why some mutated NET proteins end up in the wrong location and cause disease, which could pave the way for new treatment strategies.

NET proteins help to ensure that the genetic material of the cell, which is inside the nucleus, is properly organized, read and used. To accomplish this, they must travel to the correct location inside the nuclear envelope.

The team found that blocking peripheral tunnels halted NET transport, despite the fact that the larger nuclear pore complexes were still working.

Blocking the APN tunnels did not interrupt transport, although the team found that 10% of NET proteins are placed in both tunnels, using them simultaneously for faster and more efficient transport.

“Nobel laureate Gunter Blobel was a visionary in suggesting that NPCs (huge tunnel complexes containing hundreds of structural proteins) were very plastic and dynamic.” Schirmer says. “This is underlined in this study where simultaneous use of NET from central and peripheral channels could only occur if NETs effectively cut this dynamic protein complex.”

The results also suggest that peripheral channels are an important backup mechanism, for example, when virus infection blocks larger NPC tunnels.

The study, published in the journal. Nature’s Communications, was funded by the US National Institutes of Health and Welcome. USA