Reticulon-like REEP4 at the inner nuclear membrane promotes nuclear pore complex formation

Abstract

Nuclear pore complexes (NPCs) are channels within the nuclear envelope that mediate nucleocytoplasmic transport. NPCs form within the closed nuclear envelope during interphase or assemble concomitantly with nuclear envelope reformation in late stages of mitosis. Both interphase and mitotic NPC biogenesis require coordination of protein complex assembly and membrane deformation. During early stages of mitotic NPC assembly, a seed for new NPCs is established on chromatin, yet the factors connecting the NPC seed to the membrane of the forming nuclear envelope are unknown. Here, we report that the reticulon homology domain protein REEP4 not only localizes to high-curvature membrane of the cytoplasmic endoplasmic reticulum but is also recruited to the inner nuclear membrane by the NPC biogenesis factor ELYS. This ELYS-recruited pool of REEP4 promotes NPC assembly and appears to be particularly important for NPC formation during mitosis. These findings suggest a role for REEP4 in coordinating nuclear envelope reformation with mitotic NPC biogenesis.

Dunking into the Lipid Bilayer: How Direct Membrane Binding of Nucleoporins Can Contribute to Nuclear Pore Complex Structure and Assembly

“…In addition to nucleoporins, other membrane and membrane associated proteins affecting membrane curvature contribute to NPC assembly even if they are not part of the final NPC structure. Wedge-shaped membrane proteins important for the highly curved membrane structure of the tubular ER such as reticulons and REEP4 are involved in interphase NPC assembly [ 114 , 115 ]. In S. cerevisiae , the nuclear envelope transmembrane protein APQ12 is suggested to support NPC biogenesis by promoting phosphatidic acid accumulation with the help of an amphipathic helix, which would induce membrane curvature for the assembly process [ 116 ].…”

Puzzling out nuclear pore complex assembly

“…While NPC insertion within the NE relies on a limited set of integral membrane Nups (Box 1), which diversely contribute to Nups recruitment and NPC assembly in vivo [146,164,171,[189][190][191][192], other factors have been proposed to induce or stabilize the high level of local membrane curvature associated with pore conduits. Members of the reticulon homology domain (RHD) family, which encompasses membrane-bending proteins involved in shaping the curvature of ER cisternae and tubules [193], were found to localize to the INM or at the vicinity of NPCs, from yeasts to vertebrates [194][195][196]. Loss-of-function experiments further revealed that distinct RHD proteins contribute to interphase and postmitotic assembly [194,196,197], their recruitment possibly involving direct interactions with nucleoporins [189,196].…”

The Nuclear Pore Complex: Birth, Life, and Death of a Cellular Behemoth

“…In addition, interphase and mitotic assembly modes diverge significantly in their functional requirements. Assembly during interphase in vertebrates specifically depends on the nuclear basket NUP hsNup153 and the transmembrane NUP hsPom121, while the chromatin binding NUP ELYS and the reticulon-like protein REEP4 are important for assembly at the end of mitosis [70,111,112]. Moreover, the recruitment order of NUP subcomplexes during inside-out assembly may differ between lower and higher eukaryotes: as judged by metabolic labeling analysis, in yeast, it begins with the symmetrical core NUPs and ends with the late recruitment of the nuclear basket NUPs Mlp1/2 [102].…”

“…Furthermore, differences in assembly order and mechanistic requirements may be governed by the relation of NUPs to chromatin and membranes. While membrane and nuclear subunits take center stage in inside-out assembly [70,111,113,114], the Y complex, with its interactions to both chromatin and membranes, is a key player in the reformation of NPCs and nuclear membranes in open mitosis [52,112,115].…”

“…Both in higher and lower eukaryotes, AHs of core and linker NUPs important for NPC biogenesis have been shown to sense concave membrane curvature (reviewed in [94]) and could potentially shape the concave membrane around the NPC assembly site. In addition, reticulons and reticulon-like proteins, which are wedge-shaped membrane-curving proteins, could contribute as well [112,176,177]. Different mechanisms may generate convex curvature.…”