Annulate lamellae - an underexplored cellular structure

Annulate lamellae (AL) are endoplasmic reticulum (ER) subdomains harbouring a subset of nucleoporins (Nups), the proteins that assemble into the nuclear pore complexes (NPCs) on the nuclear envelope (NE). AL have been observed in a variety of cell types, including oocytes, spermatids, embryonic cells, somatic cells and tumour cells, as well as in multiple cell lines. Some studies propose that AL derive from the NE, whereas studies in Drosophila egg chambers indicate that AL can assemble through differential condensation of soluble Nups, thus implying that modes of AL assembly can vary depending on the cell type and cell physiology. Although little is known about the functions of AL, they have conventionally been implicated in NPC assembly and NE homeostasis. However, emerging evidence suggests additional roles for AL in the regulation of nucleocytoplasmic transport (NCT) and mRNA translation. Additionally, AL might regulate cytosolic processes such as ER-mitochondrial connectivity, ER Ca2+ release, and activation of specific proteins. AL remodelling is also associated with development, disease and infection, further emphasising a key role for AL in a variety of cellular processes and contexts.