![]() At the same time, the spaces between blebs would have enabled the gradual maturation of proteins secreted into the environment via the perinuclear space through glycosylation and proteolytic cleavage. (E) With the expansion of blebs to enclose the proto-mitochondria, a process that would have facilitated the acquisition of bacterial lipid biosynthesis machinery by the host, the site of cell growth would have progressively shifted to the cytoplasm, facilitated by the development of regulated traffic through the nuclear pore. (D) Blebs would have then been further stabilized by the development of a symmetric nuclear pore outer ring complex () and through the establishment of LINC complexes that, following the gradual loss of the S-layer, physically connected the original cell body (the nascent nuclear compartment) to the inner bleb membranes. (C) Selection for a greater area of contact between the symbionts would have led to bleb enlargement and the eventual loss of the S-layer from the protrusions. These protrusions facilitated material exchange with proto-mitochondria. (B) We envision the eocyte cell forming protrusions, aided by protein-membrane interactions at the protrusion neck. Model showing the stepwise evolution of eukaryotic cell organization from (A) an eocyte ancestor with a single bounding membrane and a glycoprotein rich cell wall (S-layer) interacting with epibiotic α-proteobacteria (proto-mitochondria). Inside-out model for the evolution of eukaryotic cell organization. It also helps to explain a number of previously enigmatic features of cell biology, including the autonomy of nuclei in syncytia and the subcellular localization of protein N-glycosylation, and makes many predictions, including a novel mechanism of interphase nuclear pore insertion. The inside-out theory is consistent with diverse kinds of data and provides an alternative framework by which to explore and understand the dynamic organization of modern eukaryotic cells. Further bleb-fusion steps yielded a continuous plasma membrane, which served to isolate the endoplasmic reticulum from the environment. The cytoplasm was then formed through the expansion of blebs around proto-mitochondria, with continuous spaces between the blebs giving rise to the endoplasmic reticulum, which later evolved into the eukaryotic secretory system. These blebs functioned to facilitate material exchange with ectosymbiotic proto-mitochondria. We propose that an ancestral prokaryotic cell, homologous to the modern-day nucleus, extruded membrane-bound blebs beyond its cell wall. Models have always assumed that the nucleus and endomembrane system evolved within the cytoplasm of a prokaryotic cell.ĭrawing on diverse aspects of cell biology and phylogenetic data, we invert the traditional interpretation of eukaryotic cell evolution. Although the origin of the eukaryotic cell has long been recognized as the single most profound change in cellular organization during the evolution of life on earth, this transition remains poorly understood. ![]()
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