Bacteria with a eukaryotic touch: A glimpse of ancient evolution?

Abstract

The discovery in 1991 by Fuerst and Webb that the chromosome of the bacterium Gemmata obscuriglobus (a member of Planctomycetales) is surrounded by a double membrane, mimicking a eukaryotic nucleus, challenged the traditional prokaryote/eukaryote classification of living organisms based on cell structure (1). Since then, such an unexpected observation has puzzled evolutionists, leading to different reactions. Most considered G. obscuriglobus a curiosity of the bacterial world. Until recently, this view has been supported by the absence of obvious eukaryotic protein signatures in the genomes of Planctomycetales and related bacteria (2). For a few others, the “nucleus” of G. obscuriglobus should, one way or another, testify as some sort of evolutionary link between Planctomycetes and modern eukaryotes (3). Two landmark papers now seem to strengthen this opinion. In PNAS, Fuerst and colleagues show that G. obscuriglobus can perform a function previously thought to be restricted to eukaryotes: endocytosis (4). Whereas in traditional bacteria, such as Escherichia coli, the capture of proteins involves their transport through the cell membrane via pore-like structures, the data obtained by Fuerst and colleagues strongly suggest that, as in eukaryotes, proteins can be internalized by G. obscuriglobus via the formation of endosome-like structures. They show that imported proteins (GFP, Ig, or streptavidin) accumulate and are later degraded in the ribosome-free region of the G. obscuriglobus cytoplasm (the so-called paryphoplasm). Ultracentrifugation of a cell extract shows that internalized proteins copurify with a cell fraction containing both membrane debris and vesicles. These vesicles are visible within the paryphoplasm by electron microscopy, and some of them appear to be formed by invagination of the cytoplasmic membrane. Immunogold labeling revealed the presence of internalized GFP lining these vesicles and membrane invagination, suggesting that vesicle formation is indeed linked to protein uptake. Although genetic experiments lack definitive proof (there are presently …