Figure 4

Evolutionary differentiation of endomembranes. (a). Schematic tree for controlling small GTPases [124, 128]. Sar-1 and Arf-1 have an extra, derived insertion, so the root cannot be in that branch. Because of disparate rates of evolution among paralogues and the shortness of the molecules it is unclear from trees whether the seven eukaryotic clades (lower) are all mutually related as shown and which of the four bacterial clades (upper) are their closest relatives. (b). After endomembranes, peroxisomes, and plasma membrane became distinct genetic membranes (Fig. 3b) most secretory ribosomes were on old DNA-bearing cisternae; the first COP/adaptin coats generated vesicles (V) from the protoendomembrane/phagosome; early SNAREs (SN, left) fused them with the plasma membrane. Endomembrane differentiation improved digestion by targeting digestive enzymes specifically to phagosomes, mediated by successive concerted duplications and divergence of coat proteins, cognate SNAREs able to bind to them, and associated small GTPases. Primary specialisation between digestion and synthesis involved clathrin vesicles (CL) associated with plasma membrane SNAREs (SNP) and COPII vesicles associated with endomembrane SNAREs (SNE). Interpolation of Golgi, by mutual fusion of uncoated COPII vesicles, stabilised by COPI-mediated recycling (right), allowed specialisation between lysosomes and surface growth. For a fuller discussion of endomembrane origins see [91].