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A8. Protocells

At some point, early genetic material must have been encapsulated in a membranous vesicles.  Would new properties emerge from this mixture that might have a competitive (evolutionary) advantage over either component alone, and thus be a step on the way to the formation of a "living" cell?  The answer appears to be yes. Chen et al. have incorporated RNA into fatty acid vesicles with interest effects.  They asked the question as whether those vesicles could grow at the expense of vesicles without encapsulated RNA.  RNA, with a high charge density and its associated counter ions would create osmotic stress on the vesicles membranes.  To relieve that stress they could acquire fatty acids from other fatty acid vesicles (or fatty acid micelles), increasing their surface area, and concomitantly reducing tension in the membrane.   

Oleic acids vesicles were first placed under stress by encapsulating 1 M sucrose in the vesicle and then diluting it in hypotonic media.  Water would enter and swell the vesicle (but without bursting and resealing, as evident from control experiments).   Then they prepared stressed and unstressed oleic acid vesicles in the presence of two nonpolar flurophores, NBD-PE (excitation at 430 nm, emission at 530 nm) and Rh-DHPE (emission at 586 nm).  These fluorophores were chosen for fluorescence resonance energy transfer measurements.  If the membrane vesicles changed size, the FRET signal would change, based on the relative concentration and proximity of the dual fluorophores.  If the separation between probe molecules increased, the FRET signal would decrease.  Conversely, if the vesicle shrunk, the FRET signal would increase.  

The results showing the effect of adding unlabeled swollen vesicles to labeled normal vesicles, and labeled swollen vesicles to unlabeled normal are shown below.  The surface area of normal labeled vesicles decreased by about 25% when unlabeled swollen vesicles were added, but not when unlabeled normal vesicles were added. Labeled swollen vesicles increased 25% in size only if mixed with unlabeled normal vesicles, not with unlabeled swollen vesicles.  Hence swollen vesicles win the competition and "steal" lipid from normal vesicles. 

Now what about vesicles swollen with encapsulated RNA?  RNA, with its associated charge and charged counter ions also placed an osmotic stress on the vesicles.  FRET labels (the two fluorophores) were place in vesicles without RNA.  Fatty acids were removed from isotonic labeled vesicles in the presence of unlabeled tRNA swollen vesicles (left panel below).  Labeled vesicles swollen with glycerol took fatty acids from unswollen vesicles (without tRNA), but not from tRNA swollen vesicles, as both were swollen so no net drive to reduce swelling by lipid exchange was present.  

These results show the vesicles with encapsulated RNA have a competitive (evolutionary) advantage over normal  vesicles.   This data suggests that having a polyanion as the source of genetic material is actually advantageous  to the protocell.  In addition the move in modern membranes to phospholipids with esterified fatty acids (instead of free ones) may actually have stabilized membranes, given the movement of free fatty acids to different membranes.

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