Structural Biochemistry/Membrane Proteins/The Evolution of Membrane Proteins

=The Evolution of Membrane Proteins= The evolution of membrane proteins and the structure of membranes remain unclear despite the advancement of our understanding of molecular membrane mechanisms and the complex topology of the membrane. One model of understanding the origins of the membrane and membrane proteins is that the biological membrane was a co-evolution of three things: lipid bilayers, membrane proteins, and membrane bioenergetics.(Mulkidjanian, Galperin, Koonin)

The goal of this model is to provide an insight into what features would the "LUCA"(last universal common ancestor) have. The premise for this model, relies on the idea that the division between archaea and bacteria was the first among cellular organisms. Using this then one can come to compare these two organism's components and try to deduce common features present in their ancestor.

Possible Molecules for the Formation of the First Membranes
There are two proposed arguments for the nature of the "LUCA" membrane. One is that fatty acids are molecules that are simple and able to from abiogenetically to be used by primordial organisms as their membranes. The other argument is that polyprenyl phosphates are a viable molecule for the first membrane due to the fact that they have the ability to form vesicles in the presence of sodium. This would then be a primitive mechanism for cation transport.

F-Type and V-Type membrane ATPases
Both F-type and V-type ATPases thought to be present(or some form of these proteins) in the LUCA for the reason that these are common in all of modern cellular life. Both F and V-type ATPases use energy from ATP hydrolysis to transfer cations across membranes. The however do require impermeable membranes to establish ion gradients. It is postulated that sodium gradients were used in primordial membranes for the production of ATP. This is deduced from the observation that F- and V-type ATPases posses the ability to use both proton-motive force(PMF) and/or sodium-motive force(SMF) for the synthesis of ATP. However, only sodium ATPases can translocate protons and sodium but proton ATPases can only translocate protons. This is due to the fact that the binding site for sodium requires six ligands where a proton onl requires one ionisable group. This signifies that proton ATPases were evolved from sodium ATPases and therefore the common ancestor of F- and V-type ATPases had a sodium binding site. Therefore it is deduced that utilization of sodium gradients was a means for the LUCA to synthesize ATP. Along with the common sodium binding site is the structural similarity among both F- and V-ATPases in where amino acid sets are almost identical. However these ATPases' subunits of their central stalk are not close to being homologous but the catalytic hexamer are homologous to hexameric helicases. Hence, the possible ancestor would enable passive transfer of biomaterial across the cell. This with a ATP-driven helicase could potentially give rise to a protein translocase(membrane pore).

Integrating a Protein into a Membrane
One probable theory of proteins inserting into a lipid membrane is using an amphiphilic alpha-helix which dimerizes at the surface of the membrane and the oligomerizes using other alpha-helices and subsequently making a pore. This idea stems from the fact that there is spontaneous insertion of alpha-helices into a lipid-bilayer. The weakness in this theory lies in the fact that alpha-helices would stabilize onto the surface of the membrane by spreading out on the surface and not producing a pore but instead something resembling an F-type ATPase.

Primordial Membrane and Membrane bioenergetics
Using the understanding the charge inside a cell should be negative, the porous primeval membranes would be sufficient to keep charge separation intact. This could also lead to voltage dependent membrane proteins. The cell through its historically low levels of sodium inside the cell due to a compliance to keep sodium levels similar to the start of life and through a changing environment, where the ocean becomes really sodium concentrated, the cell develops mechanism to make energy from the sodium concentrations. This in essence leads to an adoption of a coupled membrane ion translocation. This mechanism would not arise until ocean salinity would rise and as a consequence would lead to sodium-tight membranes. The development of a proton-tight membrane is more complicated and although there are theories regarding this phenomena nothing is conclusive.