Comparative stability of Major Facilitator Superfamily transport proteins

Membrane transporters are a vital class of proteins for which there is little available structural and thermodynamic information. The Major Facilitator Superfamily (MFS) is a large group of transport proteins responsible for transporting a wide range of substrates in eukaryotes and prokaryotes. We have used far-UV circular dichroism (CD) to assess whether transporters from this superfamily have the same chemical and thermal stability. We have compared the stability of five different MFS transporters; PepT_So from Shewanella oneidensis and LacY, GalP, GlpT and XylE from Escherichia coli, as well as a known stable mutant of LacY, LacY-C154G. CD stability measurements revealed that these transporters fall into two broad categories. The 'urea-sensitive' category includes LacY-WT, GalP and GlpT, which each lose around a third of their secondary structure in 8 M urea and two-thirds in the harsher denaturant guanidine hydrochloride (GuHCl). The 'urea-resistant' category includes LacY-C154G, XylE and PepT_So. These resistant transporters lose very little secondary structure in 8 M urea, and LacY-C154G and PepT_So resist denaturation by GuHCl up to a concentration of 4 M. The stabilities of LacY, GlpT, XylE and PepT_So correlated with their crystal structure conformations, implying that a similar conformation is adopted in vitro. The 'urea-sensitive' transporters LacY and GlpT were crystallised inward-open states, while XylE and PepT_So were crystallised in occluded states. This study highlights the importance of studying a wide range of similar proteins, as a similar secondary structure and overall function does not necessarily confer the same stability in vitro.