Hydrophobic pockets at the membrane interface: An original mechanism for membrane protein interactions

The effect of partial digestion by trypsin and GluC protease on the association of the membrane polypeptides of LH1 from Rhodospirillum (Rsp.) rubrum was studied. Trypsin and GluC protease treatments of LH1 result in the cleavage of the first three amino acids from the a polypeptide and of the first 18 amino acids from the beta polypeptide, respectively, without any noticeable reorganization of their secondary structure, as measured by attenuated total reflectance Fourier transform IR spectroscopy. However, the enthalpy variation accompanying dimer formation was dramatically reduced by the protease attacks by as much as 80%. Our results show that the alphabeta heterodimer is mainly stabilized by hydrophobic interactions which involve the amino-terminal extensions of the participating polypeptides. Using the close homology between the polypeptides of Rsp. rubrum LH1 and that of Rsp. molischianum LH2, whose structure is known, a structural model for these "hydrophobic pockets" lying close to the membrane interface is proposed.