Experimental evidence that the membrane-spanning helix of PufX adopts a bent conformation that facilitates dimerisation of the Rhodobacter sphaeroides RC–LH1 complex through N-terminal interactions

The PufX polypeptide is an integral component of some photosynthetic bacterial reaction center-light harvesting 1 (RC–LH1) core complexes. Many aspects of the structure of PufX are unresolved, including the conformation of its long membrane-spanning helix and whether C-terminal processing occurs. In the present report, NMR data recorded on the Rhodobacter sphaeroides PufX in a detergent micelle confirmed previous conclusions derived from equivalent data obtained in organic solvent, that the α-helix of PufX adopts a bent conformation that would allow the entire helix to reside in the membrane interior or at its surface. In support of this, it was found through the use of site-directed mutagenesis that increasing the size of a conserved glycine on the inside of the bend in the helix was not tolerated. Possible consequences of this bent helical structure were explored using a series of N-terminal deletions. The N-terminal sequence ADKTIFNDHLN on the cytoplasmic face of the membrane was found to be critical for the formation of dimers of the RC–LH1 complex. It was further shown that the C-terminus of PufX is processed at an early stage in the development of the photosynthetic membrane. A model in which two bent PufX polypeptides stabilise a dimeric RC–LH1 complex is presented, and it is proposed that the N-terminus of PufX from one half of the dimer engages in electrostatic interactions with charged residues on the cytoplasmic surface of the LH1α and β polypeptides on the other half of the dimer.

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► NMR on the Rba. sphaeroides PufX polypeptide in a micelle reveals a bent structure.
► The bend allows the entire PufX helix to reside in the membrane or at its surface.
► Gly29 on the inside of the bend is sufficiently small to allow the bend to form.
► The N-terminal sequence ADKTIFNDHLN is critical for the formation of RC–LH1 dimers.
► The C-terminus of PufX is processed at an early stage in membrane development.