Putative interhelical interactions within the PheP protein revealed by second-site suppressor analysis

Highly conserved glycine residues within span I and span II of the phenylalanine and tyrosine transporter PheP were shown to be important for the function of the wild-type protein. Replacement by amino acids with increasing side chain volume led to progressive loss of transport activity. Second-site suppression studies performed with a number of the primary mutants revealed a tight packing arrangement between spans I and II that is important for function and an additional interaction between spans I and III. We also postulate that a third motif, GXXIG, present in span I and highly conserved within different members of the amino acid-polyamine-organocation family, may function as a dimerization motif. Surprisingly, other highly conserved residues, such as Y60 and L41, could be replaced by various residues with no apparent loss of activity.