The identification of a minimal dimerization motif QXXS that enables homo- and hetero-association of transmembrane helices in vivo

Assembly of transmembrane (TM) domains is a critical step in the function of membrane proteins, and therefore, determining the amino acid motifs that mediate this process is important. Studies along this line have shown that the GXXXG motif is involved in TM assembly. In this study we characterized the minimal dimerization motif in the bacterial Tar-1 homodimer TM domain, which does not contain a GXXXG sequence. We found that a short polar motif QXXS is sufficient to induce stable TM-TM interactions. Statistical analysis revealed that this motif appears to be significantly over-represented in a bacterial TM data base compared with its theoretical expectancy, suggesting a general role for this motif in TM assembly. A truncated short TM peptide (9 residues) that contains the QXXS motif interacted slightly with the wild-type Tar-1. However, the same short TM peptide regained wild-type-like activity when conjugated to an octanoyl aliphatic moiety. Biophysical studies indicated that this modification compensated for the missing hydrophobicity, stabilized alpha-helical structure, and enabled insertion of the peptide into the membrane core. These findings serve as direct evidence that even a short peptide containing a minimal recognition motif is sufficient to inhibit the proper assembly of TM domains. Interestingly, electron microscopy revealed that above the critical micellar concentration, the TM lipopeptide forms a network of nanofibers, which can serve for the slow release of the active lipopeptide.