Identification of Helical Packing Motifs Common to Bacteriorhodopsin and G Protein-Coupled Receptors

A sequence analysis and comparison of transmembrane helices in bacteriorhodopsin (BR) and G protein-coupled receptors (GPCRs) is presented to identify potential regions of homology across protein families. The results show a common pattern of residues is conserved within the interhelical contact regions of BR that fit a knob-into-hole structural motif previously postulated for globular proteins and photosynthetic reaction centers. Based on an alignment of conserved prolines in transmembrane helices, it is inferred that analogous helix packing arrangements are possible in the rhodopsin-like GPCRs. Molecular models of GPCR helices V and VI indicate these interactions occur between aromatic and hydrophobic residues flanking the highly conserved prolines in these sequences. A similar packing arrangement is shown to occur in the X-ray structure of the melittin which also displays a unique pairing of proline-linked helices. The contact pattern identified is further applied to predict the packing of pairs of proline-containing helices in the pheromone-like and cAMP GPCRs. A potential role in stabilizing structure formation is also suggested for the contacts. The results and conclusions are supported by recent biophysical studies of zinc binding to kappa-opioid receptor mutants.