M13 major coat protein, a 50-amino-acid-long protein, was incorporated into DOPC/DOPG (80/20 molar ratio) unilamellar vesicles. Over 60% of all amino acid residues was replaced with cysteine residues, and the single cysteine mutants were labeled with the fluorescent label I-AEDANS. The coat protein has a single tryptophan residue that is used as a donor in fluorescence (or Förster) resonance energy transfer (FRET) experiments, using AEDANS-labeled cysteines as acceptors. Based on FRET-derived constraints, a straight alpha-helix is proposed as the membrane-bound conformation of the coat protein. Different models were tested to represent the molecular conformations of the donor and acceptor moieties. The best model was used to make a quantitative comparison of the FRET data to the structures of M13 coat protein and related coat proteins in the Protein Data Bank. This shows that the membrane-bound conformation of the coat protein is similar to the structure of the coat protein in the bacteriophage that was obtained from x-ray diffraction. Coat protein embedded in stacked, oriented bilayers and in micelles turns out to be strongly affected by the environmental stress of these membrane-mimicking environments. Our findings emphasize the need to study membrane proteins in a suitable environment, such as in fully hydrated unilamellar vesicles. Although larger proteins than M13 major coat protein may be able to handle environmental stress in a different way, any membrane protein with water exposed parts in the C or N termini and hydrophilic loop regions should be treated with care.