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Biophys J, May 2001, p. 2273-2283, Vol. 80, No. 5
-Melanocyte Stimulating Hormone with Binary
Phospholipid Membranes: Structural Changes and Relevance of Phase
Behavior
and
*Centro de Biología Molecular y Celular, Universidad
"Miguel Hernández," E-03206 Elche-Alicante, Spain; and
Centro de Química-Física Molecular,
Instituto Superior Técnico, P-1049-001 Lisboa, Portugal
The interaction of 
-melanocyte stimulating hormone
(-MSH) with negatively charged binary membrane systems composed of
either 1,2-dimyristoyl-sn-glycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)], (DMPC/DMPG) or
DMPC/1,2-dimyristoyl-sn-glycero-3-phosphate (DMPC/DMPA), both at a 3:1 ratio, was studied using complementary techniques (differential scanning calorimetry, infrared and ultraviolet absorption spectroscopy, and steady-state and time-resolved fluorescence). The
peptide structure in buffer, at medium to high concentrations, is a
mixture of aggregated 
-strands and random coil, and upon increasing
the temperature the random coil configuration becomes predominant. At
low concentrations (micromolar) there are essentially no aggregates.
When in interaction with the lipidic systems this transition is
prevented and the peptide is stabilized in a specific conformation
different from the one in solution. The incorporation of -MSH into
phosphatidic acid-containing systems produced a significant alteration
of the calorimetric data. Lateral heterogeneity can be induced by the
peptide in the DMPA-containing mixture, at variance with the one of
DMPG. In addition, the lipid/water partition coefficient for the
peptide in the presence of DMPC/DMPA is greater in the gel phase as
compared to the fluid phase. From the high values of limiting
anisotropies it can be concluded that the peptide presents a very
reduced rotational dynamics when in interaction with the lipids,
pointing out to a strong interaction. Overall, these results show that
the structure and stability of -MSH in a negatively charged membrane
environment are substantially different from those of the peptide in
solution, being stabilized in a specific conformation that could be
important to eliciting its biological activity.
Biophys J, May 2001, p. 2273-2283, Vol. 80, No. 5
© 2001 by the Biophysical Society 0006-3495/01/05/2273/11 $2.00
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