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present in myelin require detergent for their extraction. Only some MBP
(about 5%) is extracted by salts, as occurs in the case of extrinsic mem-
brane proteins. MBP is found also in detergent-resistant domains, a find-
ing that is not expected for a water-soluble protein such as classic MBP.
What determines the interaction of MBP with a specific myelin domain
are the post-translational modifications and not the innate adaptation of
MBP to a certain environment.
Another important aspect to be taken into consideration is that the
detergent-extracted MBPs, purified with their original lipid environment,
were found to have a noticeable amount of ordered secondary structure and
to differ not only in functional but also in structural aspects from the cor-
responding lipid-free form. The data on the conformation of MBP in its
native lipid environment in solution point toward a compact, but not spher-
ical, C-shaped protein-lipid complex with regions of different electron den-
sity (Haas
et al
., 2004). This is in accord with the idea that the isolation of
specific protein-lipid complexes may be the more desirable goal for struc-
tural and functional studies of membrane proteins (Garavito, Ferguson-
Miller, 2001), supposing they are structured and stable (Rosenbusch, 2001).
Since the presence of MBP in the different regions of myelin depends
on its chemical modifications and these are certainly reproducible, it
may be suggested that the extraction and the purification of MBP-lipid-
detergent complexes may represent a better (more homogeneous with
respect to the acid-extracted MBP) starting material for crystallization
attempts. Moreover, removal of detergent and incorporation of MBP into
liposomes may represent a further promising step in these studies.
Acknowledgments
This study is an outcome of the MARIE Network of the European
Science Foundation on
Myelin Structure and Its Role in Autoimmunity
,
2004-2006.
The author gratefully acknowledges the work of Anna Fasano, Grazia
Maria Liuzzi and Giulia Carlone, University of Bari, Italy; Antonella
Bobba, IBBE, CNR, Bari, Italy; Eugenia Polverini, Paolo Cavatorta and
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