A. Gow et al., CONSERVATION OF TOPOLOGY, BUT NOT CONFORMATION, OF THE PROTEOLIPID PROTEINS OF THE MYELIN SHEATH, The Journal of neuroscience, 17(1), 1997, pp. 181-189
The proteolipid protein gene products DM-20 and PLP are adhesive intri
nsic membrane proteins that make up greater than or equal to 50% of th
e protein in myelin and serve to stabilize compact myelin sheaths at t
he extracellular surfaces of apposed membrane lamellae. To identify wh
ich domains of DM-20 and PLP are positioned topologically in the extra
cellular space to participate in adhesion, we engineered N-glycosylati
on consensus sites into the hydrophilic segments and determined the ex
tent of glycosylation. In addition, we assessed the presence of two tr
anslocation stop-transfer signals and, finally, mapped the extracellul
ar and cytoplasmic dispositions of four antibody epitopes. We find tha
t the topologies of DM-20 and PLP are identical, with both proteins po
ssessing four transmembrane domains and N and C termini exposed to the
cytoplasm. Consistent with this notion, DM-20 and PLP contain within
their N- and C-terminal halves independent stop-transfer signals for i
nsertion into the bilayer of the rough endoplasmic reticulum during de
novo synthesis. Surprisingly, the conformation (as opposed to topolog
y) of DM-20 and PLP may differ, which has been inferred from the diver
gent effects that many missense mutations have on the intracellular tr
afficking of these two isoforms. The 35 amino acid cytoplasmic peptide
in PLP, which distinguishes this protein from DM-20, imparts a sensit
ivity to mutations in extracellular domains, This peptide may normally
function during myelinogenesis to detect conformational changes origi
nating across the bilayer from extracellular PLP interactions in trans
and trigger intracellular events such as membrane compaction in the c
ytoplasmic compartment.