[I 51, it had been speculated that the segment PrP:156-
160 (YPNQV) would also have a p-turn structure [12].
In fact, recent data [ 161 acquired using 2D nuclear magnetic
resonance to study the conformation of a synthetic
cognate peptide, PrP: 1455165, revealed the presence of
a p-turn involving the region PrP:1577160 (YPNQ).
This region is conserved in all mammalian PrP’s, and
is located at the proposed boundary between the cytoplasmic
and putative second transmembrane (TM II)
domains (PrP: 156-l 59).
An analysis of the sequences of the regions predicted
or found to have p-turn structure showed that the regions
PrP:141l152 (WEDRYY) and PrP:155-160
(YPNQVY) are similar to the six residue internalization
signal (FDNPVY) in the low-density lipoprotein receptor
(LDL-R). The consensus sequence reported for this
internalization signal, as mentioned earlier, is
FXNPXY [17]; and the tetrapeptide, NPVY, contained
within the LDL-R sequence was shown by 2D NMR to
adopt a /?-turn structure in solution [18]. An aminoterminal
aromatic residue and a carboxy-terminal aromatic
or large hydrophobic residue, and a turn in the
intervening region, have been suggested [17] to provide
sufficient structural information for this signal. A somewhat
different motif for internalization signals has also
been suggested [9] based on an analysis of the cytoplasmic
domains of several integral membrane proteins
known to be internalized, including the LDL-R. This
motif requires the presence of basic or polar amino acid
residues (+) at certain preferred positions relative to a
tyrosine residue: XX+X+XX+Y++X. The PrP hexapeptide
segments and adjacent residues also share similarities
with this motif. These homologies are shown in