2. IDENTIFICATION OF HOMOLOGIES TO INTERNALIZATION
SIGNALS
An analysis of the PrP primary structure previously
identified a well conserved ‘aromatic palindrome’ region
(PrP:1477163 RYYRENMYRYPNQVYYR) containing
six of the 14 tyrosine residues in murine PrP [ 121.
Based on the topography of PrP [5], this region is located
mainly in the cytoplasmic (PrP: 1377155) domain.
Although the function of this region is not known, it
was suggested that it may act as a binding site for other
proteins [12]. The presence of conserved tyrosine residues
in this region, as well as in the adjacent region of
the cytoplasmic domain, prompted an analysis of structural
homologies to internalization signals.
Secondary structure analysis using the methods of
Chou and Fasman [13] and of Garnier et al. [ 141 predicted
a probability for turn structures at PrP:142 and
146, and in the region PrP:151-160. Based on thep-turn
structure exhibited by the model pentapeptide, YPNDV
[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