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CHAPTER
14
Recessive Osteogenesis Imperfecta
Due to Mutations in
CRTAP
,
LEPRE1
and
PPIB
Erica P. Homan
1
, Brendan Lee
1,2
and Roy Morello
3
1
Baylor College of Medicine, Houston, TX, USA,
2
Howard Hughes Medical Institute, Houston, TX,
USA,
3
University of Arkansas for Medical Sciences, Little Rock, AR, USA
COLLAGEN POST-TRANSLATIONAL
MODIFICATIONS AND THE PROTEIN
COMPONENTS OF THE PROLYL
3-H
YDROXYLATION COMPLE
X
registry with the collagen repeats of the other two chains.
In addition, in the lumen of the ER, proper procollagen
chain recognition needs to be assisted along with the
inhibition of inappropriate intracellular polymerization
of procollagen monomers into fibrils.5
5
Until recently, the
least understood among the collagen post-translational
modifications was prolyl 3-hydroxylation where a
proline residue is converted into a 3-hydroxyproline
(3-Hyp). This enzymatic activity was already described
in the 1970s
6
and similar to prolyl 4-hydroxylase, it
requires Fe
2+
, O
2
, 2-oxoglutarate and ascorbate as cofac-
tors. However, it was not until 2004 that the prolyl
3-hydroxylase 1 (P3h1) protein was purified and shown
to have prolyl 3-hydroxylase activity
in vitro
.
7
It turned
out that P3h1 is encoded by Leprecan, a previously
described gene thought to encode leucine- and proline-
enriched proteoglycan associated with basement mem-
branes
8
and with growth suppressor activity.
9
Database
searches identified two additional proteins with signifi-
cant protein sequence homology to P3h1 and conserved
dioxygenase domain; these were named prolyl 3-hydroxy-
lase2 (P3h2) and prolyl 3-hydroxylase 3 (P3h3).
7
P3h1
and these two homologs are encoded by three different
genes called
Lepre1
,
Leprel1
(
Leprecan-like1
) and
Leprel2
(
Leprecan-like2
), respectively. Their activity appears to be
specific to different proline residues and
/
or collagen sub-
strates.
10
Importantly, the isolation of P3h1 resulted in
the co-purification of two additional proteins which were
initially thought to be contaminants, since P3h1 alone
was sufficient for 3-Hyp formation
in vitro
.
7
However,
Collagens are secreted proteins and constitute the
most abundant organic component of the extracellu-
lar matrix (ECM). Their intracellular synthesis pathway
is similar to that of the majority of secreted proteins,
albeit it faces additional challenges. Upon exit from the
nucleus, procollagen mRNAs are targeted to the translo-
cation machinery on the membrane of the endoplasmic
reticulum (ER).
1
The translating nascent procollagen
chains traverse the lipid bilayer of the ER via the trans-
location channel and enter the ER lumen. Here, they
undergo several post-translational modifications includ-
ing signal peptide cleavage, hydroxylation of prolyl and
lysyl residues, glycosylation and others
1
(see Chapter 6
“The Collagen Folding Machinery: Biosynthesis and
Post-Translational Modifications of Collagens” by H.P.
Bachinger et al.). The direct interaction with post-transla-
tional modification enzymes is facilitated by a number of
ER-resident chaperone and co-chaperone proteins which
also assist and facilitate protein folding to reach the
native conformation.
2-4
This process is more challenging
for collagens relative to most other proteins, especially
for the fibrillar collagens, due to the fact that they are tri-
mers (e.g., a1(I)
2
a2(I) in the case of type I collagen). Each
of the three procollagen chains contains a long Gly-X-Y
collagen repeat that needs to fold into a triple helix in
