Biomedical Engineering Reference
In-Depth Information
catalytic activity of DP4
22
and DP8
23
(Figure 1.2). DP2, belonging to the
serine protease S28 family, is a non-structural homolog sharing a number
of similarities with the DP4-like gene family including the conserved non-
classical catalytic triad and similar substrate specificity with its ability to
cleave N-terminal dipeptides from peptide substrates with proline in the
penultimate position.
24
As for PEP, a number of inhibitors designed for
targeting DP4 are also known to interact and inhibit DP2.
11,24,25
Hence, when
investigating the expression and roles of the S9b family, due consideration
must also be given to the potential involvement (or targeting in the case of
inhibition) of this related enzyme. The subcellular localization of each
enzyme is diverse and sometimes overlapping; they are found membrane-
bound (DP4, FAP, PEP), in lysosomes (DP2), in the cytoplasm (DP8/9, PEP,
DP2), or in secretory vesicles (DP2), so it is likely that their substrate
degradomes will mostly differ.
Containing a trans-membrane domain within their N-termini and a number
ofglycosylationsites,DP4,FAP,DP6,andDP10arealltypeII-membrane
glycoproteins localized to the cell surface.
26
Plasma-soluble forms of both
DP4
27
and FAP
28,29
have been identified with their presence in circulation
believed to result from shedding events at the plasma membrane. DP8, DP9,
and PEP lack a transmembrane domain and are cytosolic proteins.
6,30,31
A
number of membrane associated forms of PEP have been identified.
31,32
Unique to DP9 is the presence of one of the best-known integrin-binding
motifs the RGD (arginine-glutamine-aspartic acid; Arg-Gly-Asp) motif,
within its N-termini
33
(Figure 1.2). DP9 also contains two potential N-linked
glycosylation sites;
23,34
however, there is no evidence at present of DP9
glycosylation,
35
so the significance of these features is yet to be revealed.
DP4, FAP, DP8/9, and PEP all have neutral pH optimums of pH 7-8.
6,21
In
contrast, DP2 is a smaller, 492-amino-acid, lysosomal enzyme, active across a
broad pH range with an acidic pH optimum of 5.5.
25
DP2 also localizes within
intracellular vesicles distinct from lysosomes in resting quiescent cells.
36
It is
likely that these vesicles contain a secretory component due to the release of
fully functional DP2 in response to calcium stimulation.
36
All S9b family
members and the S28 member DP2 form homodimers with dimerization being
essential for the catalytic activity of DP4,
37,38
FAP
13,39
and DP2.
40
In con-
trast, PEP is a monomeric enzyme with SDS-PAGE mobility of 70-80
kDa.
9,41
S9b proteins have a monomer mobility on SDS-PAGE gel of 90-110
kDa and dimer mobility ranging between 150 and 200 kDa.
7
Being much
smaller, DP2 migrates in its glycosylated form as a
60 kDa monomer on
B
120 kDa homodimer.
40, 42-44
A brief discussion of the homology and gene structure of the DP4-like gene
family is provided below. Although both DP2 and PEP are of importance when
discussing therapeutic targeting of DP4, FAP, and DP8/9, they will not be
discussed here in detail; thus, the reader should refer to critical reviews on DP2
by Maes et al.
24
and on PEP by Garcia-Horsman et al.
45
. The inactive protease
homologs DP6 and DP10 will be discussed in brief, as they are not a focus of
this review (recently reviewed in McNicholas et al.
46
).
SDS-PAGE and forms a
B
