Biology Reference
In-Depth Information
properties from all the known proteases. For example, in the kidney they were the
only membrane-associated proteases that cleaved proteins at alkaline pH values.
There were many known lysosomal proteases that act on proteins at acidic pH
values, and proteases in other cellular compartments and membranes that were
exopeptidases (removed one or two amino acids from the carboxy- or amino-
terminus of a protein) or could degrade small peptides, but meprins were unique in
their ability to degrade proteins such as azocasein and gelatin at neutral and alkaline
pH values. In addition, meprin specific activity against azocasein was 10
higher in
the C57Bl/6 mouse kidney than in any other tissue analyzed (including liver, brain,
spleen, muscle). In the human intestine, meprins were discovered as proteases that
could degrade
N
-benzoyl-
L
-tyrosyl-
p
-aminobenzoic acid (PABA-peptide), a sub-
strate that was used clinically at the time to determine pancreatic function because of
the ability of chymotrypsin to hydrolyze it. From these beginnings, meprins were
purified, cloned, and sequenced, and found to be rather unique in the world of
proteases.
4.2.2 Structure of the Meprin Domains and Oligomers
The meprins are composed of two subunits,
, that are approximately 50%
identical in their primary amino acid sequence and very similar in their domain
structure (Fig.
4.2
). The subunits are coded for on separate chromosomes; meprin
and
a
b
a
on human chromosome 6, and mouse chromosome 17; meprin
on chromosome 18
of both the mouse and human genome. Both subunits contain a signal sequence (S)
that targets the protein to the secretory pathway, a prosequence (Pro) that keeps the
enzyme inactive until it is proteolytically removed (usually by a trypsin-like
enzyme), a catalytic (protease) domain that contains a deep cleft with zinc at the
active site, MAM and TRAF domains that are protein-protein interaction domains
important for protein folding, an epidermal growth factor (EGF) domain, a trans-
membrane domain (TM), and a small cytoplasmic tail. The
b
a
subunit contains an
inserted domain (I) not present in the
b
subunit. The presence of the I domain
enables the
subunit to be proteolytically cleaved in the endoplasmic reticulum
during biosynthesis. Thus, the
a
subunit loses its transmembrane domain and is not
found at the plasma membrane unless associated with a
b
subunit. The
b
subunit is
found as a type 1 protein at the cell surface unless it is shed from the surface or
redistributed, as in some pathological situations (e.g., after kidney ischemia, see
Fig.
4.7
).
The mouse meprin subunits have a molecular mass of 75-85 kDa depending on
whether they are activated or not, and they are highly glycosylated (Bertenshaw
et al.
2003
; Kadowaki et al.
2000
). Glycosylation accounts for approximately 20%
of the molecular mass (Tang and Bond
1998
). The glycans contribute to folding,
activity, stability, and oligomerization (Ishmael et al.
2006
).
N
-Glycans in the
protease domain of meprin
a
are important to the formation of an active, stable
a
