1. RAWLINGS, N.D. AND BARRETT, A.J.
Evolutionary families of metallopeptidases.
METHODS ENZYMOL. 248 183-228 (1995).
2. MELLORS, A. AND LO, R.Y.C.
O-Sialoglycoprotease from Pasturella haemolytica.
METHODS ENZYMOL. 248 728-740 (1995).
Metalloproteases are the most diverse of the four main types of protease,
with more than 30 families identified to date . Of these, around
half contain the HEXXH motif, which has been shown in crystallographic
studies to form part of the metal-binding site . The HEXXH motif is
relatively common, but can be more stringently defined for metallo-
proteases as abXHEbbHbc, where a is most often valine or threonine and
forms part of the S1' subsite in thermolysin and neprilysin, b is an
uncharged residue, and c a hydrophobic residue. Proline is never found
in this site, possibly because it would break the helical structure
adopted by this motif in metalloproteases .
Metalloproteases can be split into five groups on the basis of their metal-
binding residues: the first three contain the HEXXH motif, the other two
do not . In the first group, a glutamic acid completes the active site -
these are termed HEXXH+E: all families in this group show some sequence
relationship and have been assigned to clan MA . The second group, which
have a third histidine as the extra metal-binding residue, are termed
HEXXH+H and are grouped into clan MB on the basis of their inter-relation-
ship . In the third group, the additional metal-binding residues are
unidentified. The fourth group is diverse - the metal-binding residues are
known but do not form the HEXXH motif. And the fifth group comprises the
remaining families where the metal-binding residues are as yet unknown .
O-Sialoglycoprotein endopeptidase is secreted by the bacterium Pasturella
haemolytica and digests only proteins that are heavily sialylated, in
particular those with sialylated serine and threonine residues .
Substrate proteins include glycophorin A and leukocyte surface antigens
CD34, CD43, CD44 and CD45 [1,2]. Removal of glycosylation, by treatment
with neuraminidase, completely negates susceptibility to O-sialoglycoprotein
endopeptidase digestion [1,2].
Sequence similarity searches have revealed other members of the M22 family,
from yeast, Mycobacterium, Haemophilus influenzae and the cyanobacterium
Synechocystis . The zinc-binding and catalytic residues of this family
have not been determined, although the motif HMEGH may be a zinc-binding
OSIALOPTASE is a 6-element fingerprint that provides a signature for the
O-sialoglycoprotein endopeptidase (M22) family of metalloproteases. The
fingerprint was derived from and initial alignment of 6 sequences: the
motifs were drawn from conserved regions spanning the N-terminal half of
the alignment - motif 3 includes the region encoded by PROSITE pattern
GLYCOPROTEASE (PS0106), which describes the region surrounding the HMEGH
motif; and motif 4 contains well-conserved histidines that may be involved
in zinc binding. Three iterations on OWL29.3 were required to reach
convergence, at which point a true set comprising 13 sequences was
identified. A single partial match was also found, YHSH_HALMA, a
hypothetical protein fragment from the archaebacterium Haloarcula
marismortui that makes only a weak match with motif 4 and lacks the
portion of sequence bearing motif 6.
An update on SPTR37_9f identified a true set of 23 sequences, and 1