M. Hiraiwa et al., PROTECTIVE PROTEIN IN THE BOVINE LYSOSOMAL BETA-GALACTOSIDASE COMPLEX, Biochimica et biophysica acta. Protein structure and molecular enzymology, 1341(2), 1997, pp. 189-199
Cathepsin A [EC 3.4.16.1], so called protective protein, occurs as an
enzyme complex with lysosomal beta-galactosidase [3.2.1.23] and is inv
olved in the stable enzymic expression of lysosomal sialidase [3.2.1.1
8]. In this study we investigated the enzymatic properties of cathepsi
n A in the bovine beta-galactosidase complex and how it is involved in
the molecular multiplicities of the beta-galactosidase and sialidase
complexes. Bovine protective protein homologous to the human protein h
ad a molecular weight of 48 kDa on SDS-PAGE and cathepsin A activity o
ptimum around pH 6.0. It hydrolyzed dipeptide substrates composed of h
ydrophobic amino acids much faster than any other type of substrate te
sted. This specificity was found to be conserved from human to a non-m
ammal, chicken. Immunoprecipitation using an anti beta-galactosidase a
ntibody demonstrated that cathepsin A is a component of both the siali
dase and beta-galactosidase complexes. The over 700 kDa sialidase comp
lex depolymerized by a brief incubation at pH 7.5 and the sialidase wa
s inactivated irreversibly via formation of an enzyme active smaller s
pecies of sialidase. The 669 kDa beta-galactosidase complex dissociate
d reversibly into a 120 kDa beta-galactosidase and a 170 kDa cathepsin
A, but the 120 kDa beta-galactosidase, free from the cathepsin A, for
med a 260 kDa aggregate under the same conditions, Inactivation of cat
hepsin A by heat treatment did not affect its complex forming activity
. The 170 kDa protective protein dissociated into a 50 kDa one at pH 7
.5, which no longer formed the complex. These findings indicate that t
he 170 kDa protective protein could be the minimum unit required for i
n vitro reconstitution of the complex, and that its complex forming ac
tivity is carried in a heat-stable domain. Both beta-galactosidase and
cathepsin A activities were labile under the dissociated condition, i
ndicating that it physiologically stabilizes not only beta-galactosida
se but also itself by forming the complex. (C) 1997 Elsevier Science B
.V.