Saposins are small, heat-stable
glycoproteins required for the hydrolysis of
sphingolipids by specific lysosomal
hydrolases.
Saposins A, B, C, and D are derived by proteolytic processing from a single precursor
protein named prosaposin.
Saposin B, previously known as SAP-1 and
sulfatide activator, stimulates the hydrolysis of a wide variety of substrates including
cerebroside sulfate,
GM1 ganglioside, and
globotriaosylceramide by
arylsulfatase A,
acid beta-galactosidase, and
alpha-galactosidase, respectively. Human
saposin B deficiency, transmitted as an autosomal recessive trait, results in tissue accumulation of
cerebroside sulfate and a clinical picture resembling
metachromatic leukodystrophy (activator-deficient
metachromatic leukodystrophy). We have examined transformed lymphoblasts from the initially reported
saposin B-deficient patient and found normal amounts of
saposins A, C, and D. After preparing first-strand
cDNA from lymphoblast total
RNA, we used the polymerase chain reaction to amplify the prosaposin
cDNA. The patient's
mRNA differed from the normal sequence by only one C----T transition in the 23rd
codon of
saposin B, resulting in a
threonine to
isoleucine amino acid substitution. An affected male sibling has the same mutation as the proband and their heterozygous mother carries both the normal and mutant sequences, providing additional evidence that this base change is the disease-causing mutation. This base change results in the replacement of a polar
amino acid (
threonine) with a nonpolar
amino acid (
isoleucine) and, more importantly, eliminates the glycosylation signal in this activator
protein. One explanation for the deficiency of
saposin B in this disease is that the mutation may increase the degradation of
saposin B by exposing a potential proteolytic cleavage site (
arginine) two
amino acids to the amino-terminal side of the glycosylation site when the
carbohydrate side chain is absent.