Lipid thioesters derived from acylated proteins accumulate in infantile neuronal ceroid lipofuscinosis: correction of the defect in lymphoblasts by recombinant palmitoyl-protein thioesterase

Signalling functions of protein palmitoylation

Covalent lipid modifications anchor numerous signalling proteins to the cytoplasmic face of the plasma membrane. These modifications mediate protein-membrane and protein-protein interactions and are often essential for function. Protein palmitoylation, due to its reversible nature, may be particularly important for modulating protein function during cycles of activation and deactivation. Despite intense investigation, the exact functions of protein palmitoylation are not well understood. However, it is clear that palmitoylation can affect a protein's affinity for membranes, subcellular localization, and interactions with other proteins. In this review, recent advances in understanding the functions and mechanisms of protein palmitoylation are discussed, with particular emphasis on how this lipid affects the biochemistry and cell biology of signalling proteins.

Batten disease: four genes and still counting

The neuronal ceroid lipofuscinoses (NCLs, also known as Batten disease) are the most common childhood neurodegenerative disease. They are a group of inherited neurodegenerative disorders characterized by the accumulation of autofluorescent storage material in many cell types. Clinical features include seizures, psychomotor deterioration, and blindness, the ages and order of onset of which differ for each NCL type. An increasing number of subtypes caused by mutations in different genes are now recognized. With the advent of molecular genetics the basic genetic defect underlying each NCL phenotype is being determined, thus shedding light on the molecular basis of the NCLs and opening the way for the development of effective treatment. Four genes have been identified to date. The function of two of these is known and suggests that the primary defect in the NCLs lies in lysosomal proteolysis, the first example of this type of disease. However, since the function of the other two genes remains elusive, and at least four more genes remain to be identified, the molecular basis underlying the NCLs may be more complex than originally predicted.

The dystrophic retina in multisystem disorders: the electroretinogram in neuronal ceroid lipofuscinoses

The neuronal ceroid lipofuscinoses (NCL) are neurodegenerative disorders with psychomotor deterioration, seizures, visual failure and premature death, all associated with abnormal storage of lipoproteins within lysosomes. The most common forms of NCL are an infantile form (INCL, CLN1), a late infantile form (LINCL, CLN2) and a juvenile onset form (JNCL, CLN3). The electroretinogram (ERG) is abnormal early in all three of these forms and eventually is totally ablated. The purpose of this report is to describe the ERG in INCL, LINCL and JNCL. The ERGs of 7 patients who were examined by the author over the past 15 years were reviewed. Ganzfeld ERG responses were recorded using the ISCEV standard protocol and an intensity response series over a 3.7 log unit range. The earliest ERG manifestation of INCL is a marked loss of the scotopic and photopic b-wave with relative preservation of the a-wave; this defect, which was evident for both rods and cones, suggests preservation of photoreceptor outer segment function with severe disturbance of transmission of the signal to the second-order neuron, the bipolar cells. For LINCL, the rod responses were mildly abnormal but more preserved than in INCL or JNCL. The cone b-wave amplitudes in patients with early LINCL were severely subnormal with prolonged implicit times. Patients with JNCL invariably showed severe to profound ERG abnormalities when first tested, with essentially no rod-mediated activity and marked loss of a-wave amplitudes with even greater loss of b-wave amplitudes, creating electronegative configuration waveforms. Differences in the ERG responses were thus found that provide further clues to the earliest site of pathology within the retina.

cDNA cloning and genomic organization of peroxisome proliferator-inducible long-chain acyl-CoA hydrolase from rat liver cytosol

The cDNA for a peroxisome proliferator-inducible long-chain acyl-CoA hydrolase from rat liver cytosol, referred to as rLACH2, was isolated and its genomic structure was determined. The cDNA encoded a 419-amino-acid polypeptide with a calculated molecular weight of 46,011. Sequence analysis identified an active-site serine motif (Gly-x-Ser-x-Gly) common to carboxylesterases and lipases. When expressed in Escherichia coli, the cDNA directed expression of a protein immunoreactive to an anti-rLACH2 antibody with a molecular mass of 47 kDa, identical to that of purified rLACH2. Northern blot analysis showed marked induction of rLACH2 mRNA in the liver after feeding rats with di(2-ethylhexyl)phthalate, a peroxisome proliferator. The rLACH2 gene spanned about 19 kb and comprised 3 exons, the intron/exon boundaries of which were consistent with the donor/acceptor splice rule. A putative peroxisome proliferator response element (AGGTCATGGTTCA) was identified in the 5'-flanking region, suggesting the involvement of peroxisome proliferator-activated receptors in the regulation of rLACH2 gene expression.

Molecular genetics of palmitoyl-protein thioesterase deficiency in the U.S

Mutations in a newly described lysosomal enzyme, palmitoyl-protein thioesterase (PPT), were recently shown to be responsible for an autosomal recessive neurological disorder prevalent in Finland, infantile neuronal ceroid lipofuscinosis. The disease results in blindness, motor and cognitive deterioration, and seizures. Characteristic inclusion bodies (granular osmiophilic deposits [GROD]) are found in the brain and other tissues. The vast majority of Finnish cases are homozygous for a missense mutation (R122W) that severely affects PPT enzyme activity, and the clinical course in Finnish children is uniformly rapidly progressive and fatal. To define the clinical, biochemical, and molecular genetic characteristics of subjects with PPT deficiency in a broader population, we collected blood samples from U.S. and Canadian subjects representing 32 unrelated families with neuronal ceroid lipofuscinosis who had GROD documented morphologically. We measured PPT activity and screened the coding region of the PPT gene for mutations. In 29 of the families, PPT deficiency was found to be responsible for the neurodegenerative disorder, and mutations were identified in 57 out of 58 PPT alleles. One nonsense mutation (R151X) accounted for 40% of the alleles and was associated with severe disease in the homozygous state. A second mutation (T75P) accounted for 13% of the alleles and was associated with a late onset and protracted clinical course. A total of 19 different mutations were found, resulting in a broader spectrum of clinical presentations than previously seen in the Finnish population. Symptoms first appeared at ages ranging from 3 mo to 9 yr, and about half of the subjects have survived into the second or even third decades of life.

A cytoplasmic acyl-protein thioesterase that removes palmitate from G protein alpha subunits and p21(RAS)

Thioacylation is one of a handful of reversible covalent protein modifications, but the enzymes responsible for addition and removal of long chain fatty acids from protein cysteine residues in vivo have not yet been identified. The alpha subunits of some heterotrimeric G proteins cycle between thioacylated and deacylated states in a receptor-regulated fashion. We have identified, purified, and characterized an enzyme acyl-protein thioesterase that deacylates Galpha proteins and at least some other thioacyl protein substrates, including Ha-RAS. The action of this enzyme on thioacylated heterotrimeric Gs is regulated by activation of the G protein. Although native and recombinant acyl-protein thioesterases act as both acyl-protein thioesterases and lysophospholipases in vitro, we demonstrate by transfection that the enzyme can accelerate the turnover of thioacyl groups on Gsalpha in vivo.

Biosynthesis and intracellular targeting of the CLN3 protein defective in Batten disease

Batten disease (juvenile-onset neuronal ceroid lipofuscinosis, JNCL), the most common neurodegenerative disorder of childhood, is caused by mutations in a recently identified gene ( CLN3 ) localized to chromosome 16p11.2-12.1. To elucidate the biosynthesis and localization of the CLN3 protein, we expressed CLN3 cDNA in COS-1 and HeLa cell lines. In vitro translation, immunoprecipitation and Western blotting analyses detected an approximately 43 kDa polypeptide. Pulse-chase experiments indicated that the CLN3 protein is synthesized as an N -glycosylated single-chain polypeptide, which was not detected in growth medium. Confocal immunofluorescence microscopy revealed that the CLN3 protein is localized to the lysosomal compartment. These results provide evidence that Batten disease can be classified as a member of lysosomal diseases.

Molecular cloning and expression of palmitoyl-protein thioesterase 2 (PPT2), a homolog of lysosomal palmitoyl-protein thioesterase with a distinct substrate specificity

Palmitoyl-protein thioesterase is a lysosomal hydrolase that removes long chain fatty acyl groups from modified cysteine residues in proteins. Mutations in this enzyme were recently shown to underlie the hereditary neurodegenerative disorder, infantile neuronal ceroid lipofuscinosis, and lipid thioesters derived from acylated proteins were found to accumulate in lymphoblasts from individuals with the disorder. In the current study, we describe the cloning and expression of a second lysosomal thioesterase, palmitoyl-protein thioesterase 2 (PPT2), that shares an 18% identity with palmitoyl-protein thioesterase. Transient expression of a PPT2 cDNA led to the production of a glycosylated lysosomal protein with palmitoyl-CoA hydrolase activity comparable with palmitoyl-protein thioesterase. However, PPT2 did not remove palmitate groups from palmitoylated proteins that are substrates for palmitoyl-protein thioesterase. In cross-correction experiments, PPT2 did not abolish the accumulation of protein-derived lipid thioesters in palmitoyl-protein thioesterase-deficient cell lines. These results indicate that PPT2 is a lysosomal thioesterase that possesses a substrate specificity that is distinct from that of palmitoyl-protein thioesterase.

Isolation and characterization of a prenylcysteine lyase from bovine brain

Prenylated proteins contain one of two isoprenoid lipids, either the 15-carbon farnesyl or the 20-carbon geranylgeranyl, covalently attached to cysteine residues at or near their C terminus. The cellular abundance of prenylated proteins, which can comprise up to 2% of total cellular protein, raises the question of how cells dispose of prenylcysteines produced during the normal turnover of prenylated proteins. We have identified and characterized a novel enzyme, which we term prenylcysteine lyase, that is capable of cleaving the thioether bond of prenylcysteines. The enzyme was isolated from bovine brain membranes and exhibits an apparent molecular mass of 63 kDa. The enzyme did not require NADPH as cofactor for prenylcysteine degradation, thus distinguishing it from cytochrome P450- and flavin-containing monooxygenases that catalyze S-oxidation of thioethers. Purified prenylcysteine lyase shows similar kinetics in utilization of both farnesylcysteine and geranylgeranylcysteine as substrates, although Vmax is 2-fold higher with the former compound. Interaction of prenylcysteine substrates with the enzyme requires that they possess a free amino group; N-acetylated prenylcysteines and prenyl peptides are not substrates. These findings suggest that prenylcysteine lyase is a specific enzyme involved in prenylcysteine metabolism in mammalian cells, most likely comprising the final step in the degradation of prenylated proteins.

Palmitoyl-protein thioesterase deficiency in fibroblasts of individuals with infantile neuronal ceroid lipofuscinosis and I-cell disease

Mutations in the gene encoding a recently described lysosomal enzyme, palmitoyl-protein thioesterase (PPT), have recently been shown to result in the neurodegenerative disorder, infantile neuronal ceroid lipofuscinosis (INCL). Reduced palmitoyl-protein thioesterase enzyme has been demonstrated previously in INCL brain and immortalized lymphoblasts. In the current paper, we demonstrate that: (1) PPT can be detected by immunoblotting and enzyme activity assays in normal human skin fibroblasts; (2) INCL fibroblasts are deficient in PPT activity; (3) I-cell disease fibroblasts show markedly reduced intracellular levels of PPT but markedly increased levels of PPT in cell culture medium. These data establish that PPT is transported to lysosomes via the lysosomal enzyme:lysosomal enzyme receptor phosphomannosyl recognition system under normal physiological conditions and provide the basis for a useful clinical assay for INCL.