Mitotic phosphorylation of Bloom helicase at Thr182 is required for its proteasomal degradation and maintenance of chromosomal stability.

Mutations in Bloom helicase (BLM) lead to Bloom Syndrome (BS). BS is characterized by multiple clinical manifestations including predisposition to a wide spectrum of cancers. Studies have revealed the mechanism of BLM recruitment after stalled replication and its role during the repair of DNA damage. We now provide evidence that BLM undergoes K48-linked ubiquitylation and subsequent degradation during mitosis due to the E3 ligase, Fbw7α. Fbw7α carries out its function after GSK3ß- and CDK2/cyclin A2-dependent phosphorylation events on Thr171 and Ser175 of BLM which lies within a well-defined phosphodegron, a sequence which is conserved in all primates. Phosphorylation on BLM Thr171 and Ser175 depends on prior phosphorylation at Thr182 by Chk1/Chk2. Thr182 phosphorylation not only controls BLM ubiquitylation and degradation during mitosis but is also a determinant for its localization on the ultrafine bridges. Consequently lack of Thr182 phosphorylation leads to multiple manifestations of chromosomal instability including increased levels of DNA damage, lagging chromatin, micronuclei formation, breaks and quadriradials. Hence Thr182 phosphorylation on BLM has two functions-it regulates BLM turnover during mitosis and also helps to maintain the chromosomal stability.

Stereochemical specificity of Alzheimer's disease beta-peptide assembly.

The formation and growth of insoluble amyloid deposits composed primarily of the human beta-amyloid peptide (A beta) in brain is an essentially invariant feature of Alzheimer's disease (AD) and is widely believed to contribute to the progressive neurodegeneration of the disorder. To probe the specificity of amyloid formation and growth, we synthesized and examined the self-assembly of D- and L-stereoisomers of A beta in vitro. While both enantiomers formed insoluble aggregates at similar rates with amyloid-like fibrillar morphology, deposition of soluble A beta peptide onto preexisting A beta aggregates was stereospecific. Although the L-peptide deposited readily onto immobilized L-A beta aggregates with first-order kinetic dependence on soluble peptide concentration, essentially no association between the D-peptide and L-template was observed. Similarly, the D-peptide deposited with first-order kinetics onto a D-A beta aggregate template but did not deposit onto a similar template composed of aggregates of the L-enantiomer. Furthermore, although the L-A beta isomer deposited onto authentic AD amyloid in preparations of unfixed AD brain, no focal association between the D-peptide and brain amyloid was detected. These results establish that deposition of soluble A beta onto preexisting amyloid template is stereospecific, likely involving direct docking interactions between peptide backbone and/or side chains rather than simple hydrophobic association.

Activation of signal transducers and activators of transcription 1 and 3 by leukemia inhibitory factor, oncostatin-M, and interferon-gamma in adipocytes.

We have recently demonstrated that signal transducers and activators of transcription (STATs) 1, 3, 5A, 5B, and 6 are expressed in both cultured and native adipocytes. Our current studies have focused on the activation of STATs 1 and 3 by leukemia inhibitory factor (LIF), oncostatin-M (OSM), and interferon-gamma (IFNgamma) in 3T3-L1 adipocytes. IFNgamma is shown to be a potent activator of STAT 1 as indicated by both tyrosine phosphorylation and nuclear translocation. However, LIF and OSM, which are potent inducers of STAT 3, are less potent activators of STAT 1 as measured by both tyrosine phosphorylation and nuclear translocation. Both STATs 1 and 3 were translocated to the nucleus in a time-dependent fashion following LIF treatment. In addition, IFNgamma resulted in a time- and dose-dependent effect on STATs 1 and 3 nuclear translocation. Growth hormone, a potent activator of STATs 5A and 5B, had a minimal effect on STAT 1 and STAT 3 tyrosine phosphorylation. Preincubation with either insulin or growth hormone had no detectable effects on the tyrosine phosphorylation or nuclear translocation of STATs 1 and 3 induced by LIF, OSM, or IFNgamma. The effects of LIF and IFNgamma on STAT 1 and 3 tyrosine phosphorylation and nuclear translocation were confirmed in native rat adipocytes. In 3T3-L1 adipocytes, a low level of serine phosphorylation of STAT 3 on residue 727 was observed and was markedly enhanced by insulin, LIF, or OSM. This increase in STAT 3 Ser727 phosphorylation was dependent upon the activation of MAPK, since the MAPK kinase inhibitor (PD98059) reduced STAT 3 Ser727 phosphorylation to basal levels. The inhibition of MAPK had no effect on the ability of STATs 1 and 3 to be tyrosine-phosphorylated or translocate to the nucleus. These studies demonstrate the highly specific and quantitative activation of STATs 1 and 3 by LIF, OSM, and IFNgamma in adipocytes and indicate that STAT 3 is a substrate for MAPK in adipocytes.

Activated Alzheimer disease platelets retain more beta amyloid precursor protein.

Upon activation, platelet alpha-granules' soluble contents are secreted and membrane-bound contents are translocated to the plasma membrane. Membrane-bound proteins include the beta-amyloid precursor protein (APP) from which the beta-amyloid (A beta) deposits found surrounding the cerebrovasculature of patients with Alzheimer's Disease (AD) may originate. We show here that activated platelets from AD patients exhibit less APP processing, retain more of the protein on their surface, and secrete less as soluble fragments than do controls. Surface labeling demonstrated that there is little APP or CD62 on the surface of resting platelets. Upon activation, control platelets exhibited more of both proteins on their surface, while advanced AD patients exhibited similar amounts of CD62 as controls, but retained significantly more surface APP. AD platelets secreted similar amounts of most soluble alpha-granule contents as controls, but less APP fragments. Together these results suggest a processing defect that may account for greater deposition of A beta-containing products in the vasculature to which activated platelets adhere.

Specific and effective interaction of a guanine nucleotide analogue with small G proteins.

G proteins are molecular switches that use a cycle of GTP binding and hydrolysis to regulate a wide variety of cellular biochemical processes. Because the functional state of these proteins is allosterically determined by bound guanine nucleotides, a nucleotide analogue with protein specificity might have pharmacological or biochemical value. The binding of [alpha-32P]GTP to four small G proteins immobilized on nitrocellulose was competed by a series of analogues with modifications at multiple sites. One analogue, N2-(p-n-butylphenyl)guanosine 5'-(beta,gamma-difluoromethylene)triphosphate, had a approximately 40-fold higher affinity for one small G protein than for two of the others. Systematic analysis of each modification in the synthetic nucleotide revealed that specificity was conferred by the carbon substitution in the beta,gamma-phosphoanhydride bond. These observations were then extended to purified proteins of known sequence in solution by filtration binding studies with H-ras and rab5. Ras was 9-fold more discriminant between guanosine-5'-(beta,gamma-difluoromethylene)triphosphate and guanosine-5'-O-(3-thiotriphosphate) than was rab5, and the Q79L GTPase-defective mutant of rab5 was 6-fold more discriminant than wild-type rab5. Guanosine-5'-(beta,gamma-difluoromethylene)triphosphate protected a 20-kDa fragment of rab5 from tryptic proteolysis with greater efficacy than guanosine-5'-O-(3-thiotriphosphate) or guanosine-5'-(beta,gamma-imido)triphosphate despite its lower affinity, and GMP stabilized a conformation indistinguishable from apo-rab5. These results identify a synthetic guanine nucleotide analogue with differential affinity for closely related G proteins, determine the atomic substitution in the analogue that confers specificity, demonstrate discrimination by the analogue between wild-type and a point-mutant G protein, and establish efficacy of the analogue in inducing conformational change of a target protein disproportionate to the affinity of the interaction.

Defective glomerular beta-adrenergic signal transmission in spontaneously diabetic rats.

Previous studies in experimental diabetes have demonstrated cardiovascular abnormalities of the beta-adrenergic system and reduced adrenergically stimulated renal renin secretion. To examine the defect in the beta-adrenergic signal, glomerular cyclic adenosine monophosphate (cAMP) levels were measured in response to isoproterenol and other humoral agonists (coincubated with the phosphodiesterase inhibitor isomethylxanthine) in nondiabetic and diabetic BB/Wor rats. Basal (unstimulated) levels of glomerular cAMP did not differ between control and diabetic BB/Wor rats, nor did cAMP accumulation differ on incubation with the humoral agonists PGE2 and histamine. However, on incubation with varied concentrations of the nonselective beta-adrenergic agonist isoproterenol, control glomeruli demonstrated a twofold increase in cAMP while a negligible response was observed in diabetic glomeruli. Peak levels of cAMP were higher in control (192 +/- 24 pmol/mg protein) than in diabetic (141 +/- 8 pmol/mg protein) glomeruli (p < 0.01). No differences were observed on incubation with the adenylate cyclase stimulator forskolin. Measurement of glomerular beta-adrenoreceptors by coincubation with iodine 125-labeled cyanopindolol demonstrated no differences in either receptor number (Bmax) or affinity (KD). These data indicate that a specific defect in beta-adrenergic signalling exists in glomerular tissue from spontaneously diabetic rats. Because no decrease in forskolin-stimulated adenylate cyclase was observed, defective coupling of the receptor to its effector, perhaps through the guanine nucleotide stimulatory protein, may account for these observations.

Etonitazene: an opioid selective for the mu receptor types.

Specific radioligand binding protocols were utilized to compare the affinity of morphine and the high-potency opioid etonitazene at mu 1, mu 2, delta, kappa 1 and sigma receptors. Both etonitazene and morphine displayed a mu 1-selective binding profile; however, etonitazene had a 2500-fold higher affinity at this receptor type. The latter result is consistent with the relative potencies or morphine and etonitazene in various behavioral tests.

The opioid receptor binding of dezocine, morphine, fentanyl, butorphanol and nalbuphine.

The ability of morphine, fentanyl, butorphanol, nalbuphine, and dezocine to compete with radiolabeled ligands for binding at the mu1, mu2, kappa1, and delta opioid receptors and the sigma receptor was characterized. In the absence of sodium, the potency of opioid receptor competition at each receptor site was found to be: mu1-fentanyl > butorphanol > morphine > or = dezocine = nalbuphine; mu2-butorphanol > fentanyl > nalbuphine > morphine = dezocine; kappa1-butorphanol > nalbuphine >> morphine > or = dezocine > fentanyl; and delta-butorphanol > nalbuphine > or = dezocine > morphine > fentanyl. For all five compounds, competition at the sigma receptor was weak, with nalbuphine and dezocine having Kis of approximately 0.5 microM and the other opioids having Kis of greater than 1 microM. Since the presence of 100 mM NaCl during the competitive binding decreased the K(i), to varying degrees, of all five opioids at the mu1 and delta receptors and of some of the opioids at the mu2 and kappa1 receptors, the five compounds studied appear to differ in efficacy at the five receptor sites.

Isolation of a calreticulin-like calcium binding protein from bovine brain.

Intracellular calcium levels are stringently regulated in all cells. The nature of this regulation is incompletely understood, but recent evidence indicates that the endoplasmic reticulum plays an important role in sequestering intracellular calcium. Using methods for isolating both calsequestrin and calreticulin, we have isolated a 58 kDa, high capacity calcium binding protein that exists in microsomes that shift their density in an oxalate-mediated density shift assay. This protein which we call CBP-58 bears similarities to the endoplasmic reticulum protein, calreticulin, in that it has a pI of 4.7 containing approximately 30% glutamate and aspartate, has a high capacity for calcium, and stains blue with the carbocyanine dye, 'Stains-all'. Peptide, amino acid, nucleotide and immunochemical analyses reveal further similarities between CBP-58 and calreticulin, but also some marked differences. Its tissue distribution suggests it is highly enriched in brain versus other tissues. We believe that CBP-58 is a calreticulin-like protein and that differences in the amino acid composition and sequences may reflect species diversity in calreticulin.

Modification of brain guanine nucleotide-binding regulatory proteins by tryptamine-4,5-dione, a neurotoxic derivative of serotonin.

We have recently characterized a novel oxidation product of serotonin (5-hydroxytryptamine, 5-HT), tryptamine-4,5-dione, which increases 5-HT efflux from striatum and hippocampus and causes selective neuronal death. Exposure of striatal synaptosomes or the major brain guanine nucleotide-binding regulatory proteins Gi and Go to [3H]tryptamine-4,5-dione resulted in the radiolabeling of a major band with an apparent molecular mass equivalent to that of the alpha subunits of Gi and Go (approximately 40,000). The binding of [35S]guanosine-5'-O-(3-thiotriphosphate) ([35S]GTP-gamma-S) to Gi and Go and pertussis toxin-catalyzed [32P]ADP-ribosylation of the G protein alpha subunits were both inhibited in a dose-dependent manner by tryptamine-4,5-dione. Thus, neurotoxins such as tryptamine-4,5-dione may exert their effects through specific interactions with G proteins.

Specific gangliosides increase rapidly in rat liver following partial hepatectomy.

Rat liver gangliosides (sialic acid containing glycosphingolipids) were analyzed by HPTLC and HPLC following either partial hepatectomy or sham operation. Analysis of whole liver gangliosides by HPTLC demonstrated that within 6 h after partial (68%) hepatectomy, there was a significant increase in GM1 compared to both sham and control animals. By 48 h, GM1 was further increased and the polysialylgangliosides GD1a, GD1b and GT1b had also risen significantly, whereas changes in GM3 were negligible. Gangliosides associated with the plasma membrane were increased up to 3.5-fold in regenerating liver compared to sham-hepatectomized controls as assessed by HPLC. Although elevations in membrane gangliosides were associated with hepatocyte proliferation, they did not closely follow the growth curve. The time course of changes in ganglioside biosynthesis suggests differential upregulation of GM3 synthase and GD3 synthase in regenerating livers.

Neutrophil chemotactic factors derived from conjunctival epithelial cells: preliminary biochemical characterization.

We have previously reported on the release of neutrophil chemotactic factors (NCF) from injured conjunctival tissue. The present study was designed to biochemically characterize these conjunctiva-derived chemotactic factors and determine their biological activities. Bulbar conjunctiva was surgically removed from a rabbit eye and incubated with 250 microL of minimal essential medium (MEM) for 6 hours at 37 degrees C in a 5% CO2 atmosphere. Chemotactic activity was assayed using modified Boyden chambers with rabbit peritoneal neutrophils as indicator cells. Following treatment with subtilisin protease for 90 minutes, chemotactic activity of the conjunctival factors was reduced by 74%. Similarly, activity was lost after heating at 56 degrees C for 60 minutes (41% inhibition). Using ultrafiltration techniques, we showed that the majority of the chemotactic activity remained above a 100 kilodalton filter, suggesting the existence of high molecular weight factors. We also showed that the conjunctival factors are not glycoproteins and bind to both anion and cation exchange resins. When 100 microL of conjunctival supernatant was injected in the superior tarsal conjunctiva of rabbits, significant recruitment of neutrophils was evident by 4 hours. Control rabbits injected with MEM did not show neutrophil recruitment. Results of these studies indicate that NCF from traumatized conjunctival tissue are proteins (and not glycoproteins) of high molecular weight, heat labile, exhibit anionic and cationic charges, and are active in vivo.

Cornea derived neutrophil chemotactic factors: intracellular synthesis and release.

The present study was designed to test the hypothesis that the release of neutrophil chemotactic factors (NCF) from isolated corneas following hydrogen peroxide stimulation requires specific intracellular synthesis. For these studies, the epithelial surfaces of isolated rabbit corneas were preincubated with various inhibitors of protein synthesis (cycloheximide, 10 micrograms/ml, and puromycin, 50 micrograms/ml) and transcription (actinomycin D, 5 micrograms/ml) for 60 min prior to exposure of the corneas to glucose (G, 1 mg/ml) and glucose oxidase (GO, 20 U/ml) for 6 h at 37 degrees C. All three inhibitors decreased the levels of NCF recovered in the extracorneal fluids by 80-98%, suggesting that peroxide acts to upregulate NCF production at both the transcription and translation levels. When corneas were incubated with G/GO for 6 h at 12 degrees C or 4 degrees C instead of 37 degrees C, a reduction in the levels of NCF recovered in the supernatants was noted at 12 degrees C (46-91% inhibition) and at 4 degrees C (67-96% inhibition), suggesting that the synthesis of NCF at cold temperature was only reduced but not totally inhibited. To demonstrate that the observed reduction in chemotactic activity recovered from corneas incubated at 12 degrees C or 4 degrees C is not due to a temperature-dependent inhibition of NCF biosynthesis, but rather to a disruption of intracellular vesicular transport, temperature shift experiments were performed. Corneas were incubated with G/GO overnight at 12 degrees C or 4 degrees C prior to shifting to 37 degrees C for an additional 6 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of des arginine9-bradykinin and other bradykinin fragments on the synthesis of prostacyclin and the binding of bradykinin by vascular cells in culture.

Bradykinin (BK) fragments, des arg1-BK, des arg1,pro2-BK, des phe8,arg9-BK and des pro7,phe8,arg9-BK were synthesized and along with des arginine9-BK (daBK), tested for their ability to induce prostacyclin synthesis in homogeneous cultures of cells from the calf pulmonary artery. Of the fragments daBK was the only peptide, in addition to bradykinin (BK), to activate the synthesis of prostacyclin (PGI2) and platelet activating factor (PAF) in endothelial cells and PGI2 in fibroblasts and smooth muscle cells. Half-maximal activation of PGI2 synthesis differed with the cell type. The other fragments tested did not directly affect PGI2 synthesis. These fragments also did not inhibit daBK or BK activation of PG synthesis. BK bound to endothelial cells with a dissociation constant (Kd) of 2.1 nM and a Bmax of 47.9 fmoles/10(6) cells. The Kd for the binding of BK to smooth muscle cells and fibroblasts was somewhat higher, 4.9 nM and 7.9 nM, respectively. None of the fragments tested, including daBK, altered the binding of BK. Des arg9[leu8]-BK, reported to be a competitive antagonist of the bradykinin B1 receptor, inhibited daBK induced PG of PAF synthesis in endothelial cells but had little effect of BK binding or BK induced PG synthesis. Finally, the BK antagonist [thi5,8, d-phe7]-BK blocked both BK binding and the ability of either BK or daBK to induce PG synthesis, thus substantiating that the binding of these kinins is a step in the activation of PG synthesis.

Differential coupling of smooth muscle and liver vasopressin (V1) receptors to guanine nucleotide binding proteins.

We report the reconstitution of the smooth muscle vasopressin V1 receptor functionally coupled to a pertussis toxin-insensitive guanine nucleotide-binding protein. This V1 receptor was spontaneously coupled to this guanine nucleotide-binding protein upon solubilization in the absence of agonist, in contrast to our earlier report on the liver V1 receptor, which required agonist for coupling. The smooth muscle V1 receptor was reconstituted as a high affinity receptor (Kd = 5 nM), with a slow rate of agonist dissociation. Upon the addition of guanosine 5'-thiotriphosphate, there was a decrease in receptor affinity (Kd = 30 nM) concomitant with an increase in the rate of ligand dissociation. The ability of the smooth muscle V1 receptor to spontaneously couple to a guanine nucleotide-binding protein(s) suggests that in the absence of agonist it exists as a high affinity receptor. The smooth muscle V1 receptor may, therefore, be more sensitive to plasma concentrations of vasopressin than its liver homologue.

Purification and characterization of the rat liver vasopressin (V1) receptor.

Utilizing a proteoliposome reconstitution system, we have purified the rat liver V1 vasopressin receptor to near homogeneity. The receptor was purified approximately 21,000-fold from rat liver membranes, using differential detergent solubilization, size exclusion gel filtration, lectin affinity, and ion-exchange chromatography. The purified receptor exhibits a Kd of 6 nM, when, prior to solubilization, the membranes were exposed to 1 microM vasopressin. This resulted in the association of a pertussis toxin-insensitive guanine nucleotide-binding protein with the receptor during most of the purification procedure. In the absence of this association, the receptor had a Kd of approximately 30 nM. Association of the receptor with a G-protein was confirmed by the ability of vasopressin to stimulate the hydrolysis of [gamma-32P]GTP. The specific activity of the vasopressin-stimulated hydrolysis was 25 nmol/min/mg, approximately 8,000-fold higher than values obtained with crude reconstituted receptor preparations. Cross-linking of 125I-vasopressin to a partially purified preparation of receptor demonstrated that the receptor had a molecular weight of approximately 68,000 under reducing conditions, and 58,000 under nonreducing conditions. The purification procedure may prove useful in purifying a number of small peptide hormone receptors (e.g. bradykinin, angiotensin II) and perhaps their associated G-proteins as well.

Reconstitution of the rat liver vasopressin receptor coupled to guanine nucleotide-binding proteins.

The V1 vasopressin receptor has been solubilized from rat liver membranes with the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammoniol]-1-propanesulfonate (CHAPS) and reconstituted into phospholipid vesicles. There is essentially complete solubilization of the receptor by 3% CHAPS at a protein concentration of 15 mg/ml. Reconstitution into soybean phospholipid vesicles is readily achieved either by gel filtration chromatography or by membrane dialysis. The binding of [3H]vasopressin to proteoliposomes is specific, saturable, reversible, and magnesium-dependent. In contrast, the detergent-soluble vasopressin receptor does not display specific binding. The apparent affinity of the reconstituted receptor for [3H]vasopressin is approximately 4-fold lower than that of the receptor in native membranes. In addition, the binding of [3H]vasopressin to reconstituted vesicles is not sensitive to 100 microM guanosine 5'-O-thiotriphosphate (GTP gamma S) as it is in native membranes. However, the apparent affinity of the reconstituted receptor for ligand approximates that of native membranes when membranes are prebound with vasopressin prior to solubilization and reconstitution into vesicles. Furthermore, vesicles reconstituted from membranes prebound with vasopressin show GTP gamma S sensitivity of [3H] vasopressin binding. This finding strongly suggests that vasopressin stabilizes a receptor-G-protein complex during solubilization. The rat liver vasopressin receptor is a glycoprotein, as shown by its specific binding to the lectin "wheat germ agglutinin." The vasopressin receptor can be reconstituted from the N-acetylglucosamine-eluted peak of a wheat germ agglutinin-Sepharose column, and [3H] vasopressin binding activity is purified 5-6-fold from membranes by this chromatographic procedure. The functionality of the partially purified receptor is indicated by its ability to bind ligand with high affinity and by its ability to functionally interact with a G-protein when vasopressin is bound prior to solubilization.

Receptor-mediated transcytosis of transferrin across the blood-brain barrier.

The perfusion of rat brain with 125I-transferrin resulted in a receptor-mediated uptake of transferrin into the endothelium of the blood-brain barrier followed by its detection in the brain. During a pulse-chase experiment, 125I-transferrin accumulated in the endothelial cells during the pulse, with a decrease of this intraendothelial radioactivity during the chase associated with a concomitant increase in the nonvascular elements of the brain. The receptor-mediated movement of transferrin across the blood-brain barrier suggests that the brain may derive its iron through the transcytosis of iron-loaded transferrin across the brain microvasculature. We discuss the likelihood that aluminum and other potentially toxic heavy metals, which also bind tightly to transferrin, may enter the brain by this pathway. We also discuss the possibility that other large molecules including neuroactive peptides and neurotrophic viruses may enter the brain through a similar receptor-mediated, vesicular transcytotic route.

A trans Golgi-derived exocytic coated vesicle can contain both newly synthesized cholinesterase and internalized transferrin.

We used a cholinesterase-mediated density shift protocol to investigate the movement of internalized transferrin (Tf) through endo- and exocytic coated vesicles (CVs) in the perfused rat liver. Upon internalization, exogenous 125I-Tf was found in endocytic CVs but not in cholinesterase-containing (i.e., exocytic) CVs (0-40 min). Between 1 and 2 hr, 125I-Tf began to appear in exocytic CVs. The origin of the exocytic CV was further investigated. After perfusion of the liver with asialotransferrin, the exocytic CVs were shown to contain resialylated Tf, indicating that the trans Golgi was the origin of this class of CVs. The resialylated Tf accumulated in the extracellular medium with kinetics very similar to the time course for appearance of Tf in cholinesterase-containing, exocytic CVs, suggesting that these CVs are directly involved in the transfer of material from the trans Golgi to the cell surface.

The sequential transfer of internalized, cell surface sialoglycoconjugates through the lysosomes and Golgi complex in HeLa cells.

Surface sialoglycoproteins of HeLa cells were labeled by NaB[3H]4 reduction after oxidation with NaIO4, yielding seven major radioactive bands as visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. When labeled cells are reincubated in growth medium, all of these major classes of glycoproteins are internalized and all but one (105 kDa) are recycled, i.e. subsequently reappear on the surface. The surface-labeling patterns over time remain qualitatively similar, but changes in relative specific activity of the bands suggest some preferential degradation of individual glycoproteins. Analytical fractionation at various time points after labeling suggests that the surface molecules pass through the lysosomal compartment and subsequently accumulate in the Golgi and Golgi-related compartments before returning to the surface. Inhibition of lysosomal function with chloroquine or NH4Cl prevents the accumulation and subsequent recycling. The pathway is confirmed with preparative fractionation into surface membrane, prelysosomal, lysosomal, Golgi, and Golgi-related compartments. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis demonstrates a degree of preferential handling of the glycoproteins on this pathway, e.g. the 180-kDa band is relatively reduced at the endocytic/prelysosomal stage and the 105-kDa band appears to be degraded in its first passage through the lysosomes. The other bands recycle 10-20 times before being degraded.