Fractional exponential decay of a membrane protein population due to capture by coated pits

We consider the lateral diffusion of receptors, or other membrane proteins, in the outer membranes of certain cells, and their capture by coated pits. It is shown, for the case in which the coated pits are in fixed random positions, that the long-time decay of the total number of uncaptured proteins is of the fractional exponential form, N(t) = N0 exp [-square root of (t/tau)], and not of the pure exponential form, N(t) = N0 exp(-t/tau), which is usually assumed.     

Effect of preferential insertion of LDL receptors near coated pits

Recent experiments suggest that low density lipoprotein (LDL) receptors on human fibroblasts are not inserted into the plasma membrane uniformly, as earlier experiments indicated, but are inserted into specialized regions, called plaques, where coated pits form. If the consequent reduction in the time required for LDL receptors to diffuse to coated pits were significant, this could alter conclusions drawn from previous calculations based on the assumption that LDL receptors are inserted uniformly. In particular, the conclusion could be wrong that diffusion of LDL receptors to coated pits is the rate limiting step in the interaction of cell surface LDL receptors with coated pits. Here we calculate the extent of the reduction in mean travel time of an LDL receptor to a coated pit, as a function of the plaque radius. We find that only if LDL receptor insertion is limited to a very small portion of the plasma membrane near coated pit sites is there a substantial decrease in the average time it would take an LDL receptor to diffuse to a coated pit. In order for preferential insertion of LDL receptors into plaques to cut the mean receptor travel time in half, plaques would have to take up no more than 10% of the cell surface area; to reduce the travel time by a factor of 10 plaques would have to cover only 2% of the cell surface, approximately twice the area covered by coated pits at 37°C.     

Effect of preferential insertion of LDL receptors near coated pits

Recent experiments suggest that low density lipoprotein (LDL) receptors on human fibroblasts are not inserted into the plasma membrane uniformly, as earlier experiments indicated, but are inserted into specialized regions, called plaques, where coated pits form. If the consequent reduction in the time required for LDL receptors to diffuse to coated pits were significant, this could alter conclusions drawn from previous calculations based on the assumption that LDL receptors are inserted uniformly. In particular, the conclusion could be wrong that diffusion of LDL receptors to coated pits is the rate limiting step in the interaction of cell surface LDL receptors with coated pits. Here we calculate the extent of the reduction in mean travel time of an LDL receptor to a coated pit, as a function of the plaque radius. We find that only if LDL receptor insertion is limited to a very small portion of the plasma membrane near coated pit sites is there a substantial decrease in the average time it would take an LDL receptor to diffuse to a coated pit. In order for preferential insertion of LDL receptors into plaques to cut the mean receptor travel time in half, plaques would have to take up no more than 10% of the cell surface area; to reduce the travel time by a factor of 10, plaques would have to cover only 2% of the cell surface, approximately twice the area covered by coated pits at 37 degrees C.     

Distribution of ferritin receptors and coated pits on giant HeLa cells.

HeLa cells bind horse spleen ferritin when the two are incubated at 0 degrees C. Since the majority of this bound ferritin is located in coated pits, we conclude that the ferritin binds to a specific receptor which takes part in an endocytic cycle. When substrate-attached and well-spread giant HeLa cells are briefly labelled at 0 degrees C with ferritin, ferritin particles are found to be concentrated towards the cell periphery, where they exist largely outside coated pits. This peripheral concentration is a property of circulating (and not just newly synthesized) receptors because it is not affected by prior incubation of giant cells in cycloheximide. However, coated pits are themselves roughly uniformly distributed over the surface of these cells. These results provide evidence that the membrane internalised by coated pits on these cells is returned to the cell surface at the leading edge of the cell. Because of this separation of the sites of endocytosis and exocytosis, a flow of membrane must occur across the cell surface. This flow is composed of lipid plus receptors. The implications of this for capping and for cell spreading are discussed.     

The effect of diffusion on the binding of membrane-bound receptors to coated pits.

We have formulated a kinetic model for the primary steps that occur at the cell membrane during receptor-mediated endocytosis. This model includes the diffusion of receptor molecules, the binding of receptors to coated pits, the loss of coated pits by invagination, and random reinsertion of receptors and coated pits. Using the mechanistic statistical theory of nonequilibrium thermodynamics, we employ this mechanism to calculate the two-dimensional radial distribution of receptors around coated pits at steady state. From this we obtain an equation that describes the effect of receptor diffusion on the rate of binding to coated pits. Our equation does not assume that ligand binding is instantaneous and can be used to assess the effect of diffusion on the binding rate. Using experimental data for low density lipoprotein receptors on fibroblast cells, we conclude that the effect of diffusion on the binding of these receptors to coated pits is no more than 84% diffusion controlled. This corresponds to a dissociation rate constant for receptors on coated pits (k-) that is much less than the rate constant for invagination of the pits (lambda = 3.3 X 10(-3)/s) and a correlation length for the radial distribution function of six times the radius of a coated pit. Although the existing experimental data are compatible with any value of k-, we obtain a lower bound for the value of the binding constant (k+) of 2.3 X 10(-2)(micron)2/s. Comparison of the predicted radial distributions with experiment should provide a clear indication of the effect of diffusion on k+.     

The effect of diffusion on the trapping of membrane-bound receptors by localized coated pits

Localized coated pits are considered in the primary steps of receptor-mediated endocytosis. According to the pit reinsertion mechanism, we have modified our previous kinetic model and studied the effect of diffusion on the trapping rate constant (k+). Using experimental data for low density lipoprotein (LDL) receptors on fibroblast cells, we found that the binding of receptors to coated pits is not totally diffusion controlled. For example, the process is less than 78% diffusion controlled if receptors are not allowed to escape the coated pits. However, due to the large uncertainties in the experimental parameters, a diffusion-controlled process cannot be ruled out. The greatest differences between localized and random reinsertion were found when the escaping rate constant (k-) is much greater than the rate constant for invagination of the pits (lambda 1). Under this condition, k+ for localized reinsertion is no less than 39% diffusion controlled, while k+ for random reinsertion shows no diffusion effect at all.     

Receptors compete for adaptors found in plasma membrane coated pits.

An affinity matrix of LDL receptor cytoplasmic tails binds the HA-II 100/50/16 kd complexes found in plasma membrane coated pits. Other receptors (or their cytoplasmic domains), which are localized in coated pits during endocytosis, inhibit this binding. This includes an 8 residue peptide containing tyrosine, corresponding to the cytoplasmic portion of a mutant influenza haemagglutinin. In contrast, the equivalent peptide lacking tyrosine (like the tail of the native haemagglutinin, a protein excluded from coated pits) does not compete. These results imply that the HA-II complex has a recognition site for a common signal, probably involving a tyrosine residue, carried by the LDL receptor and competing receptors also found in plasma membrane coated pits. The HA-II complex therefore fulfils the role of an 'adaptor', the name proposed for the structural units which mediate the binding of clathrin to receptors in coated vesicles. Another related complex, the HA-I adaptor, which is restricted to Golgi coated pits, probably does not recognize the 'tyrosine signal' on the LDL receptor tail. The HA-I adaptor is likely to contain a recognition site for a different signal carried by receptors, e.g. the mannose-6-phosphate receptor, which are found in Golgi coated pits.     

Diffusion-limited forward rate constants in two dimensions. Application to the trapping of cell surface receptors by coated pits.

A variety of receptors are known to aggregate in specialized cell surface structures called coated pits, prior to being internalized when the coated pits close off. At 37 degrees C on human fibroblasts, as well as on other cell types, a recycling process maintains a constant number of coated pits on the cell surface. In this paper, we explore implications for receptor aggregation and internalization of the two types of recycling models that have been proposed for the maintenance of the coated pit concentration. In one model, coated pits alternate between accessible and inaccessible states at fixed locations on the cell surface, while in the other model, coated pits recycle to random locations on the cell surface. We consider receptors that are randomly inserted in the membrane, move by pure diffusion with diffusion coefficient D, and are instantly and irreversibly trapped when they reach a coated pit boundary (the diffusion limit). For such receptors, we calculate for each of the two models: the mean time tau to reach a coated pit, the forward rate constant k+ for the interaction of a receptor with a coated pit, and the fraction phi of receptors aggregated in coated pits. We show that for the parameters that characterize coated pits on human fibroblasts, the way in which coated pits return to the surface has a negligible effect on the values of tau, k+, and phi for mobile receptors, D greater than or equal to 1.0 X 10(-11) cm2/s, but has a substantial effect for "immobile" receptors, D much less than 1 X 10(-11) cm2/s. We present numerical examples to show that it may be possible to distinguish between these models if one can monitor slowly diffusing receptors (D less than 1 X 10(-11) cm2/s) on cells whose coated pits have relatively short lifetimes (less than or equal to 1 min). Finally, we show that for the low-density lipoprotein (LDL) receptor on human fibroblasts (D = 4.5 X 10(-11) cm2/s), the predicted and observed values of K+ and phi are in close agreement. Therefore, even for slowly diffusing LDL receptor, unaided diffusion as the transport mechanism of receptors to coated pits is consistent with measured rates of LDL internalization.     

Visualization of the membrane bound cytoskeleton and coated pits of plant cells by means of dry cleaving

Summary A technique allowing the visualization of the membrane bound cytoskeleton of differentiating cells in root tips is described. The technique, called dry cleaving, consists in cleaving critical point dried cells on grids by means of adhesive tape. The preparations show a three-dimensional cytoskeletal network of 5–10nm filaments and microtubules resembling the membrane bound cytoskeleton of animal cells. In addition, many coated pits and vesicles can be observed on the membrane.Abbreviations EM electron microscope/electron microscopy - GA glutaraldehyde - PBS phosphate buffer saline - PFA paraformaldehyde     

Inhibition of endocytosis from coated pits by acidification of the cytosol

Binding and endocytosis of the ligands transferrin, epidermal growth factor (EGF), and ricin were measured in a number of different cell lines after treatment of cells with compounds that react with SH-groups and under conditions where the cytosolic pH was lowered. N-ethylmalemide and diamide irreversibly inhibited endocytosis of all ligands tested, whereas low pH in the cytosol strongly inhibited endocytosis of transferrin and EGF. Data obtained by electron microscopy indicated that the formation of coated vesicles from coated pits is inhibited in acidified cells. Entry of ricin was much less affected, and ricin endocytosed under these conditions was able to intoxicate the cells. At low pH in the cytosol there was a calcium-dependent increase in the number of transferrin receptors at the cell surface. The increase was even larger in the presence of the calcium ionophore A23187, whereas it was completely blocked by the calmodulin antagonists trifluoperazine and W7. The results show that endocytosis from coated pits can be inhibited in a reversible way by acidification of the cytosol and they suggest that a second pathway of endocytosis exists, possibly involving formation of vesicles from uncoated areas of the membrane.     

Recruitment of epidermal growth factor receptors into coated pits requires their activated tyrosine kinase

EGF-receptor (EGF-R) tyrosine kinase is required for the down- regulation of activated EGF-R. However, controversy exists as to whether ligand-induced activation of the EGF-R tyrosine kinase is required for internalization or for lysosomal targeting. We have addressed this issue using a cell-free assay that selectively measures the recruitment of EGF-R into coated pits. Here we show that EGF bound to wild-type receptors is efficiently sequestered in coated pits. In contrast, sequestration of kinase-deficient receptors occurs inefficiently and at the same basal rate of endocytosis of unoccupied receptors or receptors lacking any cytoplasmic domain. Sequestration of deletion mutants of the EGF-R that lack autophosphorylation sites also requires an active tyrosine kinase. This suggests that a tyrosine kinase substrate(s) other than the EGF-R itself, is required for its efficient ligand-induced recruitment into coated pits. Addition of a soluble EGF-R tyrosine kinase fully and specifically restores the recruitment of kinase-deficient EGF-R into coated pits providing a powerful functional assay for identification of these substrate(s).     

Redistribution of cell surface transferrin receptors prior to their concentration in coated pits as revealed by immunoferritin labels

Summary Immunocytochemistry has been used to study distribution of cell surface transferrin receptors in erythroid, leukemic (K562) cells. The cells were fixed and labelled with monoclonal (OKT-9) anti-transferrin receptor antibodies; the antibody-labelled receptors were then detected by either immunofluoresceinor immunoferritin-antimouse-antibody conjugates. Typically, the immunoferritin labels were distributed diffusely at the non-coated regions of the cell surface as well as concentrated in the clathrincoated pits. To examine further this pattern of distribution, cells were labelled at 0° C and then warmed to 37° C for zero to 30 min prior to fixation. The majority of the immunoferritin labels were initially dispersed in small groups at the non-coated regions of the cell surface (mean = 6 immunoferritin labels/cluster), but larger groups were common subsequent to incubation at 37° C (mean = 13 immunoferritin labels/cluster). However, the size of immunoferritin labels in the coated pits was unchanged (mean = 12 immunoferritin labels/pit). Immunoferritin labels were typical in coated and uncoated vesicles l min after warming to 37° C, but common in endosomes, multivesicular bodies and lysosomes by 30 min. It appears that single cell-surface receptors form large aggregates prior to their concentration in coated pits. Coated vesicles, uncoated vesicles, and endosomal vacuoles may together form the non-lysosomal compartment where the internalized receptors might be dissociated from the ligands (antibodies).     

Recruitment of epidermal growth factor and transferrin receptors into coated pits in vitro: differing biochemical requirements.

The biochemical requirements for epidermal growth factor (EGF) and transferrin receptor-mediated endocytosis were compared using perforated human A431 cells. Morphological studies showed that horseradish peroxidase (HRP)-conjugated EGF and gold-labeled antitransferrin (Tfn) receptor antibodies were colocalized during endocytosis in vitro. The sequestration of both ligands into deeply invaginated coated pits required ATP hydrolysis and cytosolic factors and was inhibited by GTP gamma S, indicating mechanistic similarities. Importantly, several differences in the biochemical requirements for sequestration of EGF and Tfn were also detected. These included differing requirements for soluble AP (clathrin assembly protein) complexes, differing cytosolic requirements, and differing sensitivities to the tyrosine kinase inhibitor, genistein. The biochemical differences detected between EGF and Tfn sequestration most likely reflect specific requirements for the recruitment of EGF-receptors (R) into coated pits. This assay provides a novel means to identify the molecular bases for these biochemical distinctions and to elucidate the mechanisms involved in ligand-induced recruitment of EGF-R into coated pits.     

Cross-linking of plasmalemmal cholesterol in lymphocytes induces capping, membrane shedding, and endocytosis through coated pits.

By use of a nicked and biotinylated perfringolysin O (BCtheta), which binds to cholesterol specifically, we studied consequences of cross-linking cholesterol in lymphocytes. When bound with BCtheta and then with labeled avidin or streptavidin, capping occurred in most cells within 30 min at 37 degrees C. It was inhibited by cytochalasin D or NaN3, but not by nocodazole. When BCtheta-cholesterol was capped, Thy-1 and transferrin receptor, a GPI-anchored protein and a transmembrane protein, respectively, remained evenly distributed. By fluorescence and electron microscopy, a cluster of small vesicles bound with BCtheta were observed in the cap. They were then shed in the medium or internalized through coated pits. The result indicates that cross-linking of cholesterol in lymphocytes induces capping, but does not affect distribution of membrane proteins, and that the capped cholesterol molecules are either shed as vesicles or endocytosed.     

Exclusion of erythrocyte-specific membrane proteins from clathrin- coated pits during differentiation of human erythroleukemic cells

When human erythroleukemic cells are induced to differentiate, they produce globin and redistribute glycophorin and spectrin to one pole of the cell. This process was accompanied by an alteration in the clathrin-coated pits at the cell surface. In nondifferentiating cells, receptors for Concanavalin A have been shown, using electron microscopy, to be concentrated into coated pits and rapidly internalized. Glycophorin was also internalized via coated pits, but was not greatly concentrated into these portions of the surface membrane. Ligands attached to glycophorin were, therefore, cleared from the cell surface more slowly than Concanavalin A. In nondifferentiating cells, immunoelectron microscopy showed that spectrin is largely excluded from coated pits. After erythroid differentiation proceeded for several days, glycophorin was totally excluded from the coated pits along with spectrin. This did not reflect a general cessation of endocytosis, however, because Concanavalin A receptors continued to be internalized. It is possible that the specific exclusion of glycophorin from coated pits is part of the remodeling process that occurs when the precursor cell membrane differentiates into that of the mature erythrocyte.     

Acidification of the cytosol inhibits endocytosis from coated pits

Acidification of the cytosol of a number of different cell lines strongly reduced the endocytic uptake of transferrin and epidermal growth factor. The number of transferrin binding sites at the cell surface was increased in acidified cells. Electron microscopic studies showed that the number of coated pits at the cell surface was not reduced in cells with acidified cytosol. Experiments with transferrin-horseradish peroxidase conjugates and a monoclonal anti-transferrin receptor antibody demonstrated that transferrin receptors were present in approximately 75% of the coated pits both in control cells and in cells with acidified cytosol. The data therefore indicate that the reason for the reduced endocytic uptake of transferrin at internal pH less than 6.5 is an inhibition of the pinching off of coated vesicles. In contrast, acidification of the cytosol had only little effect on the uptake of ricin and the fluid phase marker lucifer yellow. Ricin endocytosed by cells with acidified cytosol exhibited full toxic effect on the cells. Although the pathway of this uptake in acidified cells remains uncertain, some coated pits may still be involved. However, the data are also consistent with the possibility that an alternative endocytic pathway involving smooth (uncoated) pits exists.     

Independent variation in the number of coated pits and of coated vesicles in cultured fibroblasts

When tissue culture cells were maintained at 37 degrees C in a serum-free medium for 4 hr no change in the number of coated pits could be detected using ultrastructural techniques. However, the number of coated vesicles was highly significantly increased, being 179% more than in the control cultures. If the cells were put back into a medium supplemented with 5% calf serum, the number of coated pits was unchanged, but the number of coated vesicles decreased and returned to the control level within a few minutes. The same results were obtained when using ligands such as Low Density Lipoprotein or alpha-2-macroglobulin which are known to be internalized via coated structures. It is concluded that coated pits appear and disappear at equal rates and that coated vesicles can accumulate independently. It is suggested that this could be due to the presence of a large reserve of soluble clathrin. This pool would have a low turnover rate because cycloheximide did not block coated vesicle accumulation over the period studied.     

Endocytosis via coated pits mediated by glycoprotein receptor in which the cytoplasmic tail is replaced by unrelated sequences.

Rat 6 fibroblast cell lines expressing wild-type chicken liver glycoprotein receptor (CHL) or chimeric receptors with alternate cytoplasmic tails were produced to study the role of the cytoplasmic tail in mediating receptor localization in coated pits and endocytosis of ligand. Cells expressing CHL or cells expressing a hybrid receptor that contains the cytoplasmic tail of the asialoglycoprotein receptor display high-efficiency endocytosis of N-acetylglucosamine-conjugated bovine serum albumin in experiments designed to measure an initial internalization step, as well as in studies of continuous uptake and degradation. Substitution of the cytoplasmic tail by the equivalent domain of rat Na,K-ATPase beta subunit or by a stretch of Xenopus laevis globin beta chain does not abolish endocytosis but decreases the endocytosis rate constant from 15%-16%/min to 2.4% and 6.5%/min, respectively. Electron microscopy was used to visualize the glycoprotein binding sites at the surface of Rat 6 cells transfected with the various receptors. The percentage of receptors found in coated areas ranged from 32% for CHL to 9% for the Na,K-ATPase hybrid, indicating that clustering in coated pits correlates with efficiency of endocytosis. We concluded that replacement of the CHL cytoplasmic tail with unrelated sequences does not prevent, but decreases to varying extents, coated-pit localization and endocytosis efficiency. The construct with NH2-terminal globin tail lacks a signal for high-efficiency localization in coated pits but nevertheless is directed to the pits by an alternative mechanism.     

Apical membrane endocytosis via coated pits is stimulated by removal of antidiuretic hormone from isolated,perfused rabbit cortical collecting tubule

Summary Antidiuretic hormone increases the water permeability of the cortical collecting tubule and causes the appearance of intramembrane particle aggregates in the apical plasma membrane of principal cells. Particle aggregates are located in apical membrane coated pits during stimulation of collecting ducts with ADHin situ. Removal of ADH causes a rapid decline in water permeability. We evaluated apical membrane retrieval associated with removal of ADH by studying the endocytosis of horseradish peroxidase (HRP) from an isotonic solution in the lumen. HRP uptake was quantified enzymatically and its intracellular distribution examined by electron microscopy. When tubules were perfused with HRP for 20 min in the absence of ADH, HRP uptake was 0.5±0.3 pg/min/m tubule length (n=6). The uptake of HRP in tubules exposed continuously to ADH during the 20-min HRP perfusion period was 1.3±0.8 pg/min/m (n=8). HPR uptake increased markedly to 3.2±1.1 pg/min/m (n=14), when the 20-min period of perfusion with HRP began immediately after removal of ADH from the peritubular bath. Endocytosis of HRP occurred in both principal and intercalated cells via apical membrane coated pits. We suggest that the rapid decline in cortical collecting duct water permeability which occurs following removal of ADH is mediated by retrieval of water permeable membrane via coated pits.