Conserved cassette structure of vertebrate Mr 300 kDa mannose 6-phosphate receptors: partial cDNA sequence of fish MPR 300.

The existence of two homologous mannose 6-phosphate receptors (MPRs) with overlapping, but distinct functions has raised the question of at what stage in the phylogenetic tree the two receptors have occurred for the first time. In this paper, we present a partial cDNA sequence of Mr 300 kDa MPR (MPR 300) from poeciliid fish (Xiphophorus). It contains a 5'-untranslated region followed by the initiator ATG, and an open reading frame that corresponds to cassettes 1-5 and part of cassette 6 of mammalian MPR 300. The size of the mRNA transcript for fish MPR 300 was comparable with that of other vertebrates. The amino acid sequence of fish MPR 300 displays 48-52% similarity with mammalian and chicken MPR 300. In particular, all the cysteine residues involved in disulfide bonding and an arginine residue, which is considered to be part of the mannose 6-phosphate binding site in cassette 3 of mammalian MPR 300, are conserved. Sequence similarities were significantly higher within cassette 3 and within cassette 5, to which a ligand-binding function has not yet been ascribed. Sequence similarities of the internal cassettes of MPR 300 are discussed with regard to the multifunctional nature of MPR 300.

The orientation of membrane proteins determined in situ by immunofluorescence staining.

Structural and functional characterization of membrane proteins includes the determination of their orientation within the membrane (integral proteins), or their exposure at either the cytosolic or extracytoplasmic surface of the membrane (peripheral proteins). We have developed an easily handled immunofluorescence-based method to investigate the exposure of antigenic epitopes at either surface of the membranes in situ. We present conditions for permeabilization of p-formaldehyde-fixed cells which allow the discrimination of epitopes exposed either at the cytosolic face of membranes, within the lumen of vesicles, or at the cell surface. The potential applications of this method include (1) the use of domain-specific antibodies as a tool to study integral membrane proteins with regard to the orientation of their carboxy-terminal and amino-terminal ends or the orientation of the loops of multispanning proteins, and (2) the assignment of the epitope of monoclonal antibodies to the cytosolic or luminal domain of integral membrane proteins with the established structure.

Mannose 6-phosphate receptors (MPR 300 and MPR 46) from a teleostean fish (trout).

Mannose 6-phosphate receptors (MPRs) are known to occur in mammals, birds, reptiles and amphibians. Here we provide evidence for the presence of two MPRs in fish, the earliest vertebrates. Using phosphomannan-Sepharose affinity chromatography, MPR 300 was purified from liver membrane extract of trout. The purified trout liver MPR 300 showed similar electrophoretic mobility as the goat liver receptor and a pH optimum of 7.0 for binding to phosphomannan. The presence of MPR 46 in fish was shown by metabolically labelling embryonic fish cells (Xiphophorus) and immunoprecipitation with an antibody against the cytoplasmic tail of human MPR 46 (anti-MSC1). This antibody had recently been shown to immunoprecipitate MPR 46 also from reptiles and amphibians.

Identification of the putative mannose 6-phosphate receptor protein (MPR 300) in the invertebrate unio.

In mammals, Mannose 6-phosphate receptor proteins (MPR 300 and MPR46) mediate transport of lysosomal enzymes to lysosomes. Both receptors have been found in non-mammalian vertebrates including fish. To investigate the presence of MPRs in invertebrates, MPR 300 protein was isolated from the mollusc unio by affinity chromatography. It was shown to exhibit biochemical and immunological properties similar to mammalian MPR 300.

Identification of three internalization sequences in the cytoplasmic tail of the 46 kDa mannose 6-phosphate receptor.

The cytoplasmic tail of the human 46 kDa mannose 6-phosphate receptor (MPR 46) is necessary for rapid internalization of the receptor and sufficient to mediate internalization of a resident plasma membrane protein. To localize the internalization sequences within the 67 amino acids of the cytoplasmic tail, the tail was progressively shortened from its C-terminus, internal deletions of between four and eight amino acids were introduced into the tail, and individual residues were substituted by alanine, glycine or serine. Three sequences were identified that contribute to the internalization of MPR 46. The first is located within the 23 juxtamembrane cytoplasmic residues of the tail. It contains four essential residues within a heptapeptide and does not resemble known internalization signals. The second sequence contains as a critical residue Tyr-45. The third region is located within the C-terminal seven residues and contains a di-leucine pair as essential residues. The first and third sequences were shown to function as autonomous internalization sequences. Substitution of critically important residues within a single internalization sequence was tolerated, with no or only a moderate decrease in the internalization rate. When essential residues from two or all three internalization sequences were substituted, however, the internalization rate was decreased by more than 60% and 90% respectively. This indicates that the autonomous internalization signals in the cytoplasmic tail of MPR 46 function in an additive manner, but are partly redundant.

The 46-kDa mannose 6-phosphate receptor contains multiple binding sites for clathrin adaptors.

The two known mannose 6-phosphate receptors (MPR46 and MPR300) both mediate the transport of Man-6-P-containing lysosomal proteins to lysosomes. However, the MPRs cannot be detected in lysosomes, instead they recycle between the plasma membrane and endosomes and between endosomes and the trans-Golgi network. Both, endocytosis from the plasma membrane and budding of transport vesicles from the trans-Golgi network involves the interaction of the receptor with the clathrin-coated vesicles-associated protein complexes AP1 and AP2. We have analyzed this interaction between the Golgi-restricted AP1 complex and the plasma membrane-restricted AP2 complex with the MPR46 tail in vitro by using a biosensor. AP1 and AP2 both bind to and dissociate from the MPR46 tail with similar kinetics. Using synthetic peptides corresponding to different MPR receptor tail regions in inhibition and binding studies, a common high affinity binding site for AP1 and AP2 and two separate high affinity binding sites for AP1 and AP2, respectively, were identified.

Is an ATPase involved in uncoating of plasma membrane adaptor complex AP-2?

The coat of clathrin-coated vesicles mostly consists of clathrin and adaptor complexes AP-1 or AP-2. Clathrin is released from the vesicles in an ATP-dependent fashion prior to their fusion with endosomes. In the present study we found that ATP strongly inhibits in vitro binding of cytosolic AP-2 to membranes of stripped vesicles, and promotes the release of endogenous AP-2 from clathrin-deprived coated vesicles. Both effects required hydrolysis of ATP. In contrast, binding of AP-1 to stripped vesicles was not affected by ATP, but was enhanced by GTP-gamma-S. These results point to an ATPase that promotes the release of AP-2 from clathrin-coated vesicles.

The carboxy-terminal peptides of 46 kDa and 300 kDa mannose 6-phosphate receptors share partial sequence homology and contain information for sorting in the early endosomal pathway.

Recycling of mannose 6-phosphate receptors was investigated by microinjection of F(ab) fragments against their carboxy-terminal peptides (residues 54-67 or 150-164 of the cytoplasmic domain of 46 kDa and 300 kDa mannose 6-phosphate receptor, respectively). For each receptor, masking the carboxy-terminal peptide by the corresponding F(ab) fragments resulted in complete depletion of the intracellular pool. Redistributed 300 kDa mannose 6-phosphate receptor was shown to accumulate at the plasma membrane and to internalize anti-ectodomain antibodies. Internalization of anti-ectodomain antibodies was also observed for redistributed 46 kDa mannose 6-phosphate receptor. Semiquantitative analysis suggested that for both redistributed receptors the amount of intracellularly accumulated anti-ectodomain antibodies was reduced. In addition, downstream transport along the endosomal pathway was slowed down. These data suggest that sorting information for early steps in the endocytic pathway is contained within the carboxy-terminal peptides of mannose 6-phosphate receptors.

Expression of mannose 6-phosphate receptors in chicken.

In mammals, the sorting of newly synthesized lysosomal enzymes is accomplished by two mannose 6-phosphate receptors (MPR) designated MPR46 and MPR300. MPR300 has an additional function in clearing the nonglycosylated insulin-like growth factor II (IGFII). The distinct expression pattern of the two MPR has been ascribed to the control of MPR300 expression by IGFII. In lower vertebrates, such as chickens or frogs, only MPR300 homologues have been described. These MPR300 homologues do not bind IGFII. In the present study, we examined whether lower vertebrates such as chickens also express two types of MPR and, if so, whether the expression pattern is distinct or similar. We were able to clone chicken cDNA fragments homologous to mammalian MPR46 and MPR300 and to show the synthesis of respective MPR polypeptides, thus establishing the existence of two types of MPR also in a nonmammalian species. Further, we analyzed the expression of the two MPR in chicken by Northern blotting and in situ hybridization. High levels of MPR46 and MPR300 RNA were detectable in epithelia, ganglia, and uropoietic system of chicken embryos. In a number of embryonic and adult tissues, varying ratios of MPR46 and MPR300 RNA were observed. The expression pattern for both MPR46 and MPR300 was distinct, although less pronounced than in mice. We conclude that functional differences unrelated to the additional function of the mammalian MPR300 as a receptor clearing IGFII are responsible for the distinct expression of the two MPR in nonmammalian, and probably also in mammalian, species.

The oligomeric state of 46-kDa mannose 6-phosphate receptor does not change upon intracellular recycling and binding of ligands.

46-kDa mannose-6-phosphate receptor forms homooligomers in cell membranes and in detergent solution. The quaternary structure of detergent-solubilized 46-kDa mannose 6-phosphate receptor is regulated by the presence of ligands, pH and receptor concentration [Waheed, A. & von Figura, K. (1990) Eur. J. Biochem. 193, 47-54). To find out whether the intracellular recycling of 46-kDa mannose 6-phosphate receptor is accompanied by changes in its quaternary structure, we have performed chemical cross-linking in membranes of intact cells. In all conditions tested, the dimer was the predominating form (more than 67% of total 46-kDa mannose 6-phosphate receptor). The amount of trimeric and tetrameric forms varied among cell lines and contributed up to 20% of total endogenous 46-kDa mannose 6-phosphate receptor in human and mouse fibroblasts. Within a given cell line, the ratio of the oligomers was not significantly changed upon elevating endosomal pH by bafilomycin A1, upon changes in receptor occupancy (treatment of cells with tunicamycin or use of mouse fibroblasts deficient in 300-kDa mannose 6-phosphate receptor), nor upon depletion of adaptors from clathrin-coated vesicles of the trans Golgi network by brefeldin A. At the cell surface, where 46-kDa mannose 6-phosphate receptor does not bind ligands, the percentage of dimer was similar to that observed intracellularly. Thus, the oligomeric state of 46-kDa mannose 6-phosphate receptor apparently does not change during recycling as well as binding and dissociation of ligands. In view of the abundance of the dimer of 46-kDa mannose 6-phosphate receptor in situ, our data suggest that it represents the main physiologically active form of the receptor, and therefore present indirect evidence that binding of ligands to 46-kDa mannose 6-phosphate receptor is probably regulated by conformational changes of receptor or ligand rather than by changes in the quaternary structure.

Biosynthetic transport of the asialoglycoprotein receptor H1 to the cell surface occurs via endosomes.

Signals for endocytosis and for basolateral and lysosomal sorting are closely related in a number of membrane proteins, suggesting similar sorting mechanisms at the plasma membrane and in the trans-Golgi network (TGN). We tested the hypothesis that basolateral membrane proteins are transported to the cell surface via endosomes for the asialoglycoprotein receptor H1. This protein was tagged with a tyrosine sulfation site (H1TS) to allow specific labeling with [35S]sulfate in the TGN. Madin-Darby canine kidney cells expressing H1TS were pulse-labeled and chased for a period of time insufficient for labeled H1TS to reach the cell surface. Upon homogenization and gradient centrifugation, fractions devoid of TGN were subjected to immunoisolation of compartments containing mannose 6-phosphate receptor, which served as an endosomal marker. H1TS in transit to the cell surface was efficiently coisolated, whereas a labeled secretory protein and free glycosaminoglycan chains were not. This indicates an indirect pathway for the asialoglycoprotein receptor to the plasma membrane via endosomes and has important implications for protein sorting in the TGN and endosomes.

Mannose 6-phosphate receptors in sorting and transport of lysosomal enzymes.

Mannose 6-phosphate receptors have been intensively studied with regard to their genomic organization, protein structure, ligand binding properties, intracellular trafficking and sorting functions. That their main function is sorting of newly synthesized lysosomal enzymes is commonly accepted, but much more remains to be learned about their precise recycling pathways and the mechanisms which regulate their vesicular transport. Additional functions have been reported, e.g., export of newly synthesized lysosomal enzymes from the cell by MPR 46 or a--probably indirect--participation in growth factor-mediated signal transduction by MPR 300. To understand the physiological relevance of these observations will be a challenge for future research.

Proteolytic processing of cathepsin D in prelysosomal organelles.

The initial steps of proteolytic processing of newly synthesized cathepsin D were investigated in prelysosomal membranes, which were defined by their contents of 300 kDa mannose 6-phosphate receptor (MPR 300). MPR 300-containing vesicles were immuno-isolated from a postmitochondrial supernatant of HepG2 cells using a peptide-specific antibody directed against the 15 C-terminal amino acids of the cytoplasmic domain of MPR 300. In the immunoisolated fraction, MPR 300 was enriched 11.5-fold over [35S]polypeptides, 29-fold over the lysosomal marker beta-hexosaminidase and 4.5-fold over the trans Golgi marker galactosyltransferase, when referred to the postmitochondrial supernatant. MPR 300-containing vesicles harbored, on average, 12% of the cathepsin D precursor from the postmitochondrial supernatant, suggesting that segregation of MPR 300 and cathepsin D occurs rapidly in prelysosomal organelles. Detection of low, but significant amount of mature cathepsin D in the immunoisolated fraction suggests that proteolytic processing is initiated in MPR 300-containing vesicles or in tightly associated prelysosomal vesicles, which are distinct from mature lysosomes.

Four monoclonal antibodies inhibit the recognition of arylsulphatase A by the lysosomal enzyme phosphotransferase.

The critical step in the sorting of lysosomal enzymes is their recognition by a phosphotransferase in the Golgi apparatus. The topogenic sequences responsible for the recognition by this enzyme have so far only been defined for the lysosomal protease cathepsin D. We have generated four monoclonal antibodies directed against lysosomal arylsulphatase A (ASA). These antibodies inhibit the recognition of ASA by the phosphotransferase in vitro and thus define a region of topogenic sequences in the ASA polypeptide. The antibodies do not interfere with the enzymic activity nor with pH-dependent dimerization of ASA. The epitopes recognized by the antibodies have been located in the second quarter of the ASA polypeptide using chimeric mouse-human ASA molecules. Three of the monoclonal antibodies bind to identical or closely adjacent epitopes, which are formed by the interaction of amino acid residues 165-184 and 202-240. The fourth antibody recognizes a different epitope within amino acids 256-265.

Tail-specific antibodies that block return of 46,000 M(r) mannose 6-phosphate receptor to the trans-Golgi network.

Recycling of 46,000 M(r) mannose 6-phosphate receptor (MPR 46) was investigated by microinjection of Fab fragments against small epitopes within the cytoplasmic domain of the receptor. Fab fragments against the peptide 43-47 (Ala-Tyr-Arg-Gly-Val) efficiently blocked return of MPR 46 to the TGN. Antibody-induced redistribution resulted in accumulation of MPR 46 within an endosomal compartment, from which it recycled to the plasma membrane. Rab5 and rab7, markers for early and late endosomes, respectively, were not detectable in the compartment of redistributed MPR 46, suggesting that it represents a specialized endosomal subcompartment. The bulk of redistributed MPR 46 did not colocalize with endocytosed fluid-phase marker, suggesting that it accumulates at a site where MPR 46 has been segregated from endocytosed material, which is destined for transport to lysosomes. Peptide 43-47 contains a tyrosine (residue 44) which has been shown earlier to be part of an internalization signal for MPR 46 (Johnson, K. F., W. Chan, and S. Kornfeld. 1990. Proc. Natl. Acad. Sci. USA. 87:10010-10014). The role of tyrosine residue 44 as part of a putative multifunctional sorting signal is discussed.

In vitro binding of plasma membrane-coated vesicle adaptors to the cytoplasmic domain of lysosomal acid phosphatase.

Sorting of the newly synthesized membrane-bound precursor of lysosomal acid phosphatase (LAP) involves internalization from the plasma membrane via clathrin-coated pits. Using an in vitro system, we present direct evidence for high affinity interaction of the cytoplasmic domain of LAP with the amino-terminal trunk portion of plasma membrane-coated vesicle adaptors. Coated vesicle adaptors of the trans-Golgi network displayed poor binding to LAP, but high affinity binding to the cytoplasmic tail of the 46-kDa mannose 6-phosphate receptor, which is included in clathrin-coated pits of the trans-Golgi network. Binding of plasma membrane adaptors to the tail peptide of LAP required an internalization signal that contains either tyrosine or phenylalanine.