Up to 4% of the human 46-kDa mannose 6-phosphate receptor (MPR46) expressed in Madin-Darby canine kidney (MDCK) cells are localized at the cell surface. At steady state, the expression of MPR46 on the apical surface of filter-grown MDCK cells is about sixfold lower than on the basolateral surface. The cytoplasmic domain of the MPR46 is phosphorylated on serine 56 at low stoichiometry. By expressing mutant MPR46 we have shown that the MPR46 phosphorylation site is required for delivery to the plasma membrane. In addition, mutant MPR46 expressed in MPR-deficient mouse embryonic fibroblasts were not detected at the cell surface and their ability to sort newly synthesized cathepsin D was not altered. Since the loss of MPR46 phosphorylation correlates with the lack of cell surface expression, phosphorylation of serine 56 may either function as a direct plasma membrane targeting signal or inhibit MPR46 recycling from endosomes to Golgi, resulting in trafficking to the cell surface.

The THF adduct of dichloro(2,4,6-triisopropylphenyl)aluminum(1) was prepared. Dehalogenation of 1 with Na/K alloy gave a tricyclic compound with a dioxonia-dialuminata central unit (2). The reaction of 2,6-bis{[(dimethylamino)methyl]phenyl}lithium with AIX 3 (X = CI, Br, I), EtAICI 2 or Et 2 AICI (in 1:1 ratio) yielded compounds Β and 3-6 containing pentacoordinated aluminum atoms. From 2-[(dimethylamino)methyl]phenyllithium and AICI 3 , AIBr 3 , EtAICI 2 or Et 2 AICI (1:1) the tetracoordinated Al-species 7, 8, C and D were obtained while with the same reactants in 2:1 ratio the pentacoordinated species 9, 10 and Ε are formed. The reaction of 3 equivalents of 2-[(dimethylamino)methyl]phenyllithium with AICI 3 gave 11 (coordination number five, fluxional behaviour in solution). From 8-(dimethylamino)naphthyllithium, AICI 3 , AIBr 3 and EtAICI 2 the tetracoordinated species 12 -14 were synthesized. By treating the dibromo species 6 and C with lithiated 2,6-dimethyl-N-(trimethylsilyl)aniline the correspondingly substituted monobromo species 15 and 16 were obtained. The compounds are characterized by NMR ( 1 H, 13 C and 29 Si) and mass spectrometry, most of them also by 27 AI NMR and elemental analyses. X-ray structure analyses are provided for 2, 4, 6, 7, 11 and 16. 8 " 10 . Ligand (X) was not used before. RESULTS AND DISCUSSIONBis(2,4,6-triisopropylphenyl)magnesium · 2 THF (A) 11 was prepared by a Grignard reaction from the corresponding bromide in THF followed by a Schlenk-dismutation with dioxane.

Abstract. Recycling of 46,000 Mr 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) efticiently 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 endocy-

Twofold Borylation of Benzene Derivatives with (Diisoalkylamino)boradiyl Units The reaction of subvalent boron species generated by the defluorination of difluoro(diorganylamino)borane with Na/K alloy in the presence of 1,4‐disubstituted benzene derivatives leads to products which contain two (diorganylamino)boradiyl units and two remaining double bonds which are located on the substituted C atoms. Compounds 5 to 8 exhibit structures based on a 2,8‐diborabicyclo[3,2,1]octa‐3,6‐diene skeleton while 9 shows a skeleton with a 1,3a,4,6a‐tetrahydroborolo[3,2‐b]borole structure. The latter structure corresponds to that of 1 obtained in the same reaction with benzene. The compounds are characterized by elemental analyses and spectroscopically [MS, NMR (1H, 11B, 13C, 15N, 19F, 29Si)]. X‐ray structure analyses are presented for 1, 6, 7, 8, and 9.

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.