Determination of the regions of the clathrin molecule inducing membrane fusion

NMR solution structure and dynamics of the peptidyl-prolyl cis-trans isomerase domain of the trigger factor from Mycoplasma genitalium compared to FK506-binding protein

We have solved the solution structure of the peptidyl-prolyl cis-trans isomerase (PPIase) domain of the trigger factor from Mycoplasma genitalium by homo- and heteronuclear NMR spectroscopy. Our results lead to a well-defined structure with a backbone rmsd of 0.23 A. As predicted, the PPIase domain of the trigger factor adopts the FK506 binding protein (FKBP) fold. Furthermore, our NMR relaxation data indicate that the dynamic behavior of the trigger factor PPIase domain and of FKBP are similar. Structural variations when compared to FKBP exist in the flap region and within the bulges of strand 5 of the beta sheet. Although the active-site crevice is similar to that of FKBP, subtle steric variations in this region can explain why FK506 does not bind to the trigger factor. Sequence variability (27% identity) between trigger factor and FKBP results in significant differences in surface charge distribution and the absence of the first strand of the central beta sheet. Our data indicate, however, that this strand may be partially structured as "nascent" beta strand. This makes the trigger factor PPIase domain the most minimal representative of the FKBP like protein family of PPIases.

Interaction of clathrin with large unilamellar phospholipid vesicles at neutral pH. Lipid dependence and protein penetration

The interaction of clathrin with large unilamellar vesicles of various lipid compositions has been examined at neutral pH. Clathrin induces leakage of contents of vesicles that contain the acidic phospholipid phosphatidylserine. Leakage is greatly enhanced by the presence of a relatively minor amount of cholesterol, but is inhibited by phosphatidylcholine. Resonance energy transfer measurements between tryptophan residues of the protein and a fluorescent lipid analog, N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)phosphatidylethanolamine incorporated into the liposomal bilayer, suggests a dynamic interaction of clathrin with the bilayer at neutral pH. This interaction includes a (partial) penetration of the protein into the lipid bilayer, as revealed by hydrophobic photoaffinity labeling with 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)-diazirine. The interaction of clathrin with lipid vesicles at neutral pH is inhibited when the protein is pretreated with trypsin or with the reducing agent dithiothreitol, suggesting that structural requirements govern clathrin-membrane interaction at these conditions. The physiological relevance of the present observations in light of vesiculation and endosomal maturation is discussed.

Sequence of critical events involved in fusion of phospholipid vesicles induced by clathrin

Membrane fusion induced by clathrin is accompanied by several events such as conformational change, membrane binding and association of clathrin, and membrane aggregation (Maezawa et al. (1989) Biochemistry 28, 1422-1428; Maezawa and Yoshimura (1990) Biochem. Biophys. Res. Commun. 173, 134-140). To clarify the sequence of these events, we examined their time-courses by reducing the pH of the medium from 7.4 to a given pH in the range of 3.5-5.0 at 25 degrees C or 10 degrees C. Large unilamellar vesicles composed of phosphatidylserine and phosphatidylcholine were used in most experiments. The half-time for conformational change of clathrin was less than those for membrane binding and association of clathrin. The half-times and the initial rates of membrane binding and association of clathrin were similar order of magnitude, although the pH-profiles of the initial rates of the two events were somewhat different. Membrane aggregation started after membrane binding of clathrin. A lag phase was observed in the time-course of membrane fusion, whereas there was no lag phase in membrane binding and association of clathrin and membrane aggregation. Moreover, the lag time before fusion was independent of the clathrin concentration, although the initial rates of these three events were dependent on it, suggesting that the three reactions are not responsible for the lag phase before fusion, and that there is some other event(s) in the lag time. On the other hand, there was a threshould-pH in the pH profile of the lag-time and the threshold-pH coincided with the critical pH at which the final associated state of clathrin was apparently reversed in the presence and absence of liposomes, suggesting that the event(s) in the lag phase may be related to this final associated state of clathrin molecules on the liposome membranes. These results indicate that clathrin-induced fusion of liposomes is initiated through the following sequential events: conformational change of clathrin, membrane binding and association of clathrin, which occur simultaneously but independently, membrane aggregation, an event(s) in the lag phase, and actual fusion.

In vitro, insulin receptor catalyses phosphorylation of clathrin heavy chain and a plasma membrane 180,000 molecular weight protein

Insulin receptor mutation studies indicate that the receptor tyrosine kinase activity is necessary for receptor endocytosis, and several insulin receptor-containing tissues have a plasma membrane-associated protein (Mr congruent to 180,000, p180) whose tyrosine phosphorylation is receptor catalysed. Since clathrin heavy chain (Mr congruent to 180,000 in dodecyl sulphate gel electrophoresis) is a major component of coated vesicles, the latter functioning in receptor endocytosis, we investigated whether insulin receptors can catalyse clathrin phosphorylation and whether p180 is clathrin. Bovine brain triskelion or coated vesicles and 32P-ATP were added to prephosphorylated insulin receptor preparations (wheat germ agglutinin-purified human placenta membrane proteins). Antiphosphotyrosine immunoprecipitated a phosphorylated 180,000 molecular weight protein. Insulin (10(-7) M) increased the rate of phosphorylation. Monoclonal anti-clathrin antibody immunoprecipitated the phosphorylated 180,000 molecular weight protein, whereas monoclonal anti-insulin receptor antibodies (alpha-IR1, MA10) immunoprecipitated both insulin receptors and the phosphorylated 180,000 molecular weight protein. In the absence of added clathrin, anticlathrin immunoprecipitated no proteins, and alpha-IR1 immunoprecipitated only the insulin receptor. Density gradient (glycerol 7.5-30%, w/v) centrifugation separated human placenta microsomal membrane proteins into endosomal, plasma membrane, cytoplasmic and coated vesicle fractions. Antiphosphotyrosine immunoprecipitated phosphorylated-microsomal proteins that centrifugated into endosomal and plasma membrane fractions. Addition of glycerol gradient fractions to a prephosphorylated insulin receptor preparation, however, gave a tyrosine-phosphorylated 180,000 molecular weight protein when cytoplasmic and coated vesicle fractions were added. Taken together these results suggest: (1) that, in vitro, human placenta insulin receptors can phosphorylate bovine brain and human placenta clathrin heavy chain; (2) that both assembled and unassembled clathrin can be phosphorylated; and (3) that p180, the plasma membrane-associated insulin receptor substrate, is not clathrin heavy chain.

Assembly of clathrin molecules on liposome membranes: a possible event necessary for induction of membrane fusion

Below pH6, clathrin induces fusion of liposomes containing phosphatidylserine (PS) [Maezawa et al. (1989) Biochemistry 28, 1422-1428]. Under similar conditions clathrin forms self-aggregates, suggesting that the associated form of clathrin may be involved in the fusion process. For examination of this possibility, the extent of fluorescence energy transfer from N-(p-(2-benzimidazolyl)phenyl)maleimide (BIPM)-labeled clathrin to N-(7-dimethyl-amino-4-methyl-3-coumarinyl)maleimide (DACM)-labeled clathrin in the presence of liposomes and the number of binding sites for clathrin in one liposome were examined in the pH region inducing membrane fusion. A high degree of transfer was observed, and the area on the membrane surface occupied by a clathrin molecule was estimated to be much less than that expected from its size, indicating that clathrin binds to the liposome membrane as an associated form, which may be essential for induction of membrane fusion.