Interaction of clathrin coat proteins with unilamellar and multilamellar vesicles of phosphatidylcholine

Influence of the clathrin coat on the membrane lipidic organization of endocytic vesicles: a fluorescence study

Endocytic coated vesicles (CV) were purified from bovine brain, and uncoated vesicles (UV) were obtained from the latter by dialysis against 1 M Tris. Membrane dynamics were explored in both vesicle populations using two complementary fluorescence approaches: diphenylhexatriene fluorescence anisotropy to account for rotational lipid movements, and pyrene excimerization with a phosphatidylcholine pyrene derivative for translational motion. It was concluded that membrane fluidity was considerably higher in UV than in CV, and that adding bulk coat proteins (adaptors+clathrin) to uncoated vesicles re-established the low fluidity found in coated vesicles. However, adding coat protein constituents separately had no effect.

The appendage domain of the AP-2 subunit is not required for assembly or invagination of clathrin-coated pits

Coated pits contain a resident membrane molecule(s) that binds clathrin AP-2 with high affinity. AP-2 binding to this site is likely to be the first step in coated pit assembly because this subunit functions as a template for the polymerization of clathrin into flat polygonal lattices. Integral membrane proteins involved in receptor mediated endocytosis cluster in the newly assembled pits as they invaginate and bud from the membrane. The AP-2 subunit is a multi-domain, molecular complex that can be separated by proteolysis into a brick-shaped core and ear-like appendage domains. We have used this property to identify the domain involved in the various stages of coated pit assembly and budding. We found that the core of AP-2 is the domain that binds both to membranes and to triskelions during assembly. Triskelions are perfectly capable of forming lattices on the membrane bound cores. Clathrin lattices bound only to core domains were also able to invaginate normally. Limited proteolysis was also useful for further characterizing the AP-2 binding site. Elastase treatment of the inside membrane surface released a peptide fraction that is able to bind AP-2 in solution and prevent it from interacting with membranes. Affinity purification of binding activity yielded a collection of peptides that was dominated by a 45-kD species. This is the candidate peptide for containing the AP-2-binding site. Therefore, the appendage domain does not directly participate in any of the assembly or invagination events required for coated pit function.

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.

Incorporation of spin-labeled fatty acids into bovine brain clathrin coated vesicles

Stearic acids with a nitroxide radical at selected positions have been incorporated in the phospholipid bilayers of clathrin coated vesicles, uncoated vesicles and sonicated liposomes made from the lipids extracted from the uncoated vesicles. The extent of incorporation was found minimum for stearic acids labeled on C-12 and for bilayers of uncoated vesicles. The ESR spectra of the spin-labeled fatty acids incorporated in the bilayers showed a pronounced temperature dependence (without discontinuity) and a decrease in the hyperfine splitting as the nitroxide group was inserted deeper in the hydrophobic core of the membranes. An abrupt phospholipid phase transition or a phase separation could be excluded. The presence of the external proteins (the clathrin coat) on the membranes was not found to noticeably influence the gradient of flexibility of the fatty acid chains of the phospholipids. The influence of the internal proteins embedded in the bilayers was evidenced by a detailed analysis of the ESR spectra of (7,8)SA in terms of two components: one component arising from the labels surrounded exclusively by phospholipids, the other component arising from labels of reduced mobility perturbed by the vicinity of the proteins. These results support the persistence of lipidic domains in the endocytic vesicles despite the accumulation of receptors which follows their formation.