Two regions on the clathrin heavy chain that are involved in triskelion interactions during assembly have been localized on the triskelion structure. These regions were previously identified with anti-heavy chain monoclonal antibodies X19 and X35, which disrupt clathrin assembly (Blank, G. S., and F. M. Brodsky, 1986, EMBO (Eur. Mol. Biol. Organ.) J., 5:2087-2095). Antibody-binding sites were determined based on their reactivity with truncated triskelions, and were mapped to an 8-kD region in the middle of the proximal portion of the triskelion arm (X19) and a 6-kD region at the triskelion elbow (X35). The elbow site implicated in triskelion assembly was also shown to be included within a heavy chain region involved in binding the light chains and to constitute part of the light chain-binding site. We postulate that this region of the heavy chain binds to the interaction site identified on the light chains that has homology to intermediate filament proteins (Brodsky, F. M., C. J. Galloway, G. S. Blank, A. P. Jackson, H.-F. Seow, K. Drickamer, and P. Parham, 1987, Nature (Lond.), 326:203-205). These findings suggest the existence of a heavy chain site, near the triskelion elbow, which is involved in both intramolecular and intermolecular interactions during clathrin assembly.

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