Clathrin self-assembly involves coordinated weak interactions favorable for cellular regulation.

The clathrin triskelion self-assembles into a polyhedral coat surrounding membrane vesicles that sort receptor cargo to the endocytic pathway. A triskelion comprises three clathrin heavy chains joined at their C-termini, extending into proximal and distal leg segments ending in a globular N-terminal domain. In the clathrin coat, leg segments entwine into parallel and anti-parallel interactions. Here we define the contributions of segmental interactions to the clathrin assembly reaction and measure the strength of their interactions. Proximal and distal leg segments were found to lack sufficient affinity to form stable homo- or heterodimers under assembly conditions. However, chimeric constructs of proximal or distal leg segments, trimerized by replacement of the clathrin trimerization domain with that of the invariant chain protein, were able to self-assemble in reversible reactions. Thus clathrin assembly occurs because weak leg segment affinities are coordinated through trimerization, sharing a dependence on multiple weak interactions with other biopolymers. Such polymerization is sensitive to small environmental changes and is therefore compatible with cellular regulation of assembly, disassembly and curvature during formation of clathrin-coated vesicles.