Clathrin light chains function in mannose phosphate receptor trafficking via regulation of actin assembly.

Clathrin-coated vesicles (CCVs) are major carriers for endocytic cargo and mediate important intracellular trafficking events at the trans-Golgi network (TGN) and endosomes. Whereas clathrin heavy chain provides the structural backbone of the clathrin coat, the role of clathrin light chains (CLCs) is poorly understood. We now demonstrate that CLCs are not required for clathrin-mediated endocytosis but are critical for clathrin-mediated trafficking between the TGN and the endosomal system. Specifically, CLC knockdown (KD) causes the cation-independent mannose-6 phosphate receptor (CI-MPR) to cluster near the TGN leading to a delay in processing of the lysosomal hydrolase cathepsin D. A recently identified binding partner for CLCs is huntingtin-interacting protein 1-related (HIP1R), which is required for productive interactions of CCVs with the actin cytoskeleton. CLC KD causes mislocalization of HIP1R and overassembly of actin, which accumulates in patches around the clustered CI-MPR. A dominant-negative CLC construct that disrupts HIP1R/CLC interactions causes similar alterations in CI-MPR trafficking and actin assembly. Thus, in mammalian cells CLCs function in intracellular membrane trafficking by acting as recruitment proteins for HIP1R, enabling HIP1R to regulate actin assembly on clathrin-coated structures.