Hip1R (gold) bridges actin and clathrin.

A direct link between clathrin cages and actin is described on page 1209 by Engqvist-Goldstein et al.

The protein that forms the link is huntingtin interacting protein 1 related (Hip1R), which was characterized previously as an actin-binding protein that associates with clathrin-coated pits and vesicles. The related protein Hip1 has been implicated in Huntington's disease, where loss of an interaction between normal huntingtin protein and Hip1 may cause problems with membrane– cytoskeletal integrity in the brain.Engqvist-Goldstein et al. now show that Hip1R tightly associates with clathrin during endocytosis and that purified Hip1R both binds to purified clathrin cages and stimulates cage assembly in vitro. They also show that clathrin cages assembled in vitro with Hip1R copellet with F-actin at low speed.

Other workers have induced the formation of clathrin coated pits using only clathrin, two coat proteins, and membranes. Actin and Hip1R may be needed, however, for the pits to mature into vesicles. The importance of the actin interaction can now be tested in cell culture systems. ▪