Huntingtin-interacting protein 1

Huntingtin-interacting protein 1 also known as HIP-1 is a protein that in humans is encoded by the HIP1 gene.

Hip-1 is a protein that interacts with the huntingtin protein. It is known to contain a domain homologous to the death effector domains (DED) found on proteins involved in apoptosis. It is believed that accumulation of high levels of the free form of this protein (free as in dissociated from the huntingtin and free to bind other key protein(s)) in the cell is one of the mechanisms by which neuron cell death is caused in Huntington's Disease (via the caspase-3 route). The role of Hip-1 in caspase mediated cell death remains unclear.

Discovery

Huntingtin interacting protein 1 (HIP1) was first identified by Wanker et al. in 1997.

Function

HIP1 was found to bind to Htt in an N-terminal dependent manner, and co-localise with Htt in the CNS although the nature of this interaction with respect to muHtt was not identified. It has since been found that the CAG expansion seen with muHtt results in decreased binding affinity for HIP1, thus causing disruption of HIP1’s usual function, and also an increased in free HIP1. It is likely that this decreased affinity plays a role in mediating HD pathogenesis, due to loss of cytoskeletal integrity and induction of apoptosis. HIP1’s pro apoptotic effect may involve activation of caspase-8 and a novel HIP1 protein interactor HIPPI. HIP1’s non-pathological activity includes clathrin assembly via interaction with clathrin light chains. HIP1 is the human homologue of Sla2p, a membrane protein in the periphery. Sla2p is an actin-binding protein involved in endocytosis, thus indicating HIP1 in this role. Further details suggesting an important role for Hip-1 in endocytosis comes from binding studies looking at Hip-1 binding to actin. Actin binding by Hip-1 is altered depending on whether clathrin is also bound to Hip-1.