We report the discovery of the huntingtin interacting protein I (HIP-I) which binds specifically to the N-terminus of human huntingtin, both in the two-hybrid screen and in in vitro binding experiments. For the interaction in vivo, a protein region downstream of the polyglutamine stretch in huntingtin is essential. The HIPI cDNA isolated by the two-hybrid screen encodes a 55 kDa fragment of a novel protein. Using an affinity-purified polyclonal antibody raised against recombinant HIP-I, a protein of 116 kDa was detected in brain extracts by Western blot analysis. The predicted amino acid sequence of the HIP-I fragment exhibits significant similarity to cytoskeleton proteins, suggesting that HIP-I and huntingtin play a functional role in the cell filament networks. The HIPI gene is ubiquitously expressed in different brain regions at low level. HIP-I is enriched in human brain but can also be detected in other human tissues as well as in mouse brain. HIP-I and huntingtin behave almost identically during subcellular fractiona-tion and both proteins are enriched in the membrane containing fractions.