Huntingtin-interacting protein 1 (HJPT) is a membrane associated protein which
interacts with huntingtin, the protein altered in Huntington Disease (HD). It was
determined that the H1P1 gene consists of 32 exons spanning approximately 215 kb of
genomic DNA and gives rise to two alternate splice forms termed HIPl-1 and HIP1- 2.
Additionally, a novel homolog of HIP1, termed HIP12, was identified. HIP1 and HIP12
share significant amino acid identity and both proteins contain an epsin N-terminal
homology (ENTH), coiled coil and talin-like domains. While overexpression of HTP1 is
toxic in cell culture, HIP12 did not confer toxicity in the same assay systems.
Interestingly, HIP12 did not interact with huntingtin but was able to interact with HJPT
suggesting a potential interaction in vivo which may influence the function of each
respective protein.
Tissue distribution studies indicated that HIP1 was expressed predominantly
within the brain whereas HIP12 expression was more ubiquitous. Within cells, HIP1 and
HJPT2 displayed a similar punctate distribution at the plasma membrane and underlying
cortical actin cytoskeleton. HIP1 was shown to interact with the endocytic proteins
clathrin heavy chain and AP2. However, HIP12 displayed weak binding to clathrin heavy
chain and no interaction with AP2. HIP1 and HIP12 were also shown to interact with the
clathrin light chain. In addition, it was also revealed that HJPT and HIP12 were capable
of inducing the assembly of the clathrin coat in vitro. Finally, it was determined that
HIP12 interacts with F-actin suggesting that HIP12 may interact with the cortical actin cytoskeleton underlying clathrin-coated pits and vesicles. HIP1 may be indirectly
associated with the actin cytoskeleton by forming heterodimers with HIP12. Thus, HTP1
and HTP12 may act as a molecular "link" between the endocytic machinery and the
cortical actin cytoskeleton. Taken together, these results demonstrate that HIP1 and
HIP12 are components of the endocytic machinery that participate in the recruitment,
assembly and stabilization of the clathrin lattice structure.
The observation that huntingtin associates with HIP1 suggests that it may function
in endocytosis and transport. Since mutant huntingtin displays an altered interaction with
HIP1, it raises the possibility that HD may be the result of defects in endocytosis and
transport. The determination of the cellular functions of the HJP1 family and huntingtin
in endocytosis and transport may provide valuable insights into the pathogenic
mechanism(s) underlying HD.Medicine, Faculty ofMedical Genetics, Department ofGraduat
