EXPANSION OF POLYGLUTAMINE REPEAT IN HUNTINGTIN LEADS TO ABNORMAL PROTEIN INTERACTIONS INVOLVING CALMODULIN

Huntington's disease (HD) is an inherited neurodegenerative disorder a ssociated with expansion of a CAG repeat in the IT15 gene, The IT15 ge ne is translated to a protein product termed huntingtin that contains a polyglutamine (polyGln) tract, Recent investigations indicate that t he cause of HD is expansion of the polyGln tract. However, the functio n of huntingtin and how the expanded polyGln tract causes HD is not kn own. We investigate potential protein-protein interactions of huntingt in using affinity resins. Huntingtin from brain extracts is retained o n calmodulin(CAM)-Sepharose in a calcium-dependent fashion, We purify rat huntingtin to apparent homogeneity using a combination of DEAE-cel lulose column chromatography, ammonium sulfate precipitation, and prep arative SDS/PAGE, Purified rat huntingtin does not interact with CAM d irectly as revealed by I-125-CAM overlay, Huntingtin forms a large CAM -containing complex of over 1,000 kDa in the presence of calcium, whic h partially disassociates in the absence of calcium, Furthermore, an i ncreased amount of mutant huntingtin from HD patient brains is retaine d on CAM-Sepharose compared to normal huntingtin from control patient brains, and the mutant allele is preferentially retained on CAM-Sephar ose in the absence of calcium, These results suggest that huntingtin i nteracts with other proteins including CAM and that the expansion of p olyGln alters this interaction.