A12 The cryo-electron microscopy structure of huntingtin

Huntingtin (HTT) is the protein that is altered in Huntington’s disease (HD). While HTT has been found to be essential for many cellular activities including transport of vesicles in the cell, cellular uptake of materials (endocytosis), cellular degradation of waste (autophagy), regulation of transcription, and even is important for embryonic development, an integrating understanding of HTT’s many biological functions at the molecular level is still missing. Although the HTT gene and the disease-causing mutation have been identified 25 years ago, very little data has been available on the structure of HTT. However, information on HTT’s structure would be very important for achieving an improved understand of HTT’s function in health and disease. Here we employed cryo-electron microscopy (cryo-EM) to determine the structure of full-length human HTT in a complex with another protein, HTT-associated protein 40 (HAP40). This interaction of HTT with HAP40 was instrumental in stabilizing HTT’s structure to a degree that the structure of HTT could be determined at detailed level at an overall resolution of 4 Å. HTT is largely α-helical and consists of three major domains. The N- and C-terminal domains contain multiple HEAT repeat elements that are arranged in a solenoid fashion. These domains are connected by a smaller bridge domain that contain different types of tandem repeats. HAP40 is also largely α-helical and has a tetratricopeptide repeat (TPR)-like organization. HAP40 binds in a cleft contacting the three HTT domains by hydrophobic and electrostatic interactions, thereby stabilizing HTT’s conformation. These data help in the interpretation of previous biochemical results and will pave the way for the generation and testing of new research hypotheses that ultimately will lead to an improved understanding of HTT’s diverse biological functions.