The evolution of the huntingtin-associated protein 40 (HAP40) in conjunction with huntingtin

Background The huntingtin-associated protein 40 (HAP40) abundantly interacts with huntingtin (HTT), the protein that is altered in Huntington's disease (HD). Therefore, we analysed the evolution of HAP40 and its interaction with HTT. Results We found that in amniotes HAP40 is encoded by a single-exon gene, whereas in all other organisms it is expressed from multi-exon genes. HAP40 co-occurs with HTT in unikonts, including filastereans such as Capsaspora owczarzaki and the amoebozoan Dictyostelium discoideum, but both proteins are absent from fungi. Outside unikonts, a few species, such as the free-living amoeboflagellate Naegleria gruberi, contain putative HTT and HAP40 orthologs. Biochemically we show that the interaction between HTT and HAP40 extends to fish, and bioinformatic analyses provide evidence for evolutionary conservation of this interaction. The closest homologue of HAP40 in current protein databases is the family of soluble N-ethylmaleimide-sensitive factor attachment proteins (SNAPs). Conclusion Our results indicate that the transition from a multi-exon to a single-exon gene appears to have taken place by retroposition during the divergence of amphibians and amniotes, followed by the loss of the parental multi-exon gene. Furthermore, it appears that the two proteins probably originated at the root of eukaryotes. Conservation of the interaction between HAP40 and HTT and their likely coevolution strongly indicate functional importance of this interaction

Meeting Abstract: A12

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