Four Salvage NAD⁺ Biosynthetic Pathway Enzymes Moonlight as Molecular Chaperones to Protect Against Proteotoxicity

Abstract

Human neurodegenerative proteinopathies are disorders associated with abnormal protein depositions in brain neurons. They include polyglutamine (polyQ) conditions such as Huntington’s disease (HD) and α-synucleinopathies such as Parkinson’s disease (PD). Overexpression of NMNAT/Nma1, an enzyme in the NAD⁺ biosynthetic salvage pathway, acts as a powerful suppressor of proteotoxicities in yeast, fly, and mouse models. Screens in yeast models of HD and PD in our lab allowed us to identify three additional enzymes of the same pathway that achieve similar protection against proteotoxic stress: Npt1, Pnc1, and Qns1. Here, we report that their ability to maintain proteostasis is independent of their catalytic activity. Furthermore, we show that, under proteotoxic stress, the four proteins are recruited as molecular chaperones with holdase and foldase activities. In yeast cells, the NAD⁺ salvage proteins act by preventing misfolding and, together with the Hsp90 chaperone, promoting the refolding of extended polyQ domains or α-synuclein. For Nma1, we demonstrate that its foldase activity maps to the C-terminal domain of the protein. The proteostasis activity of Nma1, Npt1, Pnc1, and Qns1 does not require cellular protein quality control systems such as the proteasome or autophagy. We conclude that the entire salvage NAD⁺ biosynthetic pathway links NAD⁺ metabolism and proteostasis and emerges as a target for therapeutics to combat age-associated neurodegenerative proteotoxicities. Our observations also illustrate the existence of an evolutionarily conserved strategy of repurposing or moonlighting housekeeping enzymes under stress conditions.