RT Journal Article
SR Electronic
T1 IMPDH polymers accommodate both catalytically active and inactive conformations
JF bioRxiv
FD Cold Spring Harbor Laboratory
SP 152173
DO 10.1101/152173
A1 Sajitha Anthony
A1 Anika L. Burrell
A1 Matthew C. Johnson
A1 Krisna C. Duong-Ly
A1 Yin-Ming Kuo
A1 Peter Michener
A1 Andrew Andrews
A1 Justin M. Kollman
A1 Jeffrey R. Peterson
YR 2017
UL http://biorxiv.org/content/early/2017/06/19/152173.abstract
AB Several metabolic enzymes undergo reversible polymerization into macromolecular assemblies. The function of these assemblies is often unclear but in some cases they regulate enzyme activity and metabolic homeostasis. The guanine nucleotide biosynthetic enzyme inosine monophosphate dehydrogenase (IMPDH) forms octamers that polymerize into helical chains. In mammalian cells, IMPDH filaments can associate into micron-length assemblies. Polymerization and enzyme activity are regulated in part by binding of purine nucleotides to an allosteric regulatory domain. ATP promotes octamer polymerization, whereas GTP promotes a compact, inactive conformation whose ability to polymerize is unknown. An open question is whether polymerization directly alters IMPDH catalytic activity. To address this, we identified point mutants of human IMPDH2 that either prevent or promote polymerization. Unexpectedly, we found that polymerized and non-assembled forms of IMPDH have comparable catalytic activity, substrate affinity, and GTP sensitivity and validated this finding in cells. Electron microscopy revealed that substrates and allosteric nucleotides shift the equilibrium between active and inactive conformations in both the octamer and the filament. Unlike other metabolic filaments, which selectively stabilize active or inactive conformations, IMPDH filaments accommodate multiple states. Thus, although polymerization alone does not impact catalytic activity, substrate availability and purine balance dramatically affect IMPDH filament architecture.