Isoform-specific Targeting and Interaction Domains in Human Nicotinamide Mononucleotide Adenylyltransferases

Abstract

Several important signaling pathways require NAD as substrate, thereby leading to significant consumption of the molecule. Because NAD is also an essential redox carrier, its continuous resynthesis is vital. In higher eukaryotes, maintenance of compartmentalized NAD pools is critical, but so far rather little is known about the regulation and subcellular distribution of NAD biosynthetic enzymes. The key step in NAD biosynthesis is the formation of the dinucleotide by nicotinamide/nicotinic acid mononucleotide adenylyltransferases (NMNATs). The three human isoforms were localized to the nucleus, the Golgi complex, and mitochondria. Here, we show that their genes contain unique exons that encode isoform-specific domains to mediate subcellular targeting and post-translational modifications. These domains are dispensable for catalytic activity, consistent with their absence from NMNATs of lower organisms. We further demonstrate that the Golgi-associated NMNAT is palmitoylated at two adjacent cysteine residues of its isoformspecific domain and thereby anchored at the cytoplasmic surface, a potential mechanism to regulate the cytosolic NAD pool. Insertion of unique domains thus provides a yet unrecognized enzyme targeting mode, which has also been adapted to modulate subcellular NAD supply.The metabolism of NAD has emerged as a complex network of reactions with immediate impact on fundamental biological processes (1-3). Even though the vital importance of this molecule has long been recognized, the molecular mechanisms of its synthesis and their regulation have moved into focus only in recent years. Although the bioenergetic role of NAD in reversible electron transfer reactions has been known for decades, only now has it become clear that this nucleotide is also a versatile component of signal transduction pathways, in which it is permanently degraded (1-3). For example, PARP1 (poly(ADPribose) polymerase-1) mediates DNA repair and cell stress responses (4 -6); NAD-dependent protein deacetylases, sirtuins, are involved in metabolic and life span regulation (7-10); and ADP-ribosyl cyclases use NAD to generate second messengers mediating calcium signaling (2,11,12). Because of the ongoing degradation in these reactions, continuous resynthesis of NAD is vital.Although the molecular details of NAD synthesis are still far from being understood, the central role of nicotinamide mononucleotide adenylyltransferases (NMNATs) 2 has been established (13-16). These enzymes catalyze the reaction at which all NAD biosynthetic pathways merge, the formation of the dinucleotide from NMN (or the nicotinic acid derivative, nicotinic acid mononucleotide) and ATP. Most NMNATs are homooligomers composed of ϳ30-kDa subunits. In mammals, there are three compartment-specific isoforms that likely maintain independent subcellular NAD pools.Three-dimensional structures of several NMNATs, including human isoforms 1 and 3, have been established (reviewed in Refs. 13 and 16). Interestingly, the subunit structures of both human NMNAT1 and NMNAT3 cont...

Pathways and Subcellular Compartmentation of NAD Biosynthesis in Human Cells

“…NMNAT3 colocalized with Mitotracker, as expected (23). The two other NMNAT isoforms, not shown here, are known to be nuclear (23) or at the cytosolic face of the Golgi apparatus (23,44).…”

NAD+/NADH and skeletal muscle mitochondrial adaptations to exercise

“…The idea behind this method is that if NAD ϩ is present in the compartment to which PARP1 is targeted, then PAR will accumulate and can be detected by immunocytochemistry (43). Using PARAPLAY, NAD ϩ was found in mitochondria (specifically the matrix but not intermembrane space) and peroxisomes, and surprisingly to the endoplasmic reticulum (ER) and Gogli complex (43,112). Cytosolic NAD ϩ was not detected in that study, most likely due to the fact that PAR glycohydrolase (PARG), which consumes PAR, is most abundant in the cytosol.…”

Study of Specific Region of Plasmodium falciparum Nicotinamide/Nicotinate Mononucleotide Adenylyl Transferase (PfNMNAT): Characterizing a Possible Therapeutic Target

“…These specific insertions or sequences have been studied in the 3 human isoenzymes (HsNMNAT) [17]. Comparing the functional effects generated by PfNMNAT insertions with those of the human orthologues, different results are obtained since in the latter, the changes of their specific domains do not affect the catalytic activity of the protein [18].…”