Differential localizations and functions of rice nucleotide pyrophosphatase/phosphodiesterase isozymes 1 and 3

Our previous investigation demonstrated that ADP-glucose hydrolytic nucleotide pyrophosphatase/phosphodiesterase (NPP) 1 is transported from the endoplasmic reticulum (ER)-Golgi system to the plastids via a secretory pathway in rice cells [Nanjo et al. (2006) Plant Cell 18: 2582–2592]. In this study, we analyzed the enzymatic characteristics and subcellular localization of its isozyme, NPP3. Unlike NPP1, NPP3 exhibited no hydrolytic activity toward ADP-glucose and no plastid-targeting ability. Furthermore, there was a clear difference between their N-terminal proteolytic processing schemes to form mature enzyme proteins. NPP1 is matured to a 70-kDa protein by two-step proteolytic processing. We detected the 72 kDa form of NPP1 in the microsomes of rice cells in addition to the 70 kDa mature protein, strongly suggesting that proprotein processing occurs post-translationally in the ER-Golgi system. To clarify the existence of the plastid-targeting signal of NPP1, the plastid localization of a series of carboxy-terminal truncated NPP1 proteins fused with green fluorescence protein was tested in rice cells. The results showed that NPP1 cannot be delivered to the plastid by the N-terminal region, including the ER signal sequence and the proprotein processing site, and that the peptide region, from 308 to 478 amino acid residues, is probably important for the transport of NPP1 into plastids in rice cells.