Mitochondrial Localization and Putative Signaling Function of Sucrose Synthase in Maize

Abstract

In many organisms, an increasing number of proteins seem to play two or more unrelated roles. Here we report that maize sucrose synthase (SUS) is distributed in organelles not involved in sucrose metabolism and may have novel roles beyond sucrose degradation. Bioinformatics analysis predicts that among the three maize SUS isoforms, SH1 protein has a putative mitochondrial targeting peptide (mTP). We validated this prediction by the immunodetection of SUS in mitochondria. Analysis with isoform-specific antisera revealed that both SH1 and SUS1 are represented in mitochondria, although the latter lacks a canonical mTP. The SUS2 isoform is not detectable in mitochondria, despite its presence in the cytosol. In maize primary roots, the mitochondrion-associated SUS (mtSUS; which includes SH1 and SUS1) is present mostly in the root tip, indicating tissue-specific regulation of SUS compartmentation. Unlike the glycolytic enzymes that occur attached to the outside of mitochondria, SH1 and SUS1 are intramitochondrial. The low abundance of SUS in mitochondria, its high K m value for sucrose, and the lack of sucrose in mitochondria suggest that mtSUS plays a non-sucrolytic role. Co-immunoprecipitation studies indicate that SUS interacts with the voltage-dependent anion channel in an isoform-specific and anoxia-enhanced manner and may be involved in the regulation of solute fluxes into and out of mitochondria. In several plant species, at least one of the SUS proteins possesses a putative mTP, indicating the conservation of the noncatalytic function across plant species. Taken together, these observations suggest that SUS has a novel noncatalytic function in plant cells.In both plant and animal cells, a small number of catalytic and structural proteins play additional roles that in some cases are regulatory in nature (1-3). These "moonlighting" proteins possess the ability to assemble into multiprotein complexes and mediate sophisticated biological functions such as integrating signals. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) 2 is the most studied among such versatile proteins (4). Besides its pivotal role in energy metabolism, GAPDH is shown to be involved in membrane fusion, microtubule assembly, RNA transport, DNA replication and repair, and cell death (5). The functional diversity of GAPDH is facilitated by its ability to interact with other proteins and translocate to multiple subcellular compartments. Sucrose synthase (SUS) catalyzes the reversible conversion of sucrose and UDP into fructose and UDP-glucose and is a key player in plant sucrose catabolism. In maize, the following three genes encoding sucrose synthase are known: sus1, sus2, and sh1. The isoforms encoded by these genes are symbolized SUS1, SUS2, and SUS-SH1, respectively (6). In this study, for the sake of simplicity, we call the SUS-SH1 isoform SH1. All the three isoforms are predominantly recovered as soluble (cytosolic) proteins from plant cells, in accordance with their predicted secondary structures. The well documented function of this en...

A Role for F-Actin in Hexokinase-Mediated Glucose Signaling

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Identification and Characterization of the Duplicate Rice Sucrose Synthase Genes OsSUS5 and OsSUS7 Which Are Associated with the Plasma Membrane

“…OsSUS5 and OsSUS7 are predominantly associated with the plasma membrane It is known that SUS proteins exist both freely in the cytosol and in association with various subcellular compartments including the plasma membrane (Duncan and Huber, 2007;Etxeberria and Gonzalez, 2003;Subbaiah et al, 2006;Winter et al, 1997;1998). Interestingly, in our subcellular localization analysis of the OsSUS proteins, we found that OsSUS5 and OsSUS7, belonging to SUS group III, are associated predominantly with the plasma membrane and exist in part in the cytosol in mesophyll cells.…”

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The Endosomal Protein CHARGED MULTIVESICULAR BODY PROTEIN1 Regulates the Autophagic Turnover of Plastids in Arabidopsis

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