Isolation of a putative receptor for KDEL-tailed cysteine proteinase (SH-EP) from cotyledons of Vigna mungo seedlings

The Arabidopsis exocyst subcomplex subunits involved in a golgi-independent transport into the vacuole possess consensus autophagy-associated atg8 interacting motifs

The exocyst complex is a multi-subunits evolutionary conserved complex, which was originally shown to be primarily associated with vesicular transport to the plasma membrane. A recent report (Kulich et al., 2013 Traffic; In Press) revealed that AtEXO70B1, one of the multiple subunits of the exocyst complex of Arabidopsis thaliana plants, is co-transported with the autophagy-associated Atg8f protein to the vacuole. This pathway does not involve the Golgi apparatus. The co-localization of AtEXO70B1 and Atg8f suggests either that both of these proteins are co-transported together to the vacuole or, alternatively, that Atg8 binds to a putative Atg8 interacting motif (AIM) located within the AtEXO70B1 polypeptide, apparently forming a tethering complex for an autophagic complex that is transported to the vacuole. In the present addendum, by tooling a bioinformatics approach, we show that AtEXO70B1 as well as the additional 20 paralogs of Arabidopsis EXO70 exocyst subunits each possess one or more AIMs whose consensus sequence implies their high fidelity binding to Atg8. This indicates that the autophagy machinery is strongly involved in the assembly, transport, and apparently also the function of AtEXO70B1 as well as the exocyst sub complex.

The Tr-cp 14 cysteine protease in white clover (Trifolium repens) is localized to the endoplasmic reticulum and is associated with programmed cell death during development of tracheary elements

Cysteine proteases are known to be associated with programmed cell death, developmental senescence and some types of pathogen and stress-induced responses. In the present study, we have characterized the cysteine protease Tr-cp 14 in white clover (Trifolium repens). Tr-cp 14 belongs to the C1A family of cysteine proteases with homology to XCP1 and XCP2 from Arabidopsis thaliana and p48h-17 from Zinnia elegans, which previously have been reported to be associated with tracheary element differentiation. The proform as well as the processed form of the protein was detected in petioles, flowers and leaves, but the processed form was more abundant in leaves and petioles than in flowers. The Tr-cp 14 protein was localized to differentiating tracheary elements within the xylem, indicating that the cysteine protease is involved in protein re-mobilization during tracheary element differentiation. Immunogold studies suggest that the protease prior to the burst of the vacuole was associated to the ER cisternae. After disruption of the tonoplast, it was found in the cytoplasm, and, in later stages, associated with disintegrating material dispersed throughout the cell.

An ER-localized form of PV72, a seed-specific vacuolar sorting receptor, interferes the transport of an NPIR-containing proteinase in Arabidopsis leaves

Putative vacuolar sorting receptors that bind to the vacuolar targeting signals have been found in various plants; pumpkin PV72, pea BP-80 and Arabidopsis AtELP. PV72 is a seed-specific receptor that is predicted to sort seed storage proteins to protein storage vacuoles. Analysis by surface plasmon resonance showed that the lumenal domain of PV72 bound to an NPIR (a typical vacuolar targeting signal)-containing peptide of the precursor of a cysteine proteinase, AtALEU, in the presence of Ca(2+) (K(D) = 0.1 micro M). To elucidate the receptor-dependent transport of vacuolar proteins in plant cells, we produced transgenic Arabidopsis plants that expressed a fusion protein (PV72-HDEL) composed of the lumenal domain of PV72 and an endoplasmic reticulum (ER)-retention signal, HDEL. The expression of PV72-HDEL induced the accumulation of the AtALEU precursor. The accumulation level of the AtALEU precursor was dependent on that of PV72-HDEL. In contrast, it did not induce the accumulation of a precursor of another cysteine proteinase, RD21, which contains no NPIR. Detailed subcellular localization revealed that both the AtALEU precursor and PV72-HDEL accumulated in the ER fraction. We found that most of the AtALEU precursor molecules formed a complex with PV72-HDEL. The AtALEU precursor might be trapped by PV72-HDEL in the ER and not transported to the vacuoles. This in-planta analysis supports the hypothesis that an Arabidopsis homolog of PV72 functions as a sorting receptor for the NPIR-containing proteinase. The overall results suggest that vacuolar sorting receptors for the protein storage vacuoles and the lytic vacuoles share the similar recognition mechanism for a vacuolar targeting signal.

Vacuolar sorting receptor for seed storage proteins in Arabidopsis thaliana

The seeds of higher plants accumulate large quantities of storage protein. During seed maturation, storage protein precursors synthesized on rough endoplasmic reticulum are sorted to protein storage vacuoles, where they are converted into the mature forms and accumulated. Previous attempts to determine the sorting machinery for storage proteins have not been successful. Here we show that a type I membrane protein, AtVSR1/AtELP, of Arabidopsis functions as a sorting receptor for storage proteins. The atvsr1 mutant missorts storage proteins by secreting them from cells, resulting in an enlarged and electron-dense extracellular space in the seeds. The atvsr1 seeds have distorted cells and smaller protein storage vacuoles than do WT seeds, and atvsr1 seeds abnormally accumulate the precursors of two major storage proteins, 12S globulin and 2S albumin, together with the mature forms of these proteins. AtVSR1 was found to bind to the C-terminal peptide of 12S globulin in a Ca2+-dependent manner. These findings demonstrate a receptor-mediated transport of seed storage proteins to protein storage vacuoles in higher plants.

C-terminal KDEL sequence of a KDEL-tailed cysteine proteinase (sulfhydryl-endopeptidase) is involved in formation of KDEL vesicle and in efficient vacuolar transport of sulfhydryl-endopeptidase

Sulfhydryl-endopeptidase (SH-EP) is a papain-type vacuolar proteinase expressed in cotyledons of germinated Vigna mungo seeds, and the enzyme possesses a C-terminal propeptide containing KDEL tail, an endoplasmic reticulum retention signal for soluble proteins. SH-EP is transported to vacuoles via a KDEL vesicle (KV) through a Golgi complex-independent route. To see the function of the KDEL sequence of SH-EP, wild-type SH-EP and its KDEL deletion mutant (SH-EPDeltaKDEL) were heterologously expressed in Arabidopsis and in cultured tobacco Bright Yellow 2 cells, and their intracellular transport pathways and localizations were analyzed. A combination of the results from analyses for transformed Arabidopsis and tobacco (Nicotiana tabacum) cells indicated that wild-type SH-EP is packed into KV-like vesicles through the KDEL sequence and is transported to vacuoles in the cells of transformants. In contrast, KV was not formed/induced in the cells expressing SH-EPDeltaKDEL, and the mutant protein was mainly secreted. Therefore, the C-terminal KDEL sequence of the KDEL-tailed cysteine proteinase is thought to be involved in the formation of KV, and in the efficient vacuolar transport of the proteins through KV.