Signal peptide peptidase and its homologs in Arabidopsis thaliana– plant tissue‐specific expression and distinct subcellular localization

Abstract

Signal peptide peptidase (SPP) is an aspartic proteinase that hydrolyses its substrate within the plane of the cellular membrane. In vertebrates, it plays crucial roles in life processes such as differentiation, embryogenesis, cell signaling and immunological response. We first found SPP in plants. An ortholog of human SPP (AtSPP), and its five AtSPP homologs (AtSPPL1-AtSPPL5), were searched for in the Arabidopsis database. These clones were grouped into three different clusters: AtSPP was grouped with human SPP (HsSPP) orthologs, AtSPPL1 with the HsSPPL3 family, and AtSPPL2-AtSPPL5 with the group of SPP-like proteins of plant origin. AtSPP, AtSPPL1 and AtSPPL2 were examined for their expression profiles by in situ hybridization. AtSPP was strongly expressed in both the shoot meristem of germinating seeds and the inflorescence meristem at the reproductive stage. On the other hand, AtSPPL1 and AtSPPL2 were expressed in the shoot meristem of germinating seeds, but at very low levels in the shoot apex at the reproductive stage. The subcellular localization of AtSPP, AtSPPL1 and AtSPPL2 was investigated using green fluorescent protein (GFP) fusion proteins in cultured 'Deep' cells. GFP-AtSPP localized to the endoplasmic reticulum (ER), and GFP-AtSPPL1 and GFP-AtSPPL2 to the endosomes. These results suggest that AtSPP mediates the cleavage of signal peptide in the ER membrane as well as HsSPP does, and also that AtSPPL1 and AtSPPL2 located in the endosomes have distinct roles in cells.Abbreviations 2SC, pumpkin 2S albumin; AP, aspartic proteinase; At, Arabidopsis thaliana; BiP, luminal binding protein; ER, endoplasmic reticulum; GFP, green fluorescent protein; Hs, Homo sapiens; I-CliP, intramembrane-cleaving protease; PIP1, plasma membrane intrinsic protein 1; SPP, signal peptide peptidase; SPPL, signal peptide peptidase-like protein; V-PPase, vacuolar-type proton pyrophosphatase.

Arabidopsis bZIP60 Is a Proteolysis-Activated Transcription Factor Involved in the Endoplasmic Reticulum Stress Response

“…It was also reported that bZIP17 is activated by S1P-mediated RIP in response to salt stress (Liu et al, 2007b). Moreover, Arabidopsis homologs of rhomboids and signal peptide peptidases, additional families of intramembrane-cleaving proteases, have recently been identified (Kanaoka et al, 2005;Tamura et al, 2008), although substrates of these proteases are unknown. These reports bear the implication that RIP is an important regulatory mechanism in a variety of cellular processes in plants, as it is in animals.…”

The Signal Peptide Peptidase Is Required for Pollen Function in Arabidopsis

“…AtSPP is predicted to encode an integral membrane protein of the ER with an N-terminal signal sequence. Subcellular fractionation and fluorescence localization data have confirmed its ER localization (Tamura et al, 2008). To examine the processing and localization of AtSPP in vivo, we transformed wild-type Arabidopsis plants with a gene expressing AtSPP fused to two tandem Flag epitope tags (Hopp et al, 1988) inserted upstream of the putative ER localization signal ( Fig.…”

“…There is a family of six related SPP genes in Arabidopsis with similarity to human SPP (Tamura et al, 2008). AtSPP (At2g03120) is the most likely ortholog of human SPP in Arabidopsis; it is the most closely related SPP at the amino acid sequence level and contains a potential ER retention signal (Fig.…”

“…1A; Weihofen et al, 2002). This gene, At2g03120, was previously designated AtSPP by Tamura et al (2008). AtSPP is predicted to encode an integral membrane protein of the ER with an N-terminal signal sequence.…”

“…Arabidopsis (Arabidopsis thaliana) contains six potential genes encoding SPP-like proteins (Ponting et al, 2002;Friedmann et al, 2004;Tamura et al, 2008). A recent study showed that AtSPP, the Arabidopsis protein most closely related to human SPP, and five other SPP-like genes are differentially expressed in Arabidopsis and that their proteins are differentially localized in subcellular organelles (Tamura et al, 2008).…”