Posttranslational modification can confer special functions to peptides. Based on exhaustive liquid chromatography mass spectrometry analysis targeting tyrosine-sulfated peptides, we identified an 18-aa tyrosine-sulfated glycopeptide in Arabidopsis cell suspension culture medium. This peptide, which we named PSY1, significantly promotes cellular proliferation and expansion at nanomolar concentrations. PSY1 is widely expressed in various Arabidopsis tissues, including shoot apical meristem, and is highly up-regulated by wounding. Perception of PSY1 depends on At1g72300, which is a leucine-rich repeat receptor kinase (LRR-RK) whose two paralogs are involved in the perception of phytosulfokine (PSK), which is a 5-aa tyrosine-sulfated peptide that primarily promotes cellular proliferation. Multiple loss-of-function mutations in these three paralogous LRR-RKs significantly enhanced phenotypes, compared with single disruptants, suggesting that these LRR-RKs have overlapping functions. Triple mutations in these LRR-RKs resulted in dwarfism because of decreases in cell number and cell size and caused insufficiency in tissue repair after wounding. The present results suggest that this paralogous LRR-RK family integrates growth-promoting signals mediated by two structurally distinct sulfated peptides: PSY1 and PSK.tyrosine sulfation ͉ leucine-rich repeat ͉ peptide hormone ͉ posttranslational modification ͉ receptor-like kinase

Tyrosine sulfation is a posttranslational modification common in peptides and proteins synthesized by the secretory pathway in most eukaryotes. In plants, this modification is critical for the biological activities of a subset of peptide hormones such as PSK and PSY1. In animals, tyrosine sulfation is catalyzed by Golgilocalized type II transmembrane proteins called tyrosylprotein sulfotransferases (TPSTs). However, no orthologs of animal TPST genes have been found in plants, suggesting that plants have evolved plant-specific TPSTs structurally distinct from their animal counterparts. To investigate the mechanisms of tyrosine sulfation in plants, we purified TPST activity from microsomal fractions of Arabidopsis MM2d cells, and identified a 62-kDa protein that specifically interacts with the sulfation motif of PSY1 precursor peptide. This protein is a 500-aa type I transmembrane protein that shows no sequence similarity to animal TPSTs. A recombinant version of this protein expressed in yeast catalyzed tyrosine sulfation of both PSY1 and PSK precursor polypeptide in vitro, indicating that the newly identified protein is indeed an Arabidopsis (At)TPST. AtTPST is expressed throughout the plant body, and the highest levels of expression are in the root apical meristem. A loss-of-function mutant of AtTPST displayed a marked dwarf phenotype accompanied by stunted roots, pale green leaves, reduction in higher order veins, early senescence, and a reduced number of flowers and siliques. Our results indicate that plants and animals independently acquired tyrosine sulfation enzymes through convergent evolution.Golgi ͉ peptide hormone ͉ posttranslational modification ͉ tyrosine sulfation ͉ sulfated tyrosine T yrosine sulfation is a common posttranslational modification found in peptides and proteins synthesized through the secretory pathway of most eukaryotes, including higher plants. This modification is thought to be mediated by an enzyme, tyrosylprotein sulfotransferase (TPST), which catalyzes the transfer of sulfate from 3Ј-phosphoadenosine 5Ј-phosphosulfate (PAPS) to the phenolic group of tyrosine (1). To date, 2 tyrosine sulfated peptide hormones, PSK and PSY1, have been found in plants. PSK is a 5-aa secreted peptide containing 2 sulfated tyrosines, and was initially identified as a growth-promoting factor involved in the ''density effect'' in plant cell cultures (2). PSK is recognized by a membrane-localized leucine-rich repeat (LRR) receptor kinase, PSKR1, and promotes cellular proliferation at nanomolar concentrations (3). Disruption of PSKR1 and its 2 homologs in Arabidopsis causes pleiotropic growth defects such as short roots, smaller leaves, and early senescence (4). PSY1 is an 18-aa secreted glycopeptide containing 1 sulfated tyrosine, and was identified by exhaustive analysis of tyrosine sulfated peptides in plant cell culture media (4). PSY1 also promotes cellular proliferation at nanomolar concentrations. There is evidence that one PSKR1 homolog is involved in the recognition of PSY1 (4). Both PSK and PSY...

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