Diversity of Arabidopsis Genes Encoding Precursors for Phytosulfokine, a Peptide Growth Factor

Abstract

Abstract Phytosulfokine-α (PSK-α), a unique plant peptide growth factor, was originally isolated from conditioned medium of asparagus (Asparagus officinalis) mesophyll cell cultures. PSK-α has several biological activities including promoting plant cell proliferation. Four genes that encode precursors of PSK-α have been identified from Arabidopsis. Analysis of cDNAs for two of these,AtPSK2 and AtPSK3, shows that both of these genes consist of two exons and one intron. The predicted precursors have N-terminal signal peptides and only a single PSK-α sequence located close to their carboxyl termini. Both precursors contain dibasic processing sites flanking PSK, analogous to animal and yeast prohormones. Although the PSK domain including the sequence of PSK-α and three amino acids preceding it are perfectly conserved, the precursors bear very limited similarity among Arabidopsis and rice (Oryza sativa), suggesting a new level of diversity among polypeptides that are processed into the same signaling molecule in plants, a scenario not found in animals and yeast. Unnatural [serine-4]PSK-β was found to be secreted by transgenic Arabidopsis cells expressing a mutant of either AtPSK2 orAtPSK3 cDNAs, suggesting that both AtPSK2and AtPSK3 encode PSK-α precursors.AtPSK2 and AtPSK3 were expressed demonstrably not only in cultured cells but also in intact plants, suggesting that PSK-α may be essential for plant cell proliferation in vivo as well as in vitro. Overexpression of either precursor gene allowed the transgenic calli to grow twice as large as the controls. However, the transgenic cells expressing either antisense cDNA did not dramatically decrease mitogenic activity, suggesting that these two genes may act redundantly.