Diversity of Arabidopsis genes encoding precursors for phytosulfokine, a peptide growth factor

Phytosulfokine-alpha (PSK-alpha), a unique plant peptide growth factor, was originally isolated from conditioned medium of asparagus (Asparagus offici nalis) mesophyll cell cultures. PSK-a has several biological activities inc luding promoting plant cell proliferation. Four genes that encode precursor s of PSK-alpha 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-alpha sequence located close to their carbox yl termini. Both precursors contain dibasic processing sites flanking PSK, analogous to animal and yeast prohormones. Although the PSK domain includin g the sequence of PSK-alpha and three amino acids preceding it are perfectl y conserved, the precursors bear very limited similarity among Arabidopsis and rice (Oryza saliva), suggesting a new level of diversity among polypept ides that are processed into the same signaling molecule in plants, a scena rio not found in animals and yeast. Unnatural [serine-4]PSK-beta was found to be secreted by transgenic Arabidopsis cells expressing a mutant of eithe r AtPSK2 or AtPSK3 cDNAs, suggesting that both AtPSK2 and AtPSK3 encode PSK -alpha precursors. AtPSK2 and AtPSK3 were expressed demonstrably not only i n cultured cells but also in intact plants, suggesting that PSK-alpha may b e essential for plant cell proliferation in vivo as well as in vitro. Overe xpression of either precursor gene allowed the transgenic calli to grow twi ce as large as the controls. However, the transgenic cells expressing eithe r antisense cDNA did not dramatically decrease mitogenic activity, suggesti ng that these two genes may act redundantly.