Structure-activity studies of RALF, Rapid Alkalinization Factor, reveal an essential--YISY--motif.

Rapid Alkalinization Factor (RALF) is a 49-amino acid peptide initially isolated from tobacco leaves that is capable of arresting both root and pollen tube growth. With suspension cells, addition of RALF causes an elevation of the pH of the extracellular media, caused by the blockage of a proton pump. RALF associates with a putative receptor(s) on the surface of the plant cell, initiating a signal transduction pathway. Although the exact function(s) of RALFs are unknown, its presence throughout the plant kingdom attests to its importance in some type of basic regulatory role. In the present study, deletion and substitution analyses of RALF reveal a specific - YISY - motif located at positions 5 through 8 from the N-terminus, highly conserved within the plant kingdom, which is a requirement for productive binding of RALF to its putative receptor. Replacement of isoleucine with alanine in the - YISY - motif caused a severe reduction in alkalinization of suspension cell media and a loss of root growth inhibition with tomato seedlings.