PHOSPHORYLATION OF STYLE S-RNASES BY CA2-DEPENDENT PROTEIN-KINASES FROM POLLEN TUBES()

Solanaceous plants with gametophytic self-incompatibility produce ribo nucleases in the transmitting tract of the style that interact with se lf-pollen and inhibit its growth. These ribonucleases are a series of allelic products of the S-locus, which controls self-incompatibility. Little is known about the pollen components involved in this interacti on or whether a signal transduction pathway is activated during the se lf-incompatibility response. We have partially purified a soluble prot ein kinase from pollen tubes of Nicotiana alata that phosphorylates th e self-incompatibility RNases (S-RNases) from N. alata but not Lycoper sicon peruvianum. The soluble protein kinase (Nak-1) has several featu res shared by the calcium-dependent protein kinase (CDPK) class of pla nt protein kinases, including substrate specificity, calcium dependenc e, inhibition by the calmodulin antagonist calmidazolium and cross-rea ction with monoclonal antibodies raised to a CDPK from soybean. Phosph orylation of S-2-RNase by Nak-1 is restricted to serine residues, but the site(s) of phosphorylation has not been determined and there is no evidence for allele-specific phosphorylation. The microsomal fraction from pollen tubes also phosphorylates S-RNases and this activity may be associated with proteins of M(r) similar to 60 K and 69 K that cros s-react with the monoclonal antibody to the soybean CDPK. These result s are discussed in the context of the involvement of phosphorylation i n other self-incompatibility systems.