PHOSPHORYLATION OF BARLEY AND WHEAT PHOSPHOLIPID TRANSFER PROTEINS BYWHEAT CALCIUM-DEPENDENT PROTEIN-KINASE

Wheat (Triticum aestivum L.) germ Ca2+-dependent protein kinase (CDPK) phosphorylates a wheat basic protein (WBP) which is a putative phosph olipid transfer protein and one of the best protein substrates yet fou nd for wheat CDPK. Reversed phase high performance liquid chromatograp hy (HPLC) of the tryptic digest of [P-32]phospho WBP resolved two [P-3 2]phosphopeptide peaks. Edman sequencing of one of these established t hat WBP is phosphorylated by CDPK on S17 within the sequence (APCISYA) -P-13. WBP may also be phosphorylated at an additional site. Wheat CDP K also phosphorylates barley (Hordeum. vulgare) lipid transfer protein (LTP1) and [P-32]phosphoLTP1 exactly comigrates with LTP1 on SDS-PAGE . Unlike wheat WBP, LTP1 is a relatively poor substrate for wheat CDPK and is phosphorylated largely on threonine. Reversed phase high perfo rmance liquid chromatography (HPLC) of the tryptic digest of [P-32]pho sphopeptide resolved one [P-32]phosphopeptide and subsequent Edman seq uencing of this phosphopeptide showed that LTP1 is phosphorylated on T 15 within the sequence (KPCLTYVQ)-P-11. This phosphorylation site amin o acid sequence motif differs from the Basic-X-X-Ser(Thr) motif found for a variety of synthetic peptide substrates of wheat germ CDPK but c orresponds to the Pro-Cys-X-Ser/Thr phosphorylation site motif found f or the phosphorylation by wheat germ CDPK of WBP, which is a homologue of barley LTP1. The phosphorylation sites determined for WBP and LTP1 are similar structurally and are within precisely aligned homologous sequences.