MS/MS spectra of poplar phosphopeptides with single or double phosphorylations. ESI-QUAD-TOF tandem MS spectra of doubly charged parent molecular ions with 780.30 m/z. b-type and y-type ions, including H3PO4 neutral loss ions (indicated as -H3PO4 and # in spectra), were labeled to determine peptide sequences and to localize phosphorylation sites. Asterisks denote phosphorylated serine, threonine, or tyrosine residues. (a) Phosphopeptide spectrum of EAVADMS*EDLSEGEKGDTVGDLSAHGDSVR with a single pSer, corresponding to glycosyltransferase (578888). (b) Phosphopeptide spectrum of EAVADMS*EDLS*EGEKGDTVGDLSAHGDSVR containing two phosphorylated Ser residues, corresponding to glycosyltransferase (578888). (c) Phosphopeptide spectrum of FGIIEGLMTTVHSITAT*QK with a single pThr, corresponding to glyceraldehyde 3-phosphate dehydrogenase (728998). (d) Phosphopeptide spectrum of MSFEDKDLTGDVSGLGPFELEALQDWEY*K with a single pTyr, corresponding to cytochrome b5 domain-containing proteins (662371 and 666994).