Strain-based sequence variations and structure analysis of murine prostatespecific spermine binding protein using mass spectrometry

Mouse spermine binding protein (SBP) has been characterized using mass spec trometry, including its localization within the prostate, sequence verifica tion, and its posttranslational modifications. MALDI (matrix-assisted laser desorption/ionization) mass spectrometry was employed for localization of proteins expressed by different lobes of the mouse prostate obtained after tissue blotting on a polyethylene membrane. The mass spectra showed complex protein profiles that were different for each lobe of the prostate. The pr ostate-specific spermine binding protein (SBP), primarily identified by its in-source decay fragment ion signals, was found predominantly expressed by the ventral lobe of the prostate. The MALDI in-source decay measurements c ombined with nanoESI (nanoelectrospay ionization) MS/MS measurements obtain ed after specific proteolysis of SBP, allowed the exact positioning of a si ngle N-linked carbohydrate group, and the identification of a pyroglutamate residue at the sequence N-terminus. The N-linked carbohydrate component wa s further investigated and the general pattern of the N-linked carbohydrate identified. The presence of a disulfide bridge between cysteine(78) and cy steine(124) was also established. The full sequence characterization of SBP showed several strain-based sequence differences when compared to the publ ished gene sequence.