Comparison of MALDI MS spectra obtained from 10 pmol of uroplakin II (UPII) after (a) 30 min on-membrane digestion, (b) 30 min in-gel digestion, (c) 16 h on-membrane digestion, (d) 16 h in-gel digestion. (e) Amino acid sequence of bovine mature UPII. The underlined amino acids correspond to the transmembrane domain of the protein. In the spectra, the stars indicate UPII peptides detected after both in-gel and on-membrane digestion, arrows indicate missed cleavage peptides that appear only after in-gel digestion, and the numbers indicate peptides from the UPII sequence shown in (e) detected only after on-membrane digestion. (Reproduced from ref. 2 with permission from the American Chemical Society).