2.3. Mass Spectrometry Analysis

For full-length mass determination by ESI-MS, the SEC-purified proteins were loaded on a C8 reverse phase column after adding formic acid to 0.1% final concentration and eluted with a gradient of acetonitrile into a Waters Synapt mass spectrometer. For protein identification from gel bands, the SEC-purified gp44 sample was separated by electrophoresis on a 10% Bis–Tris gel. The bands were excised, digested with trypsin, and subjected to LC-MS analysis, using a Thermo Finnigan LTQ XL spectrometer and a Mascot search of the UniRef100 database. Only spectra with a >80% peptide identification probability as determined by Scaffold Viewer 3 were counted for sequence coverage.