Pigments and proteins in green bacterial chlorosomes studied by matrix-assisted laser desorption ionization mass spectrometry

We have used matrix-assisted laser desorption ionization time-of-flight mas s spectrometry (MALDI-TOF-MS) for mass determination of pigments and protei ns in chlorosomes, the light-harvesting organelles from the photosynthetic green sulfur bacterium Chlorobium tepidum. By applying a small volume (1 CL L) of a concentrated suspension of isolated chlorosomes directly to the tar get of the mass spectrometer we have been able to detect bacteriochlorophyl l a and all the major homologs of bacteriochlorophyll c. The peak heights o f the different bacteriochlorophyll c homologs in the MALDI spectra were pr oportional to peak areas obtained from HPLC analysis of the same sample. Th e same result was also obtained when whole cells of Chi. tepidum were appli ed to the target, indicating that MALDI-MS can provide a rapid method for o btaining a semiquantitative determination or finger-print of the bacterioch lorophyll homologs in a small amount of green bacterial cella In addition t o information on pigments, the MALDI spectra also contained peaks from chlo rosome proteins. Thus we have been able with high precision to confirm the molecular masses of the chlorosome proteins CsmA and CsmE which have been p reviously determined by conventional biochemical and genetic methods, and d emonstrate the presence of truncated versions of CsmA and CsmB.