Subunit mass fingerprinting of mitochondrial complex I.
We have employed laser induced liquid bead ion desorption (LILBID) mass spectrometry to determine the total mass and to study the subunit composition of respiratory chain complex I from Yarrowia lipolytica.  Using 5-10 pmol of purified complex I, we could assign all 40 known subunits of this membrane bound multiprotein complex to peaks in LILBID subunit fingerprint spectra by comparing predicted protein masses to observed ion masses.  Notably, even the highly hydrophobic subunits encoded by the mitochondrial genome were easily detectable.  Moreover, the LILBID approach allowed us to spot and correct several errors in the genome-derived protein sequences of complex I subunits.  Typically, the masses of the individual subunits as determined by LILBID mass spectrometry were within 100 Da of the predicted values.  For the first time, we demonstrate that LILBID spectrometry can be successfully applied to a complex I band eluted from a blue-native polyacrylamide gel, making small amounts of large multiprotein complexes accessible for subunit mass fingerprint analysis even if they are membrane bound.  Thus, the LILBID subunit mass fingerprint method will be of great value for efficient proteomic analysis of complex I and its assembly intermediates, as well as of other water soluble and membrane bound multiprotein complexes.