Chemical Cross-Linking and High-Performance Fourier Transform Ion Cyclotron Resonance Mass Spectrometry for Protein Interaction Analysis: Application to a Calmodulin/Target Peptide Complex

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Abstract
Chemical cross-linking has proved successful in combination with mass spectrometry as a tool for low-resolution structure determination of proteins. The integration of chemical cross-linking with Fourier transform ion cyclotron resonance (FTICR) mass spectrometry to determine protein interfaces was tested on the calcium-dependent complex between calmodulin (CaM) and a 26-amino acid peptide derived from the skeletal muscle myosin light chain kinase (M13). Different amine-reactive, homobifunctional cross-linkers and a "zero-length" cross-linker were employed. The covalently attached complexes were separated from nonreacted proteins by one-dimensional gel electrophoresis, and the bands of interest were excised and in-gel digested with trypsin. Digestion of the cross-linked complexes resulted in complicated peptide mixtures, which were analyzed by nano-HPLC/nano-ESI-FTICR mass spectrometry. The distance constraints obtained by chemical cross-linking were in agreement with the published NMR structure of the CaM/M13 complex, pointing to residues Lys-18 and Lys-19 of M13 being cross-linked with the central alpha-helix of CaM. Thus, the integrated approach described herein has proven to be an efficient tool for mapping the topology of the CaM/M13 complex. As such it is applicable as a general strategy for the investigation of the spatial organization of protein complexes and complements existing techniques, such as X-ray crystallography and NMR spectroscopy.