Identification of the Beta Subunit Fas1p of Fatty Acid Synthetase as an Interacting Partner of Yeast Calcium/Calmodulin-Dependent Protein Kinase Cmk2p Through Mass Spectrometry Analysis

The calcium/calmodulin-dependent protein kinase II (CaMKII) is a mediator of calcium signals and regulates fatty acid metabolism in mammalian cells. Cmk2p is a yeast homolog of CaMKII and functions as a negative regulator of calcium signaling. However, its substrates remain to be identified. Combination of immunoprecipitation (IP) and mass spectrometry has been proven to be very useful for identification of interacting partner proteins and interactome. In this study, through these approaches, we have identified 65 and 110 potential Cmk2p-interacting proteins in yeast cells in the absence or presence of calcium stress, respectively. In yeast cells expressing both CMK2-HA and FAS1-GFP fusion proteins, in the absence or presence of calcium stress, less amounts of FAS1-GFP proteins are present in cell lysates after IP with anti-HA antibody than cell lysates before IP, while FAS1-GFP proteins are detected on both types of IP beads. However, as an internal control, similar amounts of Pgk1p proteins were detected in both after-IP and before-IP cell lysates but not on the IP beads. Therefore, our biochemical analysis demonstrates that the beta subunit Fas1p of fatty acid synthetase interacts with Cmk2p in yeast cells independent of calcium stress. It is also interesting to note that, in addition to the expected 52-kDa CMK2-HA band, a faster-moving 48-kDa CMK2-HA band is present in the calcium-stressed cell lysate but not in the cell lysate without calcium stress. Our data would provide important clues for understanding the functions of CaMKII in the regulation of fatty acid metabolism as well as related diseases such as cancers, diabetes, and obesity.