Leveraging immonium ions for identifying and targeting acyl-lysine modifications in proteomic datasets

Acyl modifications vary greatly in terms of elemental composition and site of protein modification. Developing methods to identify these modifications more confidently can help assess the scope of these modifications in large proteomic datasets. Herein we analyze the utility of acyl-lysine immonium ions for identifying the modifications in proteomic datasets. We demonstrate that the cyclized immonium ion is a strong indicator of acyl-lysine presence when its rank or relative abundance compared to other ions within a spectrum is considered. Utilizing a stepped collision energy method in a shotgun experiment highlights the immonium ion strongly. Implementing an analysis that accounted for features within each MS 2 spectra, this method allows peptides with short chain acyl-lysine modifications to be clearly identified in complex lysates. Immonium ions can also be used to validate novel acyl-modifications; in this study we report the first examples of 3- hydroxylpimelyl-lysine modification and validate them using immonium ions. Overall these results solidify the use of the immonium ion as a marker for acyl-lysine modifications in complex proteomic datasets. Statement of Significance Acyl-lysine modifications come in a variety of elemental compositions. There is increasing evidence that these modifications can have a functional effect on protein and are present in proteomes across all domains of life. Here we describe a new method that can allow for more confident identification of acyl modifications in proteomes by utilizing the immonium ion of these modifications. Our utilization of these ions allows for more comprehensive insight into the role of acyl modifications at the systems level.