Current approaches and challenges in targeted absolute quantification of membrane proteins.

Experimental determination of absolute protein amounts is becoming increasingly important for the establishment and validation of biomarkers, and systems biology approaches aimed at a quantitative description of a biological process. Residing at compartmental or cellular barriers, and acting as prominent drug targets, integral membranes proteins, being completely embedded in the lipid bilayer, possess characteristic physicochemical properties and are often in low abundance. These features challenge the quantification with targeted MS and the ability to accurately determine the amount of membrane proteins with high sensitivity. This review summarizes the current status of targeted membrane protein quantification with emphasis on sample preparation beforehand MS. From the beginning to the end of a usual sample preparation workflow, consisting essentially of reference point selection, cell lysis, digestion, and addition of suitable isotope-labeled standards, general and particular challenges for membrane proteins will be discussed step by step. Based on the presentation of current achievements, possible measures to better address these challenges and future avenues of targeted membrane proteomics are presented.