Spectral counting label-free proteomics.

Area of Science:

• Proteomics
• Mass Spectrometry
• Biochemistry

Background:

• Label-free proteome quantification is increasingly popular in bottom-up mass spectrometry proteomics.
• These methods are easily integrated into existing workflows without additional cost or effort.
• Mass spectrometry's quantitative accuracy is challenged by variable peptide ionization efficiency.

Purpose of the Study:

• To detail a protocol for spectral counting quantification using the exponentially modified protein abundance index (emPAI).
• To demonstrate absolute protein quantification using emPAI values, with and without standards.
• To provide guidelines for integrating emPAI into existing data, such as iTRAQ analysis.

Main Methods:

• Focus on spectral counting quantification, a label-free approach.
• Utilize the exponentially modified protein abundance index (emPAI) derived from Mascot search results.
• Demonstrate absolute quantification using emPAI values and discuss integration with other quantitative data like iTRAQ.

Main Results:

• Normalized emPAI allows for broad comparisons of entire proteomes.
• Absolute protein quantification can be achieved using emPAI values.
• emPAI can be calculated from existing data (e.g., iTRAQ) for absolute abundance determination without standards.

Conclusions:

• Spectral counting quantification with emPAI is a viable and accessible method for label-free proteome analysis.
• emPAI facilitates both relative and absolute protein quantification in mass spectrometry-based proteomics.
• The protocol provides a practical framework for implementing emPAI in diverse proteomic studies.