Multiplexed protein expression and activity assay

A multiplexed method of detecting a plurality of target polypeptides in a sample composition, the method comprising:

• a) contacting the sample composition with genetic packages that each display a polypeptide-binding component under conditions whereby the plurality of target polypeptides in the sample form complexes with displayed polypeptide-binding components specific therefor, wherein:
• each genetic package comprises a predetermined marker component that is indicative of the displayed polypeptide-binding component;
• and
• the polypeptide-binding component specifically binds to at least one of the target polypeptides, whereby target polypeptides that bind thereto can be identified by virtue of the marker protein;
• b) separating complexes of the plurality of target polypeptides with the displayed polypeptide-binding components on the genetic packages from the sample composition;
• c) optionally amplifying the genetic packages that have formed complexes, resulting in amplified genetic packages, or amplifying the marker components in the genetic packages that have formed complexes;
• d) identifying marker components in the genetic packages that have formed complexes, thereby detecting target polypeptides in the sample, wherein:
• identification of a marker component is effected by mass spectrometry;
• and
• identification of a marker component indicates the presence of a target polypeptide in the sample.