The present review deals with the use of combinatorial ligand libraries, composed by hexapeptides, in the capture and concentration of the low-abundance proteome. This method, first reported in 2005, is compared with other current methodologies aimed at exploring the “deep proteome”, such as: depletion of high-abundance proteins (especially in sera and cerebrospinal fluid) by individual or combined antibodies (up to 20 against the most abundant species); narrow (1-pH-unit) IPGs (zoom gels) and prefractionation with multicompartment electrolyzers or with Off-Gel electrophoresis. The physico-chemical properties of the hexapeptide library are also explored, namely in assessing the proper length of the baits and the behavior of shorter oligopeptides, down to capture elicited by single amino acids. A number of examples on the use of this library is given, such as the analysis of biological fluids (human sera, urine, bile, cerebrospinal fluid) and of cell lysates (platelets, red blood cells). In all cases, it was possible to detected from three to five times as many proteins as compared to control, untreated samples. Perhaps the most spectacular results were obtained with the erythrocyte proteome, where 1570 proteins could be identified in the “minority” proteome, representing only 2% of the total cell lysate. Another interesting area of application regards the concentration and detection of trace impurities contaminanting r-DNA proteins meant for human consumption: several host proteins, never reported before, could be revealed for the first time. Other nonhuman samples are currently under investigation, such as egg-white (where no less than 148 unique gene products could be identified), egg yolk (with 255 unique species) and latex from Hevea brasiliensis. It is anticipated that the ligand library could be a most useful tool for detecting biomarkers for different pathologies and for drug treatment. As a future outlook, one could envision the synthesis of specialized libraries able to capture given classes of proteins (e.g., phospho- and glyco-proteins). Additionally, the library could be used in association with other techniques currently in vogue, such as zoom gels, Off-Gels, and the like.
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