Introduction

The sequencing of the first complete genome sequence of a living organism in 1995 was a turning point in biology [1], the "blueprint of life" of Haemophilus influenzae followed by that of many other organisms, bacteria, lower and higher eukar-yotes included, was decoded. This genome sequence, however, does not provide life itself because it only tells us what may happen, and not what really happens, in the cell. Functional genomic approaches such as mRNA profiling and proteo-mics are required to bring the "virtual life of the genes to the real life of the proteins." Within the ensemble of functional genomics, covering from genome sequencing to bioinformatics and systems biology, proteomics will retain its crucial and privileged position because it deals - as no other discipline does - directly with the players oflife, the proteins.

The proteomics of today has profited greatly from genome sequencing, because protein identification by mass spectrometry (MS) techniques needs the genome sequence. The goal of recent proteomic projects is to visualize the entire pro-teome, which can only be achieved by a combination of gel-based and non-gel-based techniques. For physiological studies the highly sensitive two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) [2] is still the state of the art. The majority of cellular proteins can be visualized within the main proteomic window pi 4-7. However, many proteins are not located within this main window. Additional subproteomic fractions such as alkaline and acidic proteins, cell-wall-bound or extracellular proteins have to be analyzed separately on the way towards the entire proteome. Many proteins, however, escape detection by 2-D gel-based pro-teomics, among them the intrinsic membrane proteins, the most prominent pro-teome subgroup that needs the establishment of gel-free procedures. For the future, simple and feasible quantitative non-gel-based proteomic approaches that are mainly based upon a combination of multidimensional chromatography of peptides and MS/MS procedures are urgently required to make membrane protein and other subproteomic fractions available for high-throughput comparative and quantitative physiological proteomic analysis. At present, however, physiolog-

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