The regulation of complex developmental pathways involves the spatial and temporal interaction of specific gene products. Several powerful genetic approaches, including the tagging of gene products or cells, have allowed their distribution in space and time to be determined. The advent of procedures to create transgenic animals with acquired or abrogated gene functions has allowed the phenotypic consequences of genetic alteration to be analyzed by design rather than chance mutation. For example, the introduction of a portion of the Y chromosome into a female mouse can change its sex, demonstrating the presence of a sex-determining gene (1). In combination with a range of molecular biology techniques, such as RNA protection, PCR amplification, in situ hybridization, immunohistochemistry, and the use of reporter genes, to indicate how, when, and where a gene is expressed. This avalanche of information has started to provide a real insight into the molecular mechanisms that regulate developmental processes. However, studies on the gene products are still crucial for an understanding at the cellular level processes, such as pattern formation and organogenesis.
Proteins may be modified by glycosylation, phosphorylation, proteolysis, or myristylation, and each of these modifications affects the properties of the protein. For example, specific changes in the level of phosphorylation of certain proteins have been observed during the compaction of the mouse embryo, and it is a central paradigm for cell:cell signaling by tyrosine kinase receptors (2-5). To determine the presence of a particular protein or to investigate possible posttranslational modifications, such as glycosylation, phosphorylation, proteolytic processing, and protein:protein interactions, there are several basic protein separation techniques that can be applied, and some general recipes for these are described (6-9).
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