Coiled Coils in Scaffolding Proteins

As pointed out above, dimeric transcription factors represent a particularly simple and well-studied example of a-helical leucine zippers. However, coiled coils can assume more complex quaternary structures and are involved in a plethora of other molecular interactions and key cellular processes. On the basis of 29 genomes sequenced as of 2001, ten percent of all eukaryotic proteins and five percent of all prokaryotic proteins have been predicted to bear coiled-coil motifs (Liu and Rost 2001).

The most obvious advantage of coiled coils for their function as transcription factors appears to reside in their dynamic association-dissociation equilibrium (cf. Sect. 1.1). Another asset of coiled coils is their mechanical strength, and it can therefore be anticipated that they play an eminent role in many scaffolding proteins, most of which need to confer both stability and adaptability to supramo-lecular complexes, organelles, cells, and tissues. In fact, the first amino acid sequence of a coiled-coil motif was published in 1972 for tropomyosin, an actin-binding muscle contraction regulator protein (Hodges et al. 1972; Sodek et al. 1972). Other examples of scaffolding proteins containing coiled coils include cytoskeletal proteins, such as intermediate filaments (Steinert 1993), kinesin and other motor proteins, as well as occludin, an integral membrane protein implicated in the organization of epithelial tight junctions. In this section, we will focus on two members of the vast and diverse class of coiled-coil-based scaffolding proteins, namely, protein complexes involved in mitosis and presynaptic vesicle docking and fusion.

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