Proteins, amphipathic helix classes

An amphipathic helix enhances the lipoprotein lipase activation capacity of apoC-II (Catapano et a/., 1979 VmioetaL, 1983). Synthetic peptide studies localized the lipoprotein lipase activation site of this protein to residues 55-78. CNBr cleavage of this protein produced peptides apoC-II[l-9], apoC-II[10-59], and apoC-II[60-78]. Lipoprotein lipase activation studies on these fragments indicate that the enzyme-activating domain is localized to apoC-II[60-78]. Comparing these experimental results with the amphipathic helix map (Fig. 7), the lipoprotein lipase-activating domain of apoC-II corresponds precisely to the predicted carboxy-terminal class G amphipathic helical domain (residues 60-76). 

The two-stranded a-helical coiled coil is now recognized as one of natures favorite ways of creating a dimerization motif and has been predicted to occur in a diverse group of over 200 proteins.111 This structure consists of two amphipathic, right-handed a-helices that adopt a left-handed supercoil, analogous to a two-stranded rope where the nonpolar face of each a-helix is continually adjacent to that of the other helix. 2 This structure was first postulated by Crick to explain the X-ray diffraction pattern of a-keratin in the absence of sequence information.Pl The coiled-coil dimerization motif is natures way of creating a rod-like molecule that perhaps plays only a structural role in many fibrous proteins, such as the kmef (keratin, myosin, epidermis, fibrinogen) class 3,4 and the intermediate filament proteins)5 6 ... 

Basic Helix-Loop-Helix (bHLH) Proteins The DNA-binding domain of another class of dimeric transcription factors contains a structural motif very similar to the basic-zipper motif except that a nonhelical loop of the polypeptide chain separates two a-helical regions in each monomer (Figure 1 l-22b). Termed a basic helix-loop-helix (bHLH), this motif was predicted from the amino acid sequences of these proteins, which contain an N-terminal a helix with basic residues that interact with DNA, a middle loop region, and a C-termlnal region with hydrophobic amino acids spaced at intervals characteristic of an amphipathic a helix. As with basic-zipper proteins, different bHLH proteins can form heterodimers. 

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