Figure 3-9. Secondary-structure motifs.
Figure 3-9. Secondary-structure motifs.

Figure 3-9Secondary-structure motifs

(a) The coiled-coil motif (left) is characterized by two or more helices wound around one another. In some DNA-binding proteins, like c-Jun, a two-stranded coiled coil is responsible for dimerization (right). Each helix in a coiled coil has a repeated heptad sequence.

Figure 3-9. Secondary-structure motifs.
with a leucine or other hydrophobic residue (red) at positions 1 and 4, forming a hydrophobic stripe along the helix surface. The helices pair by binding along their hydrophobic stripes, as seen in both models displayed here, in which the hydro- phobic side chains are shown in red. (b) The helix-loop-helix motif occurs in many calcium-binding proteins. Oxygen-containing R groups of residues in the loop form a ring around a Ca2+ ion. The 14-aa loop sequence (right) is rich in invariant hydrophilic residues. (c) The zincfinger motif is present in many proteins that bind nucleic acids. A Zn2+ ion is held between a pair of β strands (green) and a single α helix (blue) by a pair of cysteine and histidine residues. In the 25-aa sequence of this motif the invariant cysteines usually occur at positions 3 and 6, and the invariant histidines at positions 20 and 24. [Part (a) courtesy of V. Malashkevich and S. Choe.]

From: Section 3.1, Hierarchical Structure of Proteins

Cover of Molecular Cell Biology
Molecular Cell Biology. 4th edition.
Lodish H, Berk A, Zipursky SL, et al.
New York: W. H. Freeman; 2000.
Copyright © 2000, W. H. Freeman and Company.

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