Heptad repeat

Heptad Repeat

A heptad repeat is a sequence motif found in proteins, characterized by a repeating pattern of seven amino acids. This structural motif is crucial in the formation of coiled-coil structures, which are important for the function of many proteins, including those involved in cellular signaling, structural integrity, and viral entry mechanisms.

Structure

The heptad repeat is denoted as (a-b-c-d-e-f-g)n, where each letter represents a position in the seven-residue repeat. Typically, positions 'a' and 'd' are occupied by hydrophobic amino acids, which facilitate the formation of a hydrophobic core when two or more heptad repeats align in a coiled-coil structure. The remaining positions are often occupied by polar or charged residues, which can form stabilizing interactions such as salt bridges or hydrogen bonds.

Function

Heptad repeats are integral to the function of many proteins:

• Coiled-coil domains: These are structural motifs where two or more alpha-helices are wound around each other. The heptad repeat pattern allows for the alignment of hydrophobic residues, stabilizing the coiled-coil structure through hydrophobic interactions.

• Viral fusion proteins: Many viral proteins, such as those from the influenza virus and HIV, contain heptad repeats that are crucial for the fusion of the viral envelope with host cell membranes. These repeats undergo conformational changes that bring the viral and cellular membranes into close proximity, facilitating fusion.

• Structural proteins: Proteins such as keratin and myosin contain heptad repeats that contribute to their mechanical properties and structural roles in cells.

Examples

• Leucine zipper: A type of coiled-coil domain found in transcription factors, where leucine residues at the 'd' position of the heptad repeat form a zipper-like structure that facilitates dimerization and DNA binding.

• SNARE proteins: These proteins mediate vesicle fusion in cells and contain heptad repeats that form coiled-coil structures essential for their function.

Also see

• Coiled-coil
• Leucine zipper
• Protein structure
• Viral fusion protein

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This diagram (which is interactive) of protein structure uses PCNA as an example. (PDB: 1AXC​)