Single-stranded binding protein

PDB 1v1q EBI

PDB 1urj EBI

1L1O Replication protein A

Single-stranded binding protein (SSB) is a type of protein that plays a crucial role in DNA replication, DNA repair, and recombination. These proteins bind to single-stranded DNA (ssDNA) without a preference for a particular sequence, stabilizing the DNA in its single-stranded form and protecting it from forming secondary structures or being degraded by nucleases. This function is essential during various cellular processes where the double-stranded DNA is temporarily unwound and becomes single-stranded.

Function

The primary function of single-stranded binding proteins is to bind to and stabilize single-stranded DNA. During DNA replication, the DNA helicase unwinds the double helix, resulting in regions of ssDNA that are vulnerable to degradation and the formation of secondary structures, such as hairpins. SSBs bind to these single-stranded regions, preventing the formation of these structures and protecting the DNA from nucleolytic degradation. This action ensures that the replication machinery, including DNA polymerase, can efficiently copy the DNA.

In DNA repair and recombination, SSBs also play a critical role by stabilizing the ssDNA intermediates that form during these processes. By binding to ssDNA, SSBs facilitate the proper alignment and processing of DNA strands for repair or recombination.

Structure

Single-stranded binding proteins typically function as multimers, with the tetramer being the most common form in E. coli (EcoSSB). Each monomer in the tetramer can bind to a specific number of nucleotides, covering and protecting the ssDNA. The structure of these proteins allows them to bind ssDNA with high affinity without covering the bases, thus enabling other proteins involved in DNA metabolism to access the DNA as needed.

Types

There are various types of SSBs, depending on the organism. In bacteria, such as E. coli, the SSB is well-characterized and functions as a tetramer. In eukaryotes, such as humans, the replication protein A (RPA) is the equivalent of SSB. RPA is more complex and consists of multiple subunits that together bind ssDNA. In viruses, specific SSBs can also be found, which are adapted to the particular needs of the viral life cycle.

Role in Biotechnology

Due to their ability to stabilize ssDNA, SSBs are also valuable tools in biotechnology and molecular biology research. They are used in techniques such as DNA sequencing, PCR (Polymerase Chain Reaction), and nucleic acid amplification, where maintaining the ssDNA in a stable form is crucial for the success of these methodologies.

Conclusion

Single-stranded binding proteins are essential for the maintenance of genome stability, playing a critical role in DNA replication, repair, and recombination. Their ability to stabilize ssDNA makes them a key component of the cellular machinery and a valuable tool in research and biotechnology.