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128  structures 3093  species 1  interaction 5580  sequences 15  architectures

Family: SSB (PF00436)

Summary: Single-strand binding protein family

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This is the Wikipedia entry entitled "Single-stranded binding protein". More...

Single-stranded binding protein Edit Wikipedia article

PDB 1v1q EBI.jpg
Crystal structure of PriB- a primosomal DNA replication protein of Escherichia coli
Symbol SSB
Pfam PF00436
Pfam clan CL0021
InterPro IPR000424
SCOP 1kaw
TCDB 3.A.7

Single-stranded binding proteins (not to be confused with the E. coli protein, Single-strand DNA-binding protein, SSB) are a class of proteins that have been identified in both viruses and organisms from bacteria to humans.


Single stranded binding proteins prevent premature annealing (binding of complementary DNA sequences), protect the single-stranded DNA from being digested by nucleases, and prevent secondary structure formation, thereby allowing other enzymes to function effectively on the single strand.

Viral SSB

PDB 1urj EBI.jpg
Single stranded DNA-binding protein(icp8) from herpes simplex virus-1
Symbol Viral_DNA_bp
Pfam PF00747
InterPro IPR000635

Although the overall picture of human cytomegalovirus (HHV-5) DNA synthesis appears typical of the herpesviruses, some novel features are emerging.


In ICP8, the herpes simplex virus (HSV-1) single-strand DNA-binding protein (ssDNA-binding protein (SSB)), The head consists of the eight alpha helices. The front side of the neck region consists of a five-stranded beta-sheet and two alpha helices, whereas the back side is a three-stranded beta-sheet The shoulder part of the N-terminal domain contains an alpha-helical and beta-sheet region.[1] The herpes simplex virus (HSV-1) SSB, ICP8, is a nuclear protein that, along other replication proteins is required for viral DNA replication during lytic infection.[1]


Six herpes virus-group-common genes encode proteins that likely constitute the replication fork machinery, including a two-subunit DNA polymerase, a Helicase-primase complex and a single-stranded DNA-binding protein.[2] The human herpesvirus 1 (HHV-1) single-strand DNA-binding protein ICP8 is a 128kDa zinc metalloprotein. Photoaffinity labeling has shown that the region encompassing amino acid residues 368-902 contains the single-strand DNA-binding site of ICP8.[3] The HHHV-1 UL5, UL8, and UL52 genes encode an essential heterotrimeric DNA helicase-primase that is responsible for concomitant DNA unwinding and primer synthesis at the viral DNA replication fork. ICP8 may stimulate DNA unwinding and enable bypass of cisplatin damaged DNA by recruiting the helicase-primase to the DNA.[4]

Bacterial SSB

In molecular biology, SSB protein domains in bacteria are important maintaining DNA metabolism, more specifically DNA replication, repair and recombination.[5] It has a structure of three beta-strands to a single six-stranded beta-sheet to form a dimer.[6]

Eukaryotic Replication Protein A

Replication protein A
1L1O Replication protein A.png
This is an image of human Replication protein A. From PDB: 1L1OProteopedia protein A Replication protein A
Gene Chromosomal
Replication protein A1 RPA1 Chr. 17 p13.3
Replication protein A2 RPA2 Chr. 1 p35.3
Replication protein A3 RPA3 Chr. 7 p21.3

Replication protein A is the equivalent to SSB in eukaryotic cells.

See also


  1. ^ a b Mapelli M, Panjikar S, Tucker PA (2005). "The crystal structure of the herpes simplex virus 1 ssDNA-binding protein suggests the structural basis for flexible, cooperative single-stranded DNA binding". J Biol Chem. 280 (4): 2990–7. doi:10.1074/jbc.M406780200. PMID 15507432. 
  2. ^ Anders DG, McCue LA (1996). "The human cytomegalovirus genes and proteins required for DNA synthesis". Intervirology. 39 (5-6): 378–88. PMID 9130047. 
  3. ^ White EJ, Boehmer PE (October 1999). "Photoaffinity labeling of the herpes simplex virus type-1 single-strand DNA-binding protein (ICP8) with oligodeoxyribonucleotides". Biochem. Biophys. Res. Commun. 264 (2): 493–7. doi:10.1006/bbrc.1999.1566. PMID 10529391. 
  4. ^ Tanguy Le Gac N, Villani G, Boehmer PE (May 1998). "Herpes simplex virus type-1 single-strand DNA-binding protein (ICP8) enhances the ability of the viral DNA helicase-primase to unwind cisplatin-modified DNA". J. Biol. Chem. 273 (22): 13801–7. doi:10.1074/jbc.273.22.13801. PMID 9593724. 
  5. ^ Meyer RR, Laine PS (December 1990). "The single-stranded DNA-binding protein of Escherichia coli". Microbiol. Rev. 54 (4): 342–80. PMC 372786free to read. PMID 2087220. 
  6. ^ Raghunathan S, Ricard CS, Lohman TM, Waksman G (June 1997). "Crystal structure of the homo-tetrameric DNA binding domain of Escherichia coli single-stranded DNA-binding protein determined by multiwavelength x-ray diffraction on the selenomethionyl protein at 2.9-A resolution". Proc. Natl. Acad. Sci. U.S.A. 94 (13): 6652–7. doi:10.1073/pnas.94.13.6652. PMC 21213free to read. PMID 9192620. 

This article incorporates text from the public domain Pfam and InterPro IPR000635

External links

This page is based on a Wikipedia article. The text is available under the Creative Commons Attribution/Share-Alike License.

This tab holds the annotation information that is stored in the Pfam database. As we move to using Wikipedia as our main source of annotation, the contents of this tab will be gradually replaced by the Wikipedia tab.

Single-strand binding protein family Provide feedback

This family includes single stranded binding proteins and also the primosomal replication protein N (PriB). PriB forms a complex with PriA, PriC and ssDNA.

Literature references

  1. Raghunathan S, Ricard CS, Lohman TM, Waksman G; , Proc Natl Acad Sci U S A 1997;94:6652-6657.: Crystal structure of the homo-tetrameric DNA binding domain of Escherichia coli single-stranded DNA-binding protein determined by multiwavelength x-ray diffraction on the selenomethionyl protein at 2.9-A resolution. PUBMED:9192620 EPMC:9192620

  2. Yang C, Curth U, Urbanke C, Kang C; , Nat Struct Biol 1997;4:153-157.: Crystal structure of human mitochondrial single-stranded DNA binding protein at 2.4 A resolution. PUBMED:9033597 EPMC:9033597

  3. Webster G, Genschel J, Curth U, Urbanke C, Kang C, Hilgenfeld R; , FEBS Lett 1997;411:313-316.: A common core for binding single-stranded DNA: structural comparison of the single-stranded DNA-binding proteins (SSB) from E. coli and human mitochondria. PUBMED:9271227 EPMC:9271227

Internal database links

External database links

This tab holds annotation information from the InterPro database.

InterPro entry IPR000424

The Escherichia coli single-strand binding protein [PUBMED:2087220] (gene ssb), also known as the helix-destabilising protein, is a protein of 177 amino acids. It binds tightly, as a homotetramer, to single-stranded DNA (ss-DNA) and plays an important role in DNA replication, recombination and repair. Closely related variants of SSB are encoded in the genome of a variety of large self-transmissible plasmids. SSB has also been characterised in bacteria such as Proteus mirabilis or Serratia marcescens. Eukaryotic mitochondrial proteins that bind ss-DNA and are probably involved in mitochondrial DNA replication are structurally and evolutionary related to prokaryotic SSB.

Gene Ontology

The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.

Domain organisation

Below is a listing of the unique domain organisations or architectures in which this domain is found. More...

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Pfam Clan

This family is a member of clan OB (CL0021), which has the following description:

The OB (oligonucleotide/oligosaccharide binding) was defined by Murzin [1]. The common part of the OB-fold, has a five-stranded beta-sheet coiled to form a closed beta-barrel. This barrel is capped by an alpha-helix located between the third and fourth strands [1].

The clan contains the following 61 members:

BOF BRCA-2_OB1 BRCA-2_OB3 CSD DNA_ligase_A_C DNA_ligase_OB DNA_ligase_OB_2 DUF2110 DUF223 DUF3127 EFP eIF-1a eIF-5a Elong-fact-P_C EutN_CcmL EXOSC1 MCM_OB mRNA_cap_C NlpE_C OB_aCoA_assoc OB_NTP_bind OB_RNB PCB_OB Phage_DNA_bind POT1 RecG_wedge RecO_N RecO_N_2 Rep-A_N Rep_fac-A_3 Rep_fac-A_C REPA_OB_2 Rho_RNA_bind Ribosom_S12_S23 Ribosomal_L2 Ribosomal_S17 Ribosomal_S28e RNA_pol_Rbc25 RNA_pol_Rpb8 RNA_pol_RpbG Rrp44_CSD1 Rrp44_S1 RsgA_N RuvA_N S1 S1-like S1_2 SSB SSL_OB Stn1 TEBP_beta Ten1 Ten1_2 TOBE TOBE_2 TOBE_3 TRAM tRNA_anti-codon tRNA_anti-like tRNA_anti_2 tRNA_bind


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Curation and family details

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Seed source: Prosite & COG2965
Previous IDs: none
Type: Domain
Author: Finn RD, Bateman A
Number in seed: 53
Number in full: 5580
Average length of the domain: 102.60 aa
Average identity of full alignment: 27 %
Average coverage of the sequence by the domain: 63.36 %

HMM information View help on HMM parameters

HMM build commands:
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 17690987 -E 1000 --cpu 4 HMM pfamseq
Model details:
Parameter Sequence Domain
Gathering cut-off 21.9 21.9
Trusted cut-off 21.9 21.9
Noise cut-off 21.8 21.8
Model length: 104
Family (HMM) version: 23
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Species distribution

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Archea Archea Eukaryota Eukaryota
Bacteria Bacteria Other sequences Other sequences
Viruses Viruses Unclassified Unclassified
Viroids Viroids Unclassified sequence Unclassified sequence


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For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the SSB domain has been found. There are 128 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein seqence.

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