SMC proteins represent a large family of ATPases that participate in many aspects of higher-order chromosome organization and dynamics. SMC stands for Structural Maintenance of Chromosomes.

Prokaryotic

SMC proteins are conserved from bacteria to humans. Most bacteria have a single SMC protein in individual species that forms a homodimer. In a subclass of Gram-negative

Gram-negative

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...

 bacteria including Escherichia coli

Escherichia coli

Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

, a distantly related protein known as MukB plays an equivalent role.

Eukaryotic

Eukaryotes have at least six SMC proteins in individual organisms, and they form three distinct heterodimers with specialized functions:

• A pair of SMC1 and SMC3 constitutes the core subunits of the cohesin
  Cohesin
  Cohesin is a protein complex that regulates the separation of sister chromatids during cell division, either mitosis or meiosis.- Structure :...
  
  complexes involved in sister chromatid cohesion.
• Likewise, a pair of SMC2 and SMC4 acts as the core of the condensin
  Condensin
  Condensins are large protein complexes that play a central role in chromosome assembly and segregation in eukaryotic cells.- Subunit composition :...
  
  complexes implicated in chromosome condensation.
• A dimer composed of SMC5 and SMC6 functions as part of a yet-to-be-named octameric complex implicated in DNA repair
  DNA repair
  DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...
  
   and checkpoint responses.

Each complex contains a distinct set of non-SMC regulatory subunits.

Some organisms have variants of SMC proteins. For instance, mammals have a meiosis

Meiosis

Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

-specific variant of SMC1, known as SMC1β. The nematode Caenorhabditis elegans

Caenorhabditis elegans

Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...

has an SMC4-variant that has a specialized role in dosage compensation

Dosage compensation

Dosage compensation, also known as Ohno's Hypothesis, is a hypothetical genetic regulatory mechanism which operates to equalize the phenotypic expression of characteristics determined by genes on the X chromosome so that they are equally expressed in the human XY male and the XX female. In 2006,...

.

subfamily	complex	S. cerevisiae	S. pombe	C. elegans Caenorhabditis elegans Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...	D. melanogaster	vertebrates
SMC1α	cohesin Cohesin Cohesin is a protein complex that regulates the separation of sister chromatids during cell division, either mitosis or meiosis.- Structure :...	Smc1	Psm1	SMC-1	DmSmc1	SMC1α
SMC1β	cohesin Cohesin Cohesin is a protein complex that regulates the separation of sister chromatids during cell division, either mitosis or meiosis.- Structure :...  (meiotic)	-	-	-	-	SMC1β
SMC2	condensin Condensin Condensins are large protein complexes that play a central role in chromosome assembly and segregation in eukaryotic cells.- Subunit composition :...	Smc2	Cut14	MIX-1	DmSmc2	CAP-E
SMC3	cohesin Cohesin Cohesin is a protein complex that regulates the separation of sister chromatids during cell division, either mitosis or meiosis.- Structure :...	Smc3	Psm3	SMC-3	DmSmc3	SMC3
SMC4	condensin Condensin Condensins are large protein complexes that play a central role in chromosome assembly and segregation in eukaryotic cells.- Subunit composition :...	Smc4	Cut3	SMC-4	DmSmc4	CAP-C
SMC4 variant	dosage compensation complex	-	-	DPY-27	-	-
SMC5	SMC5-6	Smc5	Smc5	C27A2.1	CG32438	SMC5
SMC6	SMC5-6	Smc6	Smc6/Rad18	C23H4.6, F54D5.14	CG5524	SMC6

Primary

SMC protein monomers have a modular structure and contain the following domains:

1. Walker A
   Walker motifs
   The Walker A and Walker B motifs are protein sequence motifs. These were first reported in ATP-binding proteins by Walker and co-workers in 1982.-Walker A motif:...
   
    ATP-binding motif
   ATP-binding motif
   An ATP-Binding Motif is a specific sequence of protein subunits that promotes the attachment of ATP to a target protein. An ATP binding site is a protein micro-environment where ATP is captured and hydrolyzed to ADP, releasing energy that is utilized by the protein to "do work" by changing the...
   
   
2. coiled-coil region I
3. hinge region
4. coiled-coil region II
5. Walker B ATP-binding motif

Tertiary

SMC dimers form a V-shaped molecule with two long coiled-coil arms, each having an ATP

Adenosine triphosphate

Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

-binding domain at its distal end. The ATP-binding domain of SMC proteins is structurally related to that of ABC transporters, a large family of transmembrane proteins that actively transport small molecules across cellular membranes. It is thought that the cycle of ATP binding and hydrolysis

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 modulates the cycle of closing and opening of the V-shaped molecule, but the detailed mechanisms of action of SMC proteins remain to be determined.

Genes

The following human genes encode SMC proteins:

• SMC1A
  SMC1A
  Structural maintenance of chromosomes protein 1A is a protein that in humans is encoded by the SMC1A gene.-Interactions:SMC1A has been shown to interact with SMC3 and Ataxia telangiectasia mutated.-External links:*...
  
  
• SMC1B
  SMC1B
  Structural maintenance of chromosomes protein 1B is a protein that in humans is encoded by the SMC1B gene. SMC-1B belongs to a family of proteins required for chromatid cohesion and DNA recombination during meiosis and mitosis....
  
  
• SMC2
  SMC2
  Structural maintenance of chromosomes protein 2 also known as chromosome-associated protein E is a protein that in humans is encoded by the SMC2 gene....
  
  
• SMC3
• SMC4
  SMC4
  Structural maintenance of chromosomes protein 4 also known as chromosome-associated polypeptide C or XCAP-C homolog is a protein that in humans is encoded by the SMC4 gene. SMC-4 is a core subunit of condensin I and II, large protein complexes involved in chromosome condensation...
  
  
• SMC5
  SMC5
  Structural maintenance of chromosomes protein 5 is a protein encoded by the SMC5 gene in human.-Further reading:...
  
  
• SMC6
  SMC6
  Structural maintenance of chromosomes protein 6 is a protein that in humans is encoded by the SMC6 gene.-Further reading:...