Solute carrier family


The solute carrier group of membrane transport proteins include over 400 members organized into 66 families. Most members of the SLC group are located in the cell membrane. The SLC gene nomenclature system was originally proposed by the HUGO Gene Nomenclature Committee and is the basis for the official HGNC names of the genes that encode these transporters. A more general transmembrane transporter classification can be found in TCDB database.
Solutes that are transported by the various SLC group members are extremely diverse and include both charged and uncharged organic molecules as well as inorganic ions and the gas ammonia.
As is typical of integral membrane proteins, SLCs contain a number of hydrophobic transmembrane alpha helices connected to each other by hydrophilic intra- and extra-cellular loops. Depending on the SLC, these transporters are functional as either monomers or obligate homo- or hetero-oligomers. Many SLC families are members of the major facilitator superfamily.

Scope

By convention of the nomenclature system, members within an individual SLC family have greater than 20-25% sequence identity to each other. In contrast, the homology between SLC families is very low to non-existent. Hence, the criteria for inclusion of a family into the SLC group is not evolutionary relatedness to other SLC families but rather functional.
The SLC group include examples of transport proteins that are:
The SLC series does not include members of transport protein families that have previously been classified by other widely accepted nomenclature systems including:

Subcellular distribution

Most members of the SLC group are located in the cell membrane, but some members are located in mitochondria or other intracellular organelles.

Nomenclature system

Names of individual SLC members have the following format:
SLCnXm
where:
  • SLC is the root name
  • n = an integer representing a family
  • X = a single letter denoting a subfamily
  • m = an integer representing an individual family member.
For example, is the first isoform of subfamily A of SLC family 1.
An exception occurs with SLC family 21, which for historical reasons have names in the format SLCOnXm where n = family number, X = subfamily letter, and m = member number.
While the HGNC only assign nomenclature to human genes, by convention vertebrate orthologs of these genes adopt the same nomenclature. For rodents, the case of the symbols differs from other vertebrates by using title case, i.e. Slc1a1 denotes the rodent ortholog of the human SLC1A1 gene.

Families

The following families are named under SLC:
  1. high-affinity glutamate and neutral amino acid transporter
  2. *
  3. facilitative GLUT transporter
  4. *
  5. heavy subunits of heterodimeric amino acid transporters
  6. *
  7. bicarbonate transporter
  8. *
  9. sodium glucose cotransporter
  10. *
  11. sodium- and chloride-dependent sodium:neurotransmitter symporters
  12. *
  13. cationic amino acid transporter/glycoprotein-associated
  14. * cationic amino acid transporters
  15. * glycoprotein-associated/light or catalytic subunits of heterodimeric amino acid transporters
  16. Na+/Ca2+ exchanger
  17. *
  18. Na+/H+ exchanger
  19. *
  20. sodium bile salt cotransport
  21. *
  22. proton coupled metal ion transporter
  23. *
  24. electroneutral cation-Cl cotransporter
  25. *
  26. Na+-sulfate/carboxylate cotransporter
  27. *
  28. urea transporter
  29. *
  30. proton oligopeptide cotransporter
  31. *
  32. monocarboxylate transporter
  33. *
  34. vesicular glutamate transporter
  35. *
  36. vesicular amine transporter
  37. *
  38. folate/thiamine transporter
  39. *
  40. type III Na+-phosphate cotransporter
  41. *
  42. organic anion transporting
  43. * subfamily 1
  44. * subfamily 2
  45. * subfamily 3
  46. * subfamily 4
  47. * subfamily 5
  48. * subfamily 6
  49. organic cation/anion/zwitterion transporter
  50. *
  51. Na+-dependent ascorbic acid transporter
  52. *
  53. Na+/(Ca2+-K+) exchanger
  54. *
  55. mitochondrial carrier
  56. *, SLC25A47, SLC25A48, MTCH1(SLC25A49), MTCH2(SLC25A50), SLC25A51, SLC25A52, SLC25A53
  57. multifunctional anion exchanger
  58. *
  59. fatty acid transport proteins
  60. *
  61. Na+-coupled nucleoside transport
  62. *
  63. facilitative nucleoside transporter
  64. *
  65. zinc transporter
  66. *
  67. copper transporter
  68. *
  69. vesicular inhibitory amino acid transporter
  70. *
  71. Acetyl-CoA transporter
  72. *
  73. type II Na+-phosphate cotransporter
  74. *
  75. nucleotide-sugar transporter
  76. * subfamily A
  77. * subfamily B
  78. * subfamily C
  79. * subfamily D
  80. * subfamily E
  81. * subfamily F
  82. * subfamily G
  83. proton-coupled amino acid transporter
  84. *
  85. sugar-phosphate/phosphate exchanger
  86. *
  87. System A & N, sodium-coupled neutral amino acid transporter
  88. *
  89. metal ion transporter
  90. *
  91. basolateral iron transporter
  92. *
  93. MgtE-like magnesium transporter
  94. *
  95. Ammonia transporter
  96. *
  97. Na+-independent, system-L like amino acid transporter
  98. *
  99. Choline-like transporter
  100. *
  101. Putative sugar transporter
  102. *
  103. Folate transporter
  104. *
  105. multidrug and toxin extrusion
  106. *
  107. Heme transporter family
  108. *
  109. Heme transporter
  110. *
  111. Sugar efflux transporters of the SWEET family
  112. *
  113. Transporters of steroid-derived molecules
  114. *
  115. Riboflavin transporter family RFVT/SLC52
  116. *
  117. Phosphate carriers
  118. *, MPC2, MPC1L)
  119. Mitochondrial cation/proton exchangers
  120. *, LETM2, LETMD1)
  121. Sideroflexins
  122. *, SFXN3, SFXN4, SFXN5)
  123. NiPA-like magnesium transporter family
  124. *, NIPAL2, NIPAL3, NIPAL4)
  125. MagT-like magnesium transporter family
  126. * )
  127. Sodium-dependent lysophosphatidylcholine symporter family
  128. *, MFSD2B)
  129. Glucose transporters
  130. *, MFSD4B)
  131. Molybdate transporter family
  132. *
  133. Pyrophosphate transporters
  134. *
  135. Sphingosine-phosphate transporters
  136. *, SPNS2, SPNS3)
  137. Golgi Ca2+/H+ exchangers
  138. *
  139. NPC-type cholesterol transporters
  140. *
  141. Cationic amino acid exporters
  142. *

Putative SLCs

Putative SLCs, also called atypical SLCs, are novel, plausible secondary active or facilitative transporter proteins that share ancestral background with the known SLCs. The atypical SLCs of MFS type can, however, be subdivided into 15 Putative MFS Transporter Families.
All the putative SLCs are plausible SLC transporters. Some are only "atypical" when it comes to their nomenclature; the genes have an SLC assignment but as an alias, and have retained their already assigned "non-SLC" gene symbol as the approved symbol.
Here are some Putative SLCs listed: OCA2, CLN3, TMEM104, SPNS1, SPNS2, SPNS3, SV2A, SV2B, SV2C, SVOP, SVOPL, MFSD1, MFSD2A, MFSD2B, MFSD3, MFSD4A, MFSD4B, MFSD5, MFSD6, MFSD6L, MFSD8, MFSD9, MFSD10, MFSD11, MFSD12, MFSD13A, MFSD14A, MFSD14B, UNC93A and UNC93B1.