Nematode chemoreceptor

{{Pfam box |Symbol = Sra_chemorcpt |Name = Nematode chemoreceptor, Sra |Pfam = PF02117 |InterPro = IPR000344 |PROSITE = |PDB = }}

Nematode chemoreceptors are chemoreceptors of nematodes. Animals recognise a wide variety of chemicals using their senses of taste and smell. The nematode Caenorhabditis elegans has only 14 types of chemosensory neuron, yet is able to respond to dozens of chemicals because each neuron detects several stimuli. More than 40 highly divergent transmembrane proteins that could contribute to this functional diversity have been described.{{cite journal | vauthors = Troemel ER, Chou JH, Dwyer ND, Colbert HA, Bargmann CI | title = Divergent seven transmembrane receptors are candidate chemosensory receptors in C. elegans | journal = Cell | volume = 83 | issue = 2 | pages = 207–218 | date = October 1995 | pmid = 7585938 | doi = 10.1016/0092-8674(95)90162-0 | doi-access = free }} Most of the candidate receptor genes are in clusters of similar genes; 11 of these appear to be expressed in small subsets of chemosensory neurons. A single type of neuron can potentially express at least 4 different receptor genes. Some of these might encode receptors for water-soluble attractants, repellents and pheromones, which are divergent members of the G-protein-coupled receptor family. Sequences of the Sra family of C. elegans receptor-like proteins contain 6-7 hydrophobic, putative transmembrane, regions. These can be distinguished from other 7TM proteins (especially those known to couple G-proteins) by their own characteristic TM signatures.

More than 1300 potential chemoreceptor genes have been identified in C. elegans, which are generally prefixed sr for serpentine receptor. The receptor superfamilies include Sra (Sra, Srb, Srab, Sre), Str (Srh, Str, Sri, Srd, Srj, Srm, Srn) and Srg (Srx, Srt, Srg, Sru, Srv, Srxa), as well as the families Srw, Srz, Srbc, Srsx and Srr.{{cite journal | vauthors = Robertson HM, Thomas JH | title = The putative chemoreceptor families of C. elegans | journal = WormBook | pages = 1–12 | date = January 2006 | pmid = 18050473 | pmc = 4781013 | doi = 10.1895/wormbook.1.66.1 }}{{cite journal | vauthors = Chen N, Pai S, Zhao Z, Mah A, Newbury R, Johnsen RC, Altun Z, Moerman DG, Baillie DL, Stein LD | display-authors = 6 | title = Identification of a nematode chemosensory gene family | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 102 | issue = 1 | pages = 146–151 | date = January 2005 | pmid = 15618405 | pmc = 539308 | doi = 10.1073/pnas.0408307102 | doi-access = free | bibcode = 2005PNAS..102..146C }} Many of these proteins have homologues in Caenorhabditis briggsae.

These receptors are distantly related to the rhodopsin-like receptors.{{cite journal | vauthors = Nordström KJ, Sällman Almén M, Edstam MM, Fredriksson R, Schiöth HB | title = Independent HHsearch, Needleman--Wunsch-based, and motif analyses reveal the overall hierarchy for most of the G protein-coupled receptor families | journal = Molecular Biology and Evolution | volume = 28 | issue = 9 | pages = 2471–2480 | date = September 2011 | pmid = 21402729 | doi = 10.1093/molbev/msr061 | doi-access = }} In contrast the receptor Sro is a true rhodopsin-like receptor. It is a member of the nemopsins a subgroup of the opsins,{{cite journal | vauthors = Gühmann M, Porter ML, Bok MJ | title = The Gluopsins: Opsins without the Retinal Binding Lysine | journal = Cells | volume = 11 | issue = 15 | pages = 2441 | date = August 2022 | pmid = 35954284 | doi = 10.3390/cells11152441 | pmc = 9368030 | doi-access = free }} but unlike most other opsins it does not have a lysine corresponding to position 296 in cattle rhodopsin. The lysine is replaced by an asparagine. The lysine is needed so that the chromophore retinal can covalently bind to the opsin{{cite journal | vauthors = Bownds D | title = Site of attachment of retinal in rhodopsin | journal = Nature | volume = 216 | issue = 5121 | pages = 1178–1181 | date = December 1967 | pmid = 4294735 | doi = 10.1038/2161178a0 | bibcode = 1967Natur.216.1178B | s2cid = 1657759 }} via a Schiff-base,{{cite journal | vauthors = Collins FD | title = Rhodopsin and indicator yellow | journal = Nature | volume = 171 | issue = 4350 | pages = 469–471 | date = March 1953 | pmid = 13046517 | doi = 10.1038/171469a0 | bibcode = 1953Natur.171..469C | s2cid = 4152360 }}{{cite journal | vauthors = Pitt GA, Collins FD, Morton RA, Stok P | title = Studies on rhodopsin. VIII. Retinylidenemethylamine, an indicator yellow analogue | journal = The Biochemical Journal | volume = 59 | issue = 1 | pages = 122–128 | date = January 1955 | pmid = 14351151 | doi = 10.1042/bj0590122 | pmc = 1216098 }} which makes the opsin light sensitive. If the lysine is replaced by another amino acid then the opsin becomes light insensitive.{{cite journal | vauthors = Leung NY, Thakur DP, Gurav AS, Kim SH, Di Pizio A, Niv MY, Montell C | title = Functions of Opsins in Drosophila Taste | journal = Current Biology | volume = 30 | issue = 8 | pages = 1367–1379.e6 | date = April 2020 | pmid = 32243853 | doi = 10.1016/j.cub.2020.01.068 | pmc = 7252503 | bibcode = 2020CBio...30E1367L }}{{cite journal | vauthors = Kumbalasiri T, Rollag MD, Isoldi MC, Castrucci AM, Provencio I | title = Melanopsin triggers the release of internal calcium stores in response to light | journal = Photochemistry and Photobiology | volume = 83 | issue = 2 | pages = 273–279 | date = March 2007 | pmid = 16961436 | doi = 10.1562/2006-07-11-RA-964 | s2cid = 23060331 }} Therefore, Sro is also thought to be a chemoreceptor.