Editing Nucleomorph (section)

== Structure ==
A cryptomonad nucleomorph is typically much smaller than the host nucleus. A relatively large portion of its size is devoted to the [[nucleolus]], which contains its own ribosomes and rRNA.<ref name=McFadden>{{cite journal |last1=McFadden |first1=Geoffrey I. |title=The cryptomonad nucleomorph |journal=Protoplasma |date=September 2017 |volume=254 |issue=5 |pages=1903–1907 |doi=10.1007/s00709-017-1153-5 |pmid=28828570 |bibcode=2017Prpls.254.1903M |url=https://www.researchgate.net/publication/319214581}}</ref> There seems to be nuclear pores observable by imaging, but genetic work has failed to find any protein appropriate for forming the nuclear pore complex.<ref>{{cite journal |last1=Irwin |first1=NAT |last2=Keeling |first2=PJ |title=Extensive Reduction of the Nuclear Pore Complex in Nucleomorphs. |journal=Genome Biology and Evolution |date=1 March 2019 |volume=11 |issue=3 |pages=678–687 |doi=10.1093/gbe/evz029 |pmid=30715330|pmc=6411479 }}</ref><ref name=pmid31124562>{{cite journal |last1=Zauner |first1=S |last2=Heimerl |first2=T |last3=Moog |first3=D |last4=Maier |first4=UG |title=The Known, the New, and a Possible Surprise: A Re-Evaluation of the Nucleomorph-Encoded Proteome of Cryptophytes. |journal=Genome Biology and Evolution |date=1 June 2019 |volume=11 |issue=6 |pages=1618–1629 |doi=10.1093/gbe/evz109 |pmid=31124562|pmc=6559170 }}</ref>

There is one nucleomorph per plastid. The nucleomorph divides before the accompanying plastid. The dividing nucleomorph lacks a mitotic spindle, and the nucleomorph envelope persists throughout division.<ref name=McFadden/>

Between the plastid and the cytoplasm of the host there are four membranes: the inner and outer membranes of the chloroplast, the periplastid membrane, and the epiplastid membrane. The epiplastid membrane is encrusted with ribosomes (in cryptomonads) and is in many ways similar to a [[endoplasmic reticulum]], hence the name "chloroplast endoplasmic reticulum" (cER). Plastid-targeted proteins encoded in the host genome must cross all four membranes to reach the plastid. First they use classic secretory [[signal peptide]]s to cross the epiplastid membrane. Then the symbiont-specific [[ERAD]]-like machinery (SELMA) &ndash; encoded in the nucleomorph as a repurposed ERAD &ndash; pulls the protein from the epiplastid space (or the lumen of the cER) into the periplastid space (the cytoplasm of the symbiote). The standard chloroplast transit peptide then acts to cross the remaining two layers via [[TIC/TOC complex]].<ref name=McFadden/>

The [[chlorarachniophytes]], on the other hand, has no such thing as a cER, hence the initial import into the epiplastid space must occur by some other mechanism. It's only known that their plastid-targeted proteins are prefixed by both a signal peptide and a chloroplast-targeting peptide much like cryptomonads. Based on research done on apicomplexa, which also has 4 membranes but no cER, it's possible that the protein is first sent into the ER, then sent to the epiplastid space by the endomembrane sorting system.<ref>{{cite journal |last1=van Dooren |first1=Giel G |last2=Schwartzbach |first2=Steven D |last3=Osafune |first3=Tetsuaki |last4=McFadden |first4=Geoffrey I |title=Translocation of proteins across the multiple membranes of complex plastids |journal=Biochimica et Biophysica Acta (BBA) - Molecular Cell Research |date=December 2001 |volume=1541 |issue=1–2 |pages=34–53 |doi=10.1016/S0167-4889(01)00154-9|pmid=11750661 }}</ref> Some sort of a pore may then move the peptide into the periplastid space, but there seems to be no SELMA-like pore in this group. It's only known that the TIC/TOC complex exists for crossing the last two layers.<ref>{{cite journal |last1=Sheiner |first1=L |last2=Striepen |first2=B |title=Protein sorting in complex plastids. |journal=Biochimica et Biophysica Acta (BBA) - Molecular Cell Research |date=February 2013 |volume=1833 |issue=2 |pages=352–9 |doi=10.1016/j.bbamcr.2012.05.030 |pmid=22683761|pmc=3494742 }}</ref>