CYP16 clan

The CYP16 family was the last new family described in vertebrates (Nelson, 2011) and related sequences have only recently gained the status of clan, distinct from the related CYP26 clan (Nelson DR and Goldstone JV, personal communication). No function has yet been described for CYP16 clan P450s.

The related but distinct CYP26 clan includes retinoic acid hydroxylases, and the related cyanobacterial CYP120A1 is also a retinoid acid hydroxylase (Alder et al., 2009).

Sequences belonging to the CYP16 clan are found in all major lineages of arthropods, but their distribution is very patchy:

CYP16 clan P450s of arthropods (in black) and of outgroup P450s (CYP120 and CYP26).

41 arthropod CYP16 clan sequences are provided here

Xiphosura is the only lineage of Chelicerates where CYP16 clan sequences are found. In myriapods, there is a CYP16 clan sequence in Strigamia maritima (but no evidence in the TSA of Scolopendra dehaani, GBIM01), and no evidence in the millipedes Chamberlinius hualienensis (TSA) Helicorthomorpha holstii or Trigoniulus corallinus (genomes, JAAFCF01 and JAAFCE01). CYP16 clan sequences are also found in copepods and decapods, as well as barnacles and ostracods. Their abundance in the TSA of Copepoda and Cirripedia suggests a relatively high expression in those groups of Crustacea. The presence of sequences from the CYP16 clan is restricted in insects to Archaeognatha and some Zygentoma (partial sequence found in Atelura formicaria) and Odonata.

In all cases, there is a single CYP16 sequence in each species, indicating that despite the multiple losses in arthropods, the CYP16 homologs are highly conserved and do not appear to bloom as seen in all other clans.

The arthropod CYP16 clan sequences are rather peculiar, in that they lack the C-helix WxxxR motif. They have a typical HWHGxxSxP motif in the helices F-G loop region and have an unusual and shifted I helix motif ALxPKxxASxLTS. However, they have a highly conserved ETLR and SRS5 motif ExxRxxPPxxGGxR, similar to CYP26 clan structures. Most notably, the arthropod CYP16 clan proteins lack the typical N-terminal membrane-anchoring sequence.