Manual transfer of experimentally-verified manual GO annotation data to homologous complexes by curator judgment of sequence, composition and function similarity

Birgit Meldal and Sandra Orchard (1). (1) European Bioinformatics Institute (EBI), Hinxton, Cambridgeshire, United Kingdom; 2018

Method for transferring manual annotations to an entry based on a curator’s judgment of its similarity to a putative homolog that has annotations that are supported with experimental evidence. Annotations are created when a curator judges that the sequence, composition and function of a complex shows high similarity to another complex that has annotation(s) supported by experimental evidence (and therefore display one of the evidence codes ECO:0000353 [IPI] or ECO:0005543). Annotations resulting from the transfer of GO terms display the ECO:0005610, ECO:0005544 or ECO:0005546 evidence codes and include an accession for the complex from which the annotation was projected in the ‘with/from’ field (column 8). This field MUST contain a Complex Portal accession identifier. Putative homologs are chosen using information combined from a variety of complementary sources. Potential homologs are initially identified using sequence similarity search programs such as BLAST. Homologous relationships are then verified manually using a combination of resources including sequence analysis tools, phylogenetic and comparative genomics databases such as Ensembl Compara, INPARANOID and OrthoMCL, as well as other specialised databases such as species-specific collections (e.g. HGNC’s HCOP). In all cases curators check the alignments for each complex component and use their experience to assess whether similarity is considered to be strong enough to infer that the two proteins have a common function so that they can confidently project an annotation. While there is no fixed cut-off point in percentage sequence similarity, generally proteins which have greater than 70% identity that covers greater than 90% of the length of both proteins are examined further. Whilst we expect subunit composition to be conserved between closely related species, this is not an absolute rule and orthologous complexes may differ if a subunit cannot be traced in one species or is experimentally shown not to be present. When there is evidence of multiple paralogs for a single species, multiple variants of the complex can be inferred.