What is the difference between Uniprot Accession Numbers that start with O, P, or Q?

Question

[ahmedmelmoselhy]

Does O, P, or Q stands for different things ?

I have looked at this webpage, but it didn't answer the quesion https://www.uniprot.org/help/accession_numbers

Answer 1

[Elisabeth Gasteiger]

Confirmation from UniProt: Indeed accession numbers have no meaning and just serve as unique identifiers.

This is the short answer, and below is some background:

Originally, accession numbers started with P, later with O, P and Q, but due to the large increase in the number of protein sequences in UniProtKB, we had to extend the existing accession number format by allowing the first character to be any of the 26 letters (instead of only O, P and Q). To avoid assigning accession numbers identical to those which have been used by the International Nucleotide Sequence Database, the extension in the first position went along with a restriction in the third position which can only be a letter. See https://www.uniprot.org/news/2006/11/14/release

The most recent extension of the UniProtKB accession number format took place in 2014 to allow for 10-character accession numbers: http://www.uniprot.org/help/2014/06/11/release

The "new-style" accession numbers do not have any different meaning at all, except that they implicitly convey the information that the entry they identify has been created since June 2014 - however the reverse is not necessarily true, i.e. not all new entries have 10-character accessions. In any case, it is best not to interpret ACs in any way, but just consider them as unique identifiers for a UniProtKB record.

[ATpoint]

Thank you Elisabeth Gasteiger for the confirmation. I will toggle your answer as accepted.

Answer 2

[ATpoint]

Apparently no rule for this beyond what is listed here: https://www.uniprot.org/help/accession_numbers