This entry includes type 2A encapsulin shell protein SrpI from Synechococcus elongatus and its homologues from Mycobacteria, such as [Swissprot:P46841] from Mycobacterium leprae, which forms a large 950 kDa complex with itself [1]. It has been described as the most antigenic protein in human leprosy patients, being useful for serodiagnosis of leprosy [1-3]. Some members of this family contain an N-terminal cyclic nucleotide binding domain Pfam:PF00027. This family includes proteins annotated as phage capsid proteins, such as UniProt:A0A1X9IAH4 suggesting that in bacteria this protein is a co-opted phage protein. There is a clear evolutionary relationship between the bacterial encapsulins and Caudovirales bacteriophages as they share the HK97 fold of the capsid proteins [2].

1: Characterization of the gene encoding the immunodominant 35 kDa protein of Mycobacterium leprae. Winter N, Triccas JA, Rivoire B, Pessolani MC, Eiglmeier K, Lim EM, Hunter SW, Brennan PJ, Britton WJ; Mol Microbiol. 1995;16:865-876. PMID:7476185

2: Discovery and characterization of a novel family of prokaryotic nanocompartments involved in sulfur metabolism. Nichols RJ, LaFrance B, Phillips NR, Radford DR, Oltrogge LM, Valentin-Alvarado LE, Bischoff AJ, Nogales E, Savage DF; Elife. 2021; [Epub ahead of print] PMID:33821786

3: Evaluation of major membrane protein-I as a serodiagnostic tool of pauci-bacillary leprosy. Tsukamoto Y, Maeda Y, Makino M; Diagn Microbiol Infect Dis. 2014;80:62-65. PMID:25041703