Evolutionary and functional studies on microsporidian ATP-binding cassettes: Insights into the adaptation of microsporidia to obligated intracellular parasitism.

ATP-binding cassette (ABC) transporters comprise the largest family of transmembrane proteins and are found in all domains of life. The ABCs are involved in a variety of biological processes and as exporters play important roles in multidrug resistance. However, the ABC transporters have not been addressed in microsporidia, which are a very large group of obligate intracellular parasites that can infect nearly all animals, including humans. Here, a total of 234 ABC transporters were identified from 18 microsporidian genomes and classified into five subfamilies, including 74 ABCBs, 2 ABCCs, 18 ABCEs, 15 ABCFs, 102 ABCGs and 23 uncategorized members. Two subfamilies, ABCA and ABCD, are found in most organisms, but lost in microsporidia. Phylogenetic analysis indicated that microsporidian ABCB and ABCG subfamilies expanded by recent gene duplications, which resulted in the two largest subfamilies in microsporidia. Functional analysis via qRT-PCR and Western blotting revealed that NoboABCG1.1, an ABCG member of Nosema bombycis, is expressed in mature spores and up-regulated from 1 dpi to 6 dpi in infected silkworm midgut. IFA and IEM analysis showed that NoboABCG1.1 is localized on the plasma membrane of the sporoplasm, meront and mature spore. The propagation of N. bombycis was significantly inhibited after the RNAi of NoboABCG1.1 expression, indicating that NoboABCG1.1 is important to the pathogen proliferation. In conclusion, our study uncovered that the ABCs evolved during microsporidia adaption to intracellular parasitism and play important roles in the pathogen development.