Kiwa
Description
The Kiwa antiviral defense system was first described in (N/A) and its exact mechanism was further described in (N/A) . It is named after one of the divine guardians of the ocean in the Māori traditions. Kiwa is a two-component bacterial antiviral system consisting of the transmembrane sensor KwaA and the DNA-binding effector KwaB. They form a large membrane-associated supercomplex. During infection, KwaA detects phage attachment, triggering KwaB to bind to ejected phage DNA, thus blocking phage replication and late transcription without killing the host cell.
Molecular mechanisms
The Kiwa defense system is made of a transmembrane supercomplex composed of units of transmembrane KwaA tetramers bound to cytoplasmic KwaB dimers. Upon sensing of the infection through phage tail fibers-induced membrane deformations and before phage DNA ejection, the Kiwa complex assembles and is recruited at the infection foci. The resulting activation of KwaA, through so far unknown molecular mechanisms, enables activation of KwaB through direct contact. KwaB then binds to the ejected phage DNA, probably to ssDNA at replication forks, thereby halting phage DNA replication and transcription of late genes without degradation of the DNA.
The phage encodes a DNA-mimicking protein, Gam, which inhibits KwaB by occupying its DNA binding site, thus acting as a competitive inhibitor. As Gam also targets RecBCD complexes that target phage DNA, coexistence of RecBCD and Kiwa in a genome allows Kiwa activity through RecBCD-mediated buffering of Gam availability. This complementarity gives the cell increased immune robustness, limiting the effectiveness of phage counterdefense mechanisms.
Example of genomic structure
The Kiwa is composed of 2 proteins: KwaA and KwaB.
Here is an example found in the RefSeq database:
The Kiwa system in Dickeya zeae (GCF_019464635.1, NZ_CP040817) is composed of 2 proteins KwaA (WP_038908257.1) KwaB_2 (WP_192991081.1)