1.A.133. The Bacterial Csx28 Pore-forming (Csx28) Family

Csx28, of type VI-B2CRISPER-Cas systems, is a transmembrane protein that assists to slow cellular metabolism upon viral infection, increasing antiviral defense. High-resolution cryo-electron microscopy revealed that Csx28 forms an octameric pore-like structure in the inner bacterial membrane in vivo (VanderWal et al. 2023). Csx28's antiviral activity in vivo requires sequence-specific cleavage of viral messenger RNAs by Cas13b, which subsequently results in membrane depolarization, slowed metabolism, and inhibition of sustained viral infection. This suggests a mechanism by which Csx28 acts as a downstream, Cas13b-dependent effector protein that uses membrane perturbation as an antiviral defense strategy (VanderWal et al. 2023).

1.A.133. The Bacterial Csx28 Pore-forming (Csx28) Family Csx28, of type VI-B2CRISPER-Cas systems, is a transmembrane protein that assists to slow cellular metabolism upon viral infection, increasing antiviral defense. High-resolution cryo-electron microscopy revealed that Csx28 forms an octameric pore-like structure in the inner bacterial membrane in vivo (VanderWal et al. 2023). Csx28's antiviral activity in vivo requires sequence-specific cleavage of viral messenger RNAs by Cas13b, which subsequently results in membrane depolarization, slowed metabolism, and inhibition of sustained viral infection. This suggests a mechanism by which Csx28 acts as a downstream, Cas13b-dependent effector protein that uses membrane perturbation as an antiviral defense strategy (VanderWal et al. 2023).
References:
VanderWal, A.R., J.U. Park, B. Polevoda, J.K. Nicosia, A.M. Molina Vargas, E.H. Kellogg, and M.R. O'Connell. (2023). Csx28 is a membrane pore that enhances CRISPR-Cas13b-dependent antiphage defense. Science 380: 410-415.
Examples:
TC#	Name	Organismal Type	Example
1.A.133.1.1	The Csx28 pore-forming protein of 177 aas with 1 N-terminal TMS and possibly a second moderately hydrkophhobic region between residues 100 - 140. It transports ions and colapses the inner membrane potential (pmf) by allowing the free flow of ions, possiblly inorganic cations, across the membrane. It has an octomeric structure with a well ordered C-terminal domain (VanderWal et al. 2023). The pore diameter is ~ 10 Å; each protomer is a 4-helix bundle (α1 to α4) where α1 and α2 form the inner lining of the pore and the two C-terminal helices form the outer surface of the pore. Cas13b senses the viral transcript and signals to Csx28.		Csx28 of Prevotella buccae

1.A.133.1.2	Uncharacterized protein of 178 aas with 1 N-terminal TMS.		*UP of Leptolyngbya* sp.**

1.A.133.1.3	Uncharacterized protein of 195 aas and 1 N-terminal TMS plus three more moderate peaks of hydrophobicity, where the degree of hydrophobicity is TMS1 > 2 > 3 > 4.		UP of Chitinophagales bacterium (activated sludge metagenome)

1.A.133.1.4	CRISPR-associated protein Csx28 OF 211 aas and kone strongly hydrophobic N-terminal TMS followed by two much less hydrophobic peaks that may be TMSs.		Csx28 of Phaeodactylibacter xiamenensis

1.A.133.1.5	Uncharacterized protein of 181 aas and 1 N-terminal TMS followed by three peaks of low degrees of hydrophobicity.		UP of Salmonella enterica

1.A.133.1.6	Uncharacterized protein of 172 aas and 1 N-terminal TMS		*UP of Colwellia* sp. (invertebrate metagenome)**

1.A.133.1.7	Uncharacterized protein of 163 aas and 1 N-terminal TMS.		UP of Anaerolineales bacterium (activated sludge metagenome)

1.A.133.1.8	Uncharacterized protein of 231 aas and 1 N-terminal TMS.		UP of Actinobacteria bacterium OV450

1.A.133.1.9	Uncharacterized protein of 187 aas and 1 N-terminal TMS.		*UP of Kutzneria* sp. CA-103260**